Hitachi Medical Systems America, Inc.

Hitachi

MRI Anatomy and Positioning Series
Module 2: Lower Extremity Orthopedic Imaging

Welcome to the Hitachi Medical Systems America, Inc. MRI Anatomy and Positioning Series. Over the coming months, we will be offering teaching modules to allow users of Hitachi MRI scanners to polish their positioning skills and review the anatomy that should be seen on some common MRI exams. Our intention is to discuss and review the anatomy that is most often seen, and the positioning that is most often used in your MRI studies. Good positioning skills are needed to ensure the best possible image quality for your studies.

In this second module, we will discuss the anatomy and positioning of the bones, joints, ligaments, muscles, blood vessels, and nerves of the lower extremity. It is not our intention to outline all the anatomy that you may need to know, but rather to inspire you to become more familiar with the common anatomical structures. MRI allows us to be very exact with our work, as we have the ability to align slices exactly parallel or perpendicular to specific anatomic structures. Increased knowledge of cross-sectional anatomy will enable you to perform your job more efficiently and with greater expertise. An increased understanding of diseases and injuries relevant to an anatomic region can provide the technologist with insight into the use of certain sequences that may provide the radiologist with improved information, and a more accurate interpretation.

Caution symbol
CAUTION: Always route coil cables away from the patient, using pads and /or cable covers to eliminate or minimize the chances of contact between the coil cable and the patient. Failure to do so could result in a thermal injury.

Within our modules, we will offer suggestions as to appropriate RF coils to be used for various MRI exams. Your facility may not have all of the coils that are pictured, as some are optional and must be purchased separately. Regardless of the RF coil that is being used, every attempt should be made to route the coil cable(s) in a manner that will avoid contact with the patient.

We will also discuss the use of the various pads that are furnished with our MRI systems (trough pads, table pads, accessory pads, coil cable pads, etc.). It is important to use the various pads that are provided to assist in eliminating, or at least minimizing, the amount of each patient’s skin-to-skin, skin-to-bore, or skin-to-cable contact. Reducing the amount of each of the aforementioned contacts reduces the patient’s chances of thermal injury. Please refer to the MR Patient Warming Prevention Plan published by Hitachi Medical Systems America, Inc. for more information concerning the prevention of patient warming.

Caution symbol
CAUTION: Always use the pads that are provided to eliminate or minimize the patient’s skin-to-skin, skin-to-bore, and skin-to-cable contact. Failure to do so could result in a thermal injury.

Lower Extremity

Hips

MRI may be requested for: Bones and Cartilage of the Hip

The hip joints join the legs to the trunk of the body, and are formed by the femurs and pelvic bones. The hips are ball-and-socket type joints, where the femoral head (ball) fits into the cup-shaped acetabulum (socket) of the pelvis (Figure 1). When compared to the shoulder, which is also a ball-and-socket joint, the acetabulum is a deeper socket, and encompasses a greater area of the ball, or femoral head. This accommodation is necessary to provide stability for the hip, as it is a major weight-bearing joint, and one of the largest joints in the body. When not weight-bearing, the ball and socket of the hip joint are not perfectly fitted. However, as the hip joint bears more weight, the surface area contact increases, and the joint becomes more stable. When in a standing position, the body’s center of gravity passes through the center of the acetabula. While walking, weight-bearing stresses on the hips can be five times a person’s body weight. Healthy hip joints can support your weight and allow for pain-free movement. Hip injuries or disease can cause changes that affect your gait, as well as changes that affect the ability of the hips to distribute weight bearing. Abnormal stress is then placed on the joints that are above and below the hips.

The three fused hip or innominate bones that form the acetabulum include the ilium, pubis, and ischium. The ilium forms the superior aspect, the pubis forms the inferior and anterior aspect, and the ischium forms the inferior and posterior aspect. The depth of the acetabulum socket is further increased by the attached fibrocartilaginous labrum (Figure 2). In addition to providing stability to the hip joint, the labrum allows flexibility and motion. Hip joint stability can be hampered by injuries resulting from playing sports, running, overuse, or falling, as well as by disease or tumor. MRI of the hip(s) may be ordered to assess the joint(s) for internal derangement, fracture, or degenerative joint disease. A blow to the hip joint or a fall can result in dislocation of the hip, or a hip fracture. Osteoporosis or low bone density can also lead to hip fractures. Successful prevention and/or treatment of osteoporosis may be achieved through nutrition (adequate amounts of calcium, vitamin D and phosphorus), exercise, safety measures, and medications.

Articular cartilage covers the femoral head and the acetabulum (Figure 3). This cartilage is thin but tough, flexible, smooth and slippery, with a rubbery consistency. It absorbs shock, and allows the bones to move against each other easily and without pain. It is kept lubricated by synovial fluid, which is made in the synovial membrane (joint lining). Synovial fluid is both viscous and sticky. This fluid is what allows us to flex our joints under great pressure without wear. The articular cartilage of the hip is typically about ¼ inch thick, except in the posterior aspect of the hip socket (Figure 4). Here, the cartilage is thicker, as this area absorbs most of the force during walking, running, and jumping. MRI of the hip joint can detect problems involving both the articular cartilage and the fibrocartilaginous ring, or labrum. Cartilage has minimal blood vessels, so it is not good at repairing itself. Fraying, fissuring, and other abnormalities or defects of the cartilage can lead to arthritis in the hip joint. Contrast can be directly injected in the hip joint for a detailed look at the cartilage and labrum.

The femurs are the longest bones in the body, with large round heads that rotate and glide within the acetabula of the pelvis. The femoral head is particularly subject to pathologic changes if there is any significant alteration of blood supply (avascular necrosis). The femoral neck connects the head of the femur to the shaft. The neck ends at the greater and lesser trochanters, which are sites of muscle and tendon attachments. A disease characterized by an inadequate blood supply to the femoral head is Legg-Calve-Perthes disease, also known as LCP or simply Perthes disease. This is a degenerative disease of the hip joint that affects children, most commonly seen in boys ages two through twelve. One of the growth plates of the femoral head, the capital femoral epiphysis, is inside the joint capsule of the hip. Blood vessels that feed this epiphysis run along the side of the femoral neck, and are in danger of being torn or “pinched off” if the growth plate is damaged. This can result in a loss of blood supply to the epiphysis, leading to a deformity of the femoral head (Figure 5). The femoral head may become unstable and break easily, which can lead to incorrect healing and deformities of the entire hip joint (Figure 6). Treatment of Perthes disease is centered on the goal of returning the femoral head to a normal shape. Surgical and non-surgical treatments are used, based on the idea of “containment”- holding the femoral head in the acetabulum as much as possible, while still allowing motion of the hip joint for cartilage nutrition and healthy growth of the joint.

High level athletes and active individuals may be susceptible to a hip condition known as Femoro-Acetabular Impingement, or FAI. FAI is characterized by excessive friction in the hip joint. The femoral head and acetabulum rub abnormally, and can create damage to the articular or labral cartilage. FAI is also associated with labral tears, early hip arthritis, hyperlaxity and low back pain. FAI generally occurs in two forms: Cam and Pincer. The Cam form results in abnormal contact between the femoral head and the socket of the hip because the femoral head and neck relationship is aspherical (Figure 7). Males and those involved in significant contact sports typically display Cam impingement. Pincer impingement occurs when the acetabulum covers too much of the femoral head, resulting in the labral cartilage being pinched between the rim of the socket and the anterior femoral head-neck junction (Figure 8). Pincer impingement may be more common in women. Typically, these two forms exist together, and are labeled as “mixed impingement” (Figure 9).

 Ewing’s sarcoma is a malignant bone tumor that may affect the pelvis and/or femur, thereby also affecting the stability of the hips. Like Perthes disease, Ewing’s sarcoma is more common in males, typically presenting in childhood or early adulthood. MRI is routinely used in the work-up of these malignant tumors to show bony and soft tissue extent of the tumor, and its relation to nearby anatomic structures (Figure 10). Contrast may be used to help determine the amount of necrosis within the tumor, which aids in determining the response to treatment before surgery.

Ligaments of the Hip

Hip stability is further increased by three strong ligaments that encompass the hip joint and form the joint capsule. These ligaments connect the femoral head to the acetabulum, with names suggestive of the bones they connect. They include the pubofemoral and iliofemoral ligaments anteriorly, and the ischiofemoral ligament posteriorly (Figure 11). The iliofemoral ligament is the strongest ligament in the body. However, sports and overuse can still result in sprains of these sturdy ligaments of the joint capsules of the hips. A smaller ligament, the ligamentum teres, is an intracapsular ligament that connects the tip of the femoral head to the acetabulum (Figure 12). A small artery within this ligament brings some of the blood supply to the femoral head. Damage to the ligamentum teres, and its enclosed artery, can result in avascular necrosis.

Muscles and Tendons of the Hip

The muscles of the thigh and lower back work together to keep the hip stable, in alignment, and able to move. The hip gains stability because the hip muscles do not attach right at the joint. Hip muscles allow the movements of flexion, extension, abduction, adduction, and medial and lateral rotation. To better understand the functions of the muscles surrounding the hip, they can be divided into groups based on their locations- anterior, posterior, and medial.

The anterior thigh muscles are the main hip flexors, and are located anterior to the hip joint. Seventy percent of the thigh’s muscle mass is made up of the quadriceps femoris muscle, so named because it arises from four muscle heads- the rectus femoris, vastus medialis, vastus intermedius, and vastus lateralis (Figures 13, 14). The rectus femoris is the only one of the “quad” muscles to cross the hip joint. The sartorius muscle is found anterior to the quadriceps, and also serves as an abductor and lateral rotator of the hip. The most powerful of the anterior thigh hip flexors is the iliopsoas, which originates in the low back and pelvis and attaches at the lesser trochanter.

Posterior hip muscles include those of both the thigh and gluteal regions. The posterior thigh muscles are also known as the hamstrings- semimembranosus, semitendinosus, and biceps femoris (Figure 15). These muscles originate at the inferior pelvis, and are the extensors for the hip. They are active in normal walking motions. When the hamstrings are “tight”, they limit hip flexion when the knee joint is extended (bending forward from the waist with knees straight), and can limit lumbar movement, leading to back pain. The gluteal muscles include the gluteus maximus, medius, and minimus, six deep muscles that serve as lateral rotators, and the tensor fasciae latae. The three gluteals and the anterior sartorius muscle are all involved in abduction. The gluteus maximus is the main hip extensor, and is the most superficial of the gluteal muscles. It is involved in running and walking uphill, and assists with normal tone of the iliotibial band, which lies lateral to it. The gluteus medius and minimus both insert at the greater trochanter of the femur. The minimus is the deepest of the three gluteal muscles. Anterior to the gluteus minimus is the tensor fasciae latae muscle. It is a flexor and medial rotator of the hip, originating from the anterior superior iliac spine (ASIS) and inserting on the iliotibial band. The term “tensor fasciae latae” defines this muscle’s job- “muscle that stretches the band on the side”. This muscle helps the iliopsoas, gluteus medius, and gluteus minimus muscles during flexion, abduction and medial rotation of the thigh by making the iliotibial band taut, thereby steadying the trunk and stabilizing the hip (Figure 16). The iliotibial band or tract is not a muscle, but a thickened, fibrous band of deep fascia, or connective tissue. It is found at the lateral aspect of the thigh, and runs from the ilium to the tibia. It encloses the muscles and helps with lateral stabilization of the knee joint, as well as helping to maintain both hip and knee extension. Tightening of the iliotibial (IT) band typically causes more problems at the knee as opposed to the hip, but hip pain can result from the IT band rubbing as it passes over the greater trochanter.

The medial thigh (groin) muscles include five muscles of adduction, and one lateral rotator (Figures 17, 18). The lone lateral rotator is the obturator externus, which covers the external surface of the obturator foramen in the deep upper medial thigh. The adductors include the gracilis, the pectineus, and the adductor brevis, longus and magnus. The gracilis is the longest adductor, extending from the medial inferior aspect of the pubic bone, to the medial aspect of the tibia. The adductor magnus is the most massive of the medial muscles of the thigh.

The tendons and muscles of the hips are very powerful and create great forces, making them prone to inflammation and irritation. Tendonitis of the hip can result from repetitive movements involving the soft tissues surrounding the hip joint. Overuse of the hip joint in fitness workouts can lead to tendonitis. Tendons lose their elasticity as we age, resulting in swelling and irritation when the tendons are no longer “gliding” on their normal paths. Iliopsoas tendonitis plays a major role in snapping hip syndrome, or dancer’s hip. A snapping sensation when the hip is flexed and extended may be accompanied by an audible snapping or popping noise, as well as pain. This can be both an extra-articular and an intra-articular occurrence. Extra-articular snapping is often found in those patients with a leg length difference (the longer leg is symptomatic), those with tightness of the iliotibial band on the involved side, and those with weak hip abductors and external rotators. Lateral extra-articular snapping can be caused by the iliotibial band, tensor fascia latae or gluteus medius tendon as they slide back and forth across the greater trochanter (Figure 19). If any of these connective tissue bands thickens, they can “catch” on the greater trochanter during the motion of hip extension, thereby creating the “snapping” sensation and sound. Medial extra-articular snapping, which is less common, can occur when the iliopsoas tendon catches on the anterior inferior iliac spine, lesser trochanter, or iliopectineal ridge during hip extension. Intra-articular snapping hip syndrome is similar in many ways to the extra-articular type, but often involves an underlying mechanical problem in the lower extremity, and more intense pain. Intra-articular snapping may be indicative of a torn acetabular labrum, recurrent hip subluxation, a tear of the ligamentum teres, loose bodies, articular cartilage damage, or synovial chondromatosis (cartilage formations in the synovial membrane of the joint). Snapping hip syndrome is usually found in those ages 15-40, often in those in training for the military. It can also affect athletes, especially those involved in dance, gymnastics, soccer, and track and field. These athletes will all be performing repeated hip flexions, which can lead to tendonitis in the hip area. The repetitive motions of those involved in weightlifting and running generally lead to a thickening of the tendons in the hip region, rather than snapping hip syndrome. Prevention, or at least a lessening, of this syndrome may be found with increased stretching of the iliopsosas muscle or the iliotibial band. Surgery is usually not required, unless intra-articular pathology is present.

Tendon or muscle strains can occur suddenly, as in sports injuries, or they can develop over time, with symptoms including pain, swelling, muscle spasms, and difficulty moving certain muscles. MRI can be used to detect tendon and muscle tears and strains, as well as bone tumors and infection. MRI has shown good accuracy for the diagnosis of tears of the gluteus medius and gluteus minimus tendons, which are both abductor tendons of the hip. An association was found between these tears and areas of high signal intensity superior or lateral to the greater trochanter on T2-weighted images, tendon elongation in the gluteus medius, and tendon discontinuity (Figure 20). STIR and fat-suppressed T2-weighted coronal images are very sensitive for detection of areas of high signal intensity superior to the greater trochanter. Coronal T1-weighted images demonstrate tendon elongation in the gluteus medius (Figure 21). Axial images may prove superior for localizing involvement to individual abductor tendons and confirming tendon discontinuity (Figure 22). Tears of the abductor tendons may be the leading cause of greater trochanteric pain syndrome.

Nerves of the Hip

The nerves of the hip supply the various muscles in the hip area. The major nerves include the femoral, obturator, and lateral femoral cutaneous nerves anteriorly, and the large sciatic nerve posteriorly (Figure 23). The femoral nerve innervates the quadriceps femoris and sartorius, and is the sensory nerve to the anterior thigh. Trauma to this nerve usually occurs in the pelvis, as it passes through or near the psoas muscle. The obturator nerve passes along the lateral pelvic wall and through the obturator foramen, then splits into branches that supply the adductor muscle group. This nerve can also be subject to trauma in the pelvis due to its passage through the obturator foramen. The lateral femoral cutaneous nerve is a sensory nerve that travels along the anterolateral aspect of the thigh. It supplies sensation to the skin surface of the thigh. This is the single nerve involved in a painful condition called meralgia paresthetica, which is characterized by tingling, numbness, and burning pain in the outer part of the thigh. Meralgia paresthetica results from focal entrapment of the lateral femoral cutaneous nerve as it passes through the tunnel formed by the lateral attachment of the inguinal ligament and the ASIS. The posterior sciatic nerve passes deep to the gluteus maximus into the posterior thigh, where it innervates the hamstring muscles, on its way down to the lower leg and foot. The sciatic nerve is approximately as big around as the thumb, and is the largest single nerve in the human body. It can be injured in cases of posterior hip dislocation. Pressure on this nerve can cause nerve pain, numbness, tingling and weakness (sciatica symptoms) in the buttocks, leg, or foot, depending on the site of origin of the sciatic nerve compression.

Arteries and Veins of the Hip

The arterial blood vessels that supply the hips are branches of the internal and external iliacs. The internal iliac artery gives off the superior and inferior gluteals, and the obturator artery. The inferior gluteal flows to the posterior aspect of the hip joint and proximal femur, where it joins a branch of the femoral artery. The obturator artery runs through the obturator foramen, and sends its acetabular branch to the ligamentum teres as part of the blood supply to the femoral head. The external iliac becomes the femoral artery, which has numerous branches that supply the hip and proximal femur. The largest femoral branch is the profunda femoris, which branches superiorly into the medial and lateral circumflex femorals (Figure 24). The circumflex femorals and the inferior gluteal artery contribute to the anastomoses to supply the femoral head, femoral neck, and the hip joint. The medial circumflex also has an acetabular branch to the ligamentum teres. Congenital anomalies in the hip anastomoses, degenerative processes, and trauma can all compromise the blood supply to the hip joint area.

Venous flow in the hip and proximal femur typically follows the arterial flow, including the same names for the vessels. The deep veins of the hip and thigh can be the origination of a deep vein thrombosis, which can result in a pulmonary embolus. This can be caused by immobility after hip surgery, sitting in cars or airplanes for extended trips, being overweight, or slow or low blood flow. These blood clots can break off, travel through the larger veins of the thigh and hip, continue through the heart, and become lodged in the smaller vessels of the lung. MRI is being used more frequently to diagnose this very serious condition.

Bursae of the Hip

The hip joint is surrounded by bursae, similar to the shoulder. These fluid-filled sacs are lined with a synovial membrane, which produces synovial fluid. Their function is to lessen the friction between tendon and bone, ligament and bone, tendons and ligaments, and between muscles. There may be as many as 20 bursae around the hip. If they become infected or inflamed, the result is a painful condition called bursitis. Common hip bursae that may become inflamed include the greater trochanteric bursa, the iliopsoas bursa, and the ischial bursa (Figure 25). The greater trochanteric bursa is sandwiched between the greater trochanter of the femur, and the muscles and tendons that cross over it. If this bursal sac becomes inflamed, patients experience pain with every step they take, as each step requires the tendon to move over the femur at the hip joint. A tight iliotibial band can also cause irritation of the greater trochanteric bursa. Iliopsoas bursitis can result from irritation of the bursa found between the hip joint and the iliopsoas muscle that passes in front of it. Another common site for bursitis is the ischial bursa, which acts as a lubricating pad between tendons and the ischial tuberosity, which is the bony prominence of the pelvis that you sit on. The ischial bursa acts to prevent destruction of the tendons as they move over the ischial tuberosity. Prolonged sitting can cause ischial bursitis. Inflammation around the ischial tuberosity can irritate the sciatic nerve, and trigger symptoms similar to sciatica. Hip bursitis is seen in runners and athletes in sports that involve excessive running (soccer, football, etc.). It can also be caused by an injury (traumatic bursitis), and is seen in post-op hip replacement and hip surgery patients. Treatment for hip bursitis typically includes rest, anti-inflammatory medications, and ice. It may become necessary to aspirate the bursa, which can be combined with a cortisone injection. MRI may be needed if the diagnosis is unclear, or if the problem does not resolve with normal treatments.

Open MRI Systems

Introduction

When positioning a patient for a hip exam in an open MRI system, it is important to consider the choices you have as far as coil selection and patient setup. Positioning that will result in both the coil and the anatomy at isocenter in all three planes will have a positive impact on the image quality of your study.

Components of positioning to consider include coil selection, table pads and accessory pads, and proper laser light centering. Coil selection will be influenced by the patient’s size and body habitus. The extensive inventory of table and accessory pads should be used for proper centering, as well as patient comfort and stability (Figure 26). The use of trough pads and thick or thin table pads are keys to achieving coronal centering of both the patient and the coil. When scanning on open MRI systems, it is extremely important to center the anatomy of interest in the laser lights (Figure 27) in all three directions: head-to-foot (axial or transverse plane), right-to-left (sagittal plane), and anterior-to-posterior (coronal plane).

Coils and Positioning

RAPID Body Coil

The coil of choice for a study of the hips is the RAPID body coil. Fit the base of the coil in the table trough. The longitudinal center mark on the coil will be centered at the midline of the table. The horizontal center of the coil should be between the marking on the patient table and the end of the table nearest the magnetic field. This will allow for sufficient table travel while still achieving isocenter positioning. The patient should be positioned on the base portion of the coil with their hips aligned with the horizontal center mark on the coil. The coil pads and additional table pads can be adjusted to help achieve coronal centering of the hips. The upper portion of the coil is then placed on the base and pushed firmly into place to lock the coil (Figure 32). For bilateral hip scanning, the midline of the patient’s body should be aligned with the longitudinal center mark on the upper portion of the coil. The patient should now be centered in all three planes- centered on the hips in the coronal and axial planes, and centered midline in the sagittal plane. If a unilateral hip scan is to be performed, the table can be moved to the right or left to achieve the best possible sagittal plane centering of the affected hip.

Flexible Body Coil

The flexible body coils can be used for hip scanning to accommodate larger patients. However, they do not have RAPID capabilities, and should not be used with protocols that are labeled as “RAPID”. Trough and/or table pads should be placed on the table to help center the flexible coil in the coronal plane. The horizontal center of the coil should be between the marking on the patient table and the end of the table nearest the magnetic field. This will allow for sufficient table travel while still achieving isocenter positioning. The longitudinal center of the coil should be aligned with the sagittal laser light. Position the patient on the flexible coil with their hips aligned with the horizontal center mark on the coil. Depending on the patient’s body habitus, table and/or accessory pads may need to be adjusted to maintain coronal centering. Close the flexible body coil around the patient and secure the latches. Accessory pads can be placed between the flex coil and the patient to maintain the anterior portion of the coil in a level position, which will minimize stress on the latches. For bilateral hip scanning, the midline of the patient’s body should be aligned with the longitudinal center mark or stitching found on the anterior aspect of the coil (Figures 33, 34). The patient should now be centered in all three planes- centered on the hips in the coronal and axial planes, and centered midline in the sagittal plane. If a unilateral hip scan is to be performed, the table can be moved to the right or left to achieve the best possible sagittal plane centering of the affected hip.

Regardless of the coil being used, your radiologist’s preference may be to have you secure the patient’s feet with their toes slightly inverted, as seen in Figure 34. This is done to minimize rotation of the femoral heads, so images can be acquired with the hips in true anatomic position. Sponges, tape, and MR-safe sandbags can be used to secure the patient’s feet in the desired position. The patient’s arms and hands can be placed at their sides, above their head, or on their chest, with no interlocking of their fingers. Their arms and hands should remain outside of the RAPID or flex coils. The table straps may be used to further secure both the RAPID and flexible coils. Figure 34 demonstrates use of the table strap with the flexible coil.

Echelon MRI System

Introduction

The 1.5T Echelon system comes equipped with a variety of trough and table pads, as well as various positioning pads and sponges (Figure 35). These positioning aids can be used to support the position of both the patient and the coil, as well as to keep your patient comfortable and secure. The magnet bore of the Echelon does not permit lateral patient table movement. However, accurate patient and coil positioning, and correct use of the sagittal and axial laser lights (Figure 36) to center the patient’s anatomy ensures that high quality images will be acquired through isocenter scanning.

Coils and Positioning

RAPID Torso/Body Coil

The coil of choice for a study of the hips is the RAPID Torso/Body coil. Trough and/or table pads are placed on the table as needed, depending on the patient’s size and body habitus. The lower portion of the coil should be positioned in the middle of the table on top of the pads. The exact placement of the coil on the table, and the direction it faces, are based on the patient’s preference to enter the scanner head first or feet first. The patient should be in a supine position on the lower portion of the coil with their hips aligned with the coil’s horizontal center marks. Their feet should be pointed in the same direction as the coil cables and plugs. The upper portion of the coil is then placed on the base and secured with the Velcro® straps (Figure 38). The midline of the patient’s body should be aligned with the longitudinal center mark on the upper portion of the coil.

Transmit / Receive Body Coil

This integrated coil is the coil of choice for very large patients, as well as high-anxiety or claustrophobic patients. These patients can be scanned without a coil being placed over or around their bodies. However, use of the T/R body coil in this manner is not recommended for routine day-to-day scanning. Specific parameter changes should be made before using the integrated coil. The patient can be positioned on the table either head first or feet first. Trough and/or table pads are placed on the table as needed, depending on the patient’s size and body habitus. The patient’s midline should be centered in the sagittal laser light, and their hips should be centered in the axial laser light. 

Regardless of the coil being used, your radiologist’s preference may be to have you secure the patient’s feet with their toes slightly inverted. This is done to minimize rotation of the femoral heads, so images can be acquired with the hips in true anatomic position. Sponges, tape, and MR-safe sandbags can be used to secure the patient’s feet in the desired position. The patient’s arms and hands can be placed at their sides, above their head, or on their chest, with no interlocking of their fingers. Their arms and hands should remain outside of the RAPID torso/body coil (Figure 38).

Scan Setups

The following are HMSA suggestions for hip imaging. Always check with your radiologist for his/her imaging preferences. You may be required to scan all hips bilaterally, or to scan bilaterally in certain planes using specific sequences, with the remainder of the scans performed only on the affected hip.

Axial Scans

When positioning unilateral axial slices for the hip, a coronal image can be used to ensure inclusion of all pertinent anatomy. The slices should extend superiorly to include the entire femoral head and acetabulum, and inferiorly to include anatomy below the lesser trochanter. The slices should be aligned perpendicular to the shaft of the femur, as seen in the coronal image in Figure 39.

For bilateral axial hip slice setup, parameters may have to be altered to maintain adequate resolution with the larger FOV that is required (Figure 40). The slice group may require angulation to maintain alignment of the femoral heads on the resultant images.

Coronal Scans

Coronal slices of the hip should cover the area from the posterior margin to the anterior margin of the femoral head. The area from the proximal margin of the femoral shaft to the greater sciatic notch should be included in the image (Figure 41). Slices may be angled so that they are parallel to the femoral neck. Thinner slices may be requested for coronal scanning.

Sagittal Scans

Sagittal slices of the hip should extend past the greater trochanter laterally, and through the acetabulum medially. The slices should be aligned along the long axis of the femur, and perpendicular to the coronal slices, as seen in the coronal image in Figure 42. Two different slice groups will be necessary when performing bilateral sagittal scans.

Hip Arthrography

MR hip arthrography is often times referred to as the gold standard for assessment of the labrum of the hip. The most clinically significant abnormal findings that result from hip arthrography are labral detachments and tears. Detachment of the labrum, which is more common than a labral tear, can be diagnosed from the appearance of the injected contrast at the acetabular-labral interface (Figure 43). A labral tear can result in injected contrast appearing within the substance of the labrum (Figure 44). Contrast injection is necessary to differentiate torn or detached labra from other pathologic conditions, which may have separate signal intensities. The sensitivity and accuracy for the diagnosis of labral tears and detachment with MR arthrography vs. nonarthrographic MR is 90%. Hip arthrography with MR can also depict intrarticular loose bodies, osteochondral abnormalities, and abnormalities of soft-tissue structures.

Hip arthrography can be performed under fluoro in the x-ray dept., with the patient being moved to the MRI dept. for further imaging, or the entire procedure can be performed in the MRI suite, if MR compatible supplies are available for interventional techniques. The patient should be securely positioned with the hips in internal rotation.

T1-weighted imaging is performed post-contrast to visualize the high signal of the intraarticular contrast. T1 gradient echo sequences offer the benefits of thin sections, elimination of partial volume averaging, and increased detection of small tears. Fatsat sequences are helpful in increasing the contrast between the injected contrast and the adjacent soft tissue. STIR or fatsat T2 sequences performed in the coronal plane may help to detect unsuspected pathologic conditions in the soft tissue and adjacent osseous structures.

Post-contrast axial oblique images have been shown to optimize the detection of the most common sports-related acetabular labral tears, which are anterior or anterosuperior in location. Using a mid-coronal localizer, the axial oblique slices should be prescribed parallel to the long axis of the femoral neck.

Femur (Thigh)

MRI may be requested for: Bone Anatomy and Pathology of the Femur (Thigh)

The two femurs are the largest, longest, and strongest bones in the body. Superiorly, the femoral head articulates with the acetabulum of the pelvis to form the hip joint. This is a multiaxial joint, and includes the motions of flexion, extension, adduction, abduction, and medial and lateral rotation. The femoral head, neck, and greater and lesser trochanters have been discussed previously with the anatomy of the hip. Along the posterior aspect of the shaft of the femur is found the linea aspera, which is a ridge with a roughened surface (Figure 45). This is the insertion site for the adductors and intermuscular septa. The ridge forms due to the tension generated by the muscles that are attached to the femur. Inferiorly, the femoral condyles articulate with the flat, plateau-like surfaces of the tibial condyles to form the knee joints, whose principle movements are flexion and extension. The knee joints also include the patellofemoral joints, which are saddle-type joints between the patellas and femurs. The femoral condyles are separated anteriorly by a smooth, shallow articular depression called the patellar surface. Posteriorly, the condyles are separated by the intercondylar fossa, which is the area that the anterior and posterior cruciate ligaments pass through before attaching to the lateral and medial walls of the fossa, respectively. The medial and lateral femoral epicondyles are located superiorly to the condyles, and are sites for muscle and ligament attachments. The medial femoral epicondyle is the point of attachment for the tibial collateral ligament of the knee, and is also the site of the adductor tubercle, which is the insertion site for the adductor magnus muscle. The lateral femoral epicondyle is the point of attachment for the fibular collateral ligament, also a knee support.

Various pathologies that affect the femoral head and neck were discussed previously with the hip anatomy. These included avascular necrosis, Perthes disease, and FAI. MRI may play a role in the diagnosis of these conditions and diseases, but already has a large part in the diagnosis of femoral diaphysis injuries. Many injuries to the shaft of the femur are misdiagnosed as muscle or tendon strains, as the patient may report vague symptoms of medial or anterior thigh pain. Studies have shown that the femur may be the fourth most common site for stress fractures. MRI has become the imaging modality of choice, as it has excellent specificity and sensitivity to the early stress changes seen in bone. Repetitive submaximal stresses can create an exaggerated bone-remodeling response, and a net weakening of the bone. A true stress fracture is actually the final stage of the preceding grades of injury. Radiologists may use a 5-stage grading system, with Grade 0 representing a normal stage, and Grade 4 representing a stress fracture, with a discrete fracture line that can be seen on MRI. Stress injuries may increase along a continuum of severity with these grades, beginning with periosteal edema, progressing to marrow edema, and on to a cortical fracture. Diaphyseal stress injuries are common in middle and long distance runners, and are typically seen in the medial aspect of the proximal and middle thirds of the femur. This area may be more susceptible to stress injuries due to the biomechanical forces from weight bearing and muscle exertion. With weight bearing, the lateral aspect of the bone undergoes tension, while the medial aspect undergoes compression. This medial compression is increased with muscle actions, resulting in more stress injuries in this area. MRI scans for stress fracture of the femoral diaphysis should be centered on the area of the worst pain. Recommended sequences include coronal and axial T1 and T2 weighted sequences, with fat suppression or STIR (Figures 46-48).

A condition that affects the distal femur and benefits from MRI for diagnosis is osteochondritis dissecans. This is a type of osteochondral lesion. It differs from Perthes disease, which was discussed previously in the hip anatomy, in that Perthes disease is a secondary process of an osteochondral abnormality. Osteochondritis dissecans can result from physical trauma, ischemia, avascular necrosis, and/or repetitive trauma. Cracks develop in the articular cartilage and underlying subchondral bone. The subchondral bone is deprived of blood, and dies off (avascular necrosis). The bone is then reabsorbed, leaving the articular cartilage prone to damage. This results in fragmentation of the cartilage and the bone, and osteochondral fragments in the joint space. Seventy-five percent of osteochondritis occurrences are in the knee. This condition may be seen in physically active adolescents, but is rare, and they typically have a quicker recovery. An adolescent’s bones are still growing, and they have superior bone-remodeling capabilities- their bones repair damaged or dead bone tissue and cartilage. Amongst juveniles and adults, this condition is usually trauma-related. High-impact sports (gymnastics, soccer, basketball, football) may put participants at a higher risk for this condition due to stresses on the joints. Patients report symptoms including pain, swelling, joint effusion and tenderness, and joints that “catch” or lock. Non-surgical treatment can be difficult, as articular cartilage has limited capacity for healing. Surgical treatments include arthroscopic drilling of intact lesions, and drilling and replacement of cartilage plugs. MRI is beneficial for staging osteochondritis dissecans lesions, evaluating the integrity of the joint surface, and showing edema, fractures, fluid interfaces, and fragment displacement (Figure 49). Bone and cartilage edema that can be seen on MRI can help to distinguish osteochondritis dissecans from normal bone variants. The lateral aspect of the medial femoral condyle is often affected first, followed by the weight-bearing surface of the lateral femoral condyle. Fluid insinuation or junctional cysts between fragments and parent bones suggest instability, and may require orthopedic intervention.

Ligaments of the Femur

Ligaments of the superior aspect of the femur were discussed previously in the hip anatomy. The superior femoral ligaments connect the femoral head to the acetabulum. Inferior femoral ligaments are associated with the knee joint, and will be discussed here only to the extent of their attachment to a femoral epicondyle or condyle. Ligaments that are considered intracapsular for the knee joint include the anterior and posterior cruciates, and the anterior and posterior meniscofemoral ligaments (Figure 50). The anterior cruciate ligament stretches from the lateral femoral condyle to the anterior intercondylar area. The posterior cruciate ligament stretches from the medial femoral condyle to the posterior intercondylar area. The cruciate ligaments help to control anterior/posterior displacement of the tibia relative to the femur. The anterior and posterior meniscofemoral ligaments (or Ligaments of Humphrey and Wrisberg, respectively) stretch from the posterior horn of the lateral meniscus to the medial femoral condyle. The ligament of Humphrey passes anterior to the posterior cruciate ligament, while the ligament of Wrisberg passes posterior to the posterior cruciate ligament (Figures 51, 52). These ligaments may be the only attachments for the posterior horn of the lateral meniscus.

Extracapsular ligaments that involve the femur include the medial and lateral collateral ligaments, and the oblique popliteal ligament. The medial (tibial) collateral ligament extends from the medial epicondyle of the femur to the medial tibial condyle. The lateral (fibular) collateral ligament extends from the lateral epicondyle of the femur to the head of the fibula. The medial and lateral collateral ligaments lend to the stability of the knee joint by resisting sideward displacement and rotation. The oblique popliteal ligament attaches to the posterior surface of the femur, and acts as a posterior stabilizer for the knee joint.

Muscles and Tendons of the Femur (Thigh)

The muscles of the femur have been discussed previously in regard to their affect on the hip joint. These muscles will be reviewed with focus on their distal attachments, and their actions on the knee joint. Beginning with the muscles of the anterior thigh, the sartorius muscle is a knee flexor (Figure 53). It begins at the anterior superior iliac spine, and inserts on the medial aspect of the tibia. The quadriceps femoris is the main knee extensor. Its vastus lateralis and vastus medialis muscles arise from the linea aspera, while the vastus intermedius arises from the femoral shaft. The four heads of this muscle (including rectus femoris) converge on the superior aspect of the patella (which is the base of the patella) to form the patellar tendon. Some fibers continue over the patellar surface to join the patellar ligament below. The vasti lateralis and medialis also serve to strengthen the knee joint capsule distally. If the patella is cut away from the quadriceps femoris, the articularis genu muscle is seen. This small muscle arises from the lower anterior surface of the femur and attaches in the upper part of the knee joint capsule. It serves to elevate the capsule and synovial membrane of the knee joint, preventing any pinching of these structures during leg extension. The articularis genu plays an important role in the erect posture of humans.

The muscles of the posterior thigh are the hamstring muscles, which are knee flexors (Figure 54). The biceps femoris inserts on the head of the fibula, the semitendinosus inserts on the medial aspect of the tibia, and the semimembranosus inserts on the medial condyle of the tibia. Additional posterior muscles that are knee flexors include the popliteus and gastrocnemius of the lower leg. The popliteus originates on the lateral femoral condyle, and pulls on that condyle to release the knee from full extension so flexion can begin. It inserts on the superior and medial aspect of the tibia. The two heads of the gastrocnemius originate from the medial and lateral femoral condyles, respectively. This muscle covers the full extent of the posterior lower leg. 

The gracilis muscle, located in the medial thigh, is the longest of the adductor muscles, and is also a knee flexor. It crosses the medial knee, and inserts on the medial tibia. The tendon of the gracilis joins the tendons of the sartorius (anterior thigh muscle) and the semitendinosus (hamstring muscle) to insert on the anteromedial surface of the proximal tibia. This conjoined tendon insertion is called “pes anserinus”, as it is shaped like a goose’s foot. The pectineus, adductor brevis, adductor longus and adductor magnus medial thigh muscles all insert on the linea aspera of the femur. In the lower half of the adductor magnus, the muscle fibers give way to the adductor hiatus, which is the passage for the femoral artery and vein. 

Although not included in the muscle groups of the femur, the iliotibial tract plays a part in knee motion. The tensor fasciae latae muscle “tenses” the iliotibial band, and works with the gluteus maximus to lock the knee in full extension. The iliotibial tract runs along the lateral aspect of the thigh, attaching at the lateral epicondyle of the tibia.

Strains of the thigh muscles are common injuries, especially amongst those who participate in sports. Strains are stretched or torn muscles or tendons. The hamstring and quadriceps muscle groups are particularly at risk, as they cross both the hip and knee joints. They are also the muscle groups that are used for high-speed activities, such as track and field events, football, basketball, and soccer. A person experiencing a muscle strain in the thigh will frequently describe a popping or snapping sensation as the muscle tears. Pain is typically sudden and may be severe. The area around the injury may be tender to the touch, and bruising may be visible if blood vessels are broken. Contusions to the quadricep muscles can result from a direct blow to the anterior thigh from an object or person, such as a helmet or competitor’s knee. The quadriceps are in contact with the femur throughout its length, making these muscles susceptible to compression forces. The muscles are more resistant to injury if struck while contracted and nonfatigued. The rectus femoris is the most commonly injured quadriceps muscle, as it is the most anterior. Partial tears most commonly affect the distal head of the rectus femoris. The classic quadriceps strain occurs at the conjoined muscle tendon junction, often referred to as “jumper’s knee”.

The posterior thigh muscles, known as the hamstrings, are important in “power” activities, such as running, jumping, and climbing. Adolescent athletes who are still growing are prone to hamstring strains because their bones and muscles do not grow at the same rate. During a growth spurt, a child’s bones may grow faster than their muscles, and the growing bone can pull the muscle tight. A sudden impact, jump, or stretch can tear the muscle away from the bone. Injuries typically occur in the thick part of the muscle, or where muscle fibers join tendon fibers. Frequently, hamstring strains are the result of an imbalance between opposing muscle groups; the opposing muscle group for the hamstrings are the quadriceps. The anterior quadriceps muscles are usually more powerful, and do not fatigue as quickly. The hamstrings will fatigue faster during high-speed activities, which leaves them prone to strains. If the hamstrings are strained while sprinting in full stride, there is typically sudden, sharp pain in the back of the thigh. The injured person comes to a quick stop, and either falls or hops on their good leg. Additional symptoms of hamstring injuries include swelling, bruising of the back of the leg, and weakness of the hamstring muscles. Hamstring injuries are often labeled as pulls, partial tears, or complete tears. Complete tears, or avulsions, occur when the tendon tears away from the bone (Figure 55). The tendon may pull a piece of the bone away with it. Injuries of this severity will usually require surgical treatment. MRI is useful in helping to determine the severity of hamstring injuries, as it offers high quality images of soft tissues.

Nerves of the Femur (Thigh)

The major nerves of the proximal femur were discussed with the hip anatomy. These nerves will be reviewed and discussed with relation to additional branches and functions throughout the thigh region. Nerves from the lumbar plexus that descend into the thigh include the femoral, obturator, and lateral femoral cutaneous nerves (Figures 56, 57). The femoral nerve has an anterior and a posterior division. The anterior division of the femoral nerve is further divided into anterior cutaneous branches and muscular branches. The anterior cutaneous branches include the intermediate and medial cutaneous nerves. These nerves continue to branch out on their descent through the thigh, eventually communicating with the saphenous nerve in the region of the patella. The muscular branches of the anterior division of the femoral nerve include the nerve to the pectineus, which is the superior medial thigh muscle, and the nerve to the sartorius, one of the anterior thigh muscles. The posterior division of the femoral nerve supplies the four muscles that make up the quadriceps femoris (anterior thigh muscles). The largest and longest of the femoral nerve branches is the saphenous nerve. It branches from the femoral nerve near the lateral aspect of the pubic bone, then passes between the gracilis and sartorius muscles and becomes superficial. The saphenous nerve passes anteroinferiorly to supply the skin and fascia of the anterior and medial aspects of the knee, leg, and foot. The obturator nerve was previously mentioned as the nerve supply to the adductor muscle group. Its cutaneous branch also supplies the medial aspect of the thigh. The lateral femoral cutaneous nerve of the lumbar plexus was previously discussed in hip anatomy as the nerve involved in meralgia paresthetica. It is a sensory nerve only, with its branches supplying the skin on the lateral aspect of the thigh.

Nerves that supply the thigh from the sacral plexus include the posterior femoral cutaneous nerve and the sciatic nerve. Branches of the posterior femoral cutaneous nerve supply the skin of the posterior thigh and proximal leg (Figure 57). The large sciatic nerve was previously mentioned in the hip anatomy as the innervator of the hamstring muscles. The sciatic nerve descends and splits into the common peroneal and tibial nerves (Figure 58). These two nerves are bound together by the same connective tissue sheath. They typically separate approximately halfway down the thigh, superior and posterior to the knee, but they occasionally separate as they leave the pelvis. The common peroneal, or fibular nerve, is the smaller and more lateral of the two terminal sciatic branches (Figure 59). It passes over the posterior aspect of the head of the fibula, then winds around the lateral surface of the fibular neck, where it is palpable. The tibial nerve is the more medial of the two terminal branches of the sciatic nerve. It supplies all the muscles in the posterior compartment of the lower leg. The tibial nerve is the most superficial component of the popliteal fossa (the depression or “pit” at the back of the knee), which also contains the popliteal artery and vein.

Arteries and Veins of the Femur (Thigh)

The femoral artery is the main blood supply for the thigh and femur. It is sometimes referred to as the common femoral artery at its superior location, from which it divides into the deep femoral (profunda femoris) and the superficial femoral. The common femoral portion is the site of catheter placement for interventional and diagnostic tests. A catheter or needle can be placed to oppose the blood flow (retrograde) for cardiac studies and testing on the opposite lower extremity, or in the same direction as the blood flow (antegrade) for studies of the same lower extremity. Common femoral arteries can be a location for endarterectomy procedures, which involve a surgical cutdown of the vessel, and the removal of plaque from within the vessel.

The femoral artery is often a site of peripheral artery disease, brought on by arteriosclerosis. Plaque builds on the arterial walls, causing them to narrow and stiffen. The arteries can then no longer dilate to allow proper blood flow to the lower extremities. The muscles of the lower legs suffer from decreases in both blood flow and oxygen, leading to pain, weakness, and numbness in the lower extremities. In its early stages, the patient may have symptoms of intermittent claudication, where they have pain while exercising, but not when they are at rest. As the femoral artery blockage increases, patients may progress to ischemic rest pain, in which they suffer from constant pain, and cannot tolerate even a bed sheet touching their legs. The ischemia, or insufficient blood flow to the tissues of the lower extremity, can result in sores on the feet, leading to ulcers and even gangrene. Peripheral arterial disease is most common in men over 50, and increases with age. It often accompanies other conditions such as hypertension, diabetes, high cholesterol, and stroke. Treatment of peripheral artery disease includes angioplasty, peripheral artery bypass surgery, or stents in the affected leg (Figures 60, 61). MRI plays an important role in the realm of diagnostic testing for peripheral vascular disease. For post-op MR exams, safety precautions must be researched and observed for patients who have received stents, regarding if and when it is safe to scan the patient in an MR system.

The deep femoral artery, or profunda femoris, is a large artery with multiple perforating branches (Figure 62). It perforates the adductor magnus muscle to gain access to the medial and posterior compartments of the thigh. It descends very close to the femur, and does not leave the thigh region. The medial and lateral circumflex femorals are branches of the profunda femoris that supply the femoral head and neck. The lateral circumflex gives off the descending branch artery, which descends along the lateral aspect of the femur, connecting to the lateral superior genicular artery above the knee. The descending branch artery is used for bypass and reconstruction surgery.

The superficial femoral artery takes blood to the arteries that supply the knee, lower leg, and foot. It travels down the thigh in the adductor canal, which is an intermuscular passage for the femoral vessels. The canal ends at the adductor hiatus, which is in the tendon of the adductor magnus muscle. Here, the femoral artery becomes the popliteal artery, and enters the popliteal fossa. Just superior to the adductor hiatus is the split where the descending genicular artery branches off from the superficial femoral artery. The genicular arteries are an important source of blood flow to the knee.

As with the arteries of the femur and thigh region, the major veins that move blood back towards the heart include both deep and superficial veins. Typically, deep veins run alongside arteries, and share their names. Superficial veins usually travel with cutaneous nerves, and are the vessels that are visualized in the extremities. Blood moves from the superficial veins to the deep veins by way of communicating veins or perforators. Perforating and superficial veins have valves that ensure the uni-directional flow of blood back towards the heart. The blood in the veins of the legs must overcome gravity for quite a distance, causing the venous valves to come under weight-bearing stress. Incompetent valves can result in a pooling of the blood, and swelling in the lower superficial veins. This can lead to conditions such as varicose and spider veins, or superficial thrombophlebitis. Clots that form in the superficial veins typically do not enter the deep venous system, as the valves in the perforating or communicating veins act as sieves to filter them out. However, clots in the deep veins, or deep vein thromboses (DVT) can be life threatening. Blood moves back to the heart through normal body movement, so prolonged immobility can lead to slower blood flow (stasis) and venous distention and inflammation. Clots can form, and possibly release small fragments (emboli) into the circulation (Figure 63). These fragments can flow through the larger veins that lead to the heart, into the right side of the heart, and out to the smaller vessels of the lungs, where they can create pulmonary embolisms. They are often diagnosed through the use of CTA (computed tomography angiography). Treatment of DVTs can help prevent pulmonary embolisms, as more than 90% of pulmonary embolisms come from the legs, specifically the thigh region. DVTs below the knee do not usually break off. There is an increased risk of clot development during periods of immobility (hospitalization, prolonged travel), or due to conditions such as obesity, pregnancy, or hypercoagulability (blood coagulates too quickly). Trauma to a vein, whether it is from a fractured bone, a bruise, or a complication of an invasive procedure, can also increase the risk for a DVT. Ultrasound is typically the imaging modality of choice for a DVT diagnosis. CTA, or Computed Tomography Angiography, is also used, but involves radiation. Once diagnosed, patients are typically placed on anticoagulant therapy, unless they have had recent surgery or are already receiving anticoagulants. In those cases, an IVC filter can be placed to prevent the emboli from reaching the lungs.

The major deep veins of the femur and thigh include the popliteal, femoral, profunda femoris, and medial and lateral circumflex. These are listed and discussed in ascending order, or from distal to proximal, following the flow of blood in the vein. The popliteal vein begins at the juncture of the anterior and posterior tibial veins, inferior to the knee joint. The small saphenous vein joins the popliteal vein just superior to the knee. At the adductor canal, the popliteal vein continues as the femoral vein. The femoral vein runs medially up the thigh, and is joined by the profunda femoris vein near the hip. Although it is truly a deep vein, the femoral vein is sometimes referred to as the superficial femoral vein, as it runs alongside the superficial femoral artery. The profunda femoris, or deep femoral vein, drains the posterior thigh muscles, and ascends the thigh to join the (superficial) femoral vein. The medial and lateral circumflex femoral veins accompany the arteries of the same names, and empty into the femoral vein just superior to the point where the profunda femoris joins the femoral. In the groin area, the femoral vein is referred to as the common femoral vein, as this region is superior to the point where the profunda femoris and circumflex veins have joined the femoral vein. The great saphenous vein also communicates with the common femoral vein in this area.

The great saphenous vein is the main superficial vein of the thigh and femur (Figure 64). It is the longest vein in the body, beginning in the medial foot, and ascending to the hip, with many branches along the way. In the area of the thigh, the great saphenous vein ascends over the posterior border of the medial epicondyle of the femur, heading anteriorly to lie on the anterior surface of the thigh. It receives blood from numerous tributaries. Occasionally, the tributaries from the medial and posterior thigh unite to form an accessory saphenous vein, which will join the great saphenous vein at varying levels along the thigh. The great saphenous joins the femoral vein in the femoral triangle, at the saphenofemoral junction. The great saphenous can be used for auto transplantation in coronary artery bypass operations when arterial grafts are not available, or numerous grafts are needed (triple or quadruple bypass). It is also used for peripheral artery bypass, as the vein has better long term patency than synthetic grafts. However, extreme caution must be used when excising the saphenous vein. Damage to the saphenous nerve can occur during extraction of the saphenous vein due to their close proximity in the thigh.

Open MRI Systems

Introduction

When positioning a patient for a femur exam in an open MRI system, it is important to consider the choices you have as far as coil selection and patient setup. Positioning that will result in both the coil and the anatomy at isocenter in all three planes will have a positive impact on the image quality of your study.

Components of positioning to consider include coil selection, table pads and accessory pads, and proper laser light centering. Coil selection will be influenced by the patient’s size and body habitus. The extensive inventory of table and accessory pads should be used for proper centering, as well as patient comfort and stability (Figure 65). The use of trough pads and thick or thin table pads are keys to achieving coronal centering of both the patient and the coil. When scanning on open MRI systems, it is extremely important to center the anatomy of interest in the laser lights (Figure 66) in all three directions: head-to-foot (axial or transverse plane), right-to-left (sagittal plane), and anterior-to-posterior (coronal plane).

Coils and Positioning

RAPID Body Coil

The coil of choice for a study of the femurs is the RAPID body coil. Fit the base of the coil in the table trough. The longitudinal center mark on the coil will be centered at the midline of the table. The horizontal center mark on the coil should be between the marking on the patient table and the end of the table nearest the magnetic field. This will allow for sufficient table travel while still achieving isocenter positioning. The patient should be positioned on the base portion of the coil with the midpoint of their femurs (in the head to foot direction) aligned with the horizontal center mark on the coil. The coil pads and additional table pads can be adjusted to help achieve coronal centering of the femurs. The upper portion of the coil is then placed on the base and pushed firmly into place to lock the coil (Figure 71). For bilateral femur scanning, the midline of the patient’s body should be aligned with the longitudinal center mark on the upper portion of the coil. The patient should now be centered in all three planes- centered on the middle of the femurs in the coronal and axial planes, and centered midline in the sagittal plane. If a unilateral femur scan is to be performed, the table can be moved to the right or left to achieve the best possible sagittal plane centering of the affected femur.

Flexible Body Coil

The flexible body coils can be used for femur scanning to accommodate larger patients. However, they do not have RAPID capabilities, and should not be used with protocols that are labeled as “RAPID”. Trough and/or table pads should be placed on the table to help center the flexible coil in the coronal plane. The horizontal center of the flexible body coil should be between the marking on the patient table and the end of the table nearest the magnetic field. This will allow for sufficient table travel while still achieving isocenter positioning. The longitudinal center mark on the bottom of the coil should be aligned with the sagittal laser light. Position the patient on the flexible coil with the midpoint of their femurs aligned with the horizontal center mark on the coil. Depending on the patient’s body habitus, table and/or accessory pads may have to be adjusted to maintain coronal centering (Figure 72). Close the flexible body coil around the patient and secure the latches. Accessory pads can be placed between the flex coil and the patient to maintain the anterior portion of the coil in a level position, which will minimize stress on the latches. For bilateral femur scanning, the midline of the patient’s body should be aligned with the longitudinal center mark or stitching found on the anterior aspect of the coil (Figure 73). The patient should now be centered in all three planes- centered on the femurs in the coronal and axial planes, and centered midline in the sagittal plane. If a unilateral femur scan is to be performed, the table can be moved to the right or left to achieve the best possible sagittal plane centering of the affected femur.

Regardless of the coil being used, your radiologist’s preference may be to have you secure the patient’s feet with their toes slightly inverted. This is done to minimize rotation of the femoral heads, so images can be acquired with the femurs in true anatomic position. Sponges, tape, and MR-safe sandbags can be used to secure the patient’s feet in the desired position. The patient’s arms and hands can be placed at their sides, above their head, or on their chest, with no interlocking of their fingers. Their arms and hands should remain outside of the RAPID or flex coils. The table straps may be used to further secure both the RAPID and flexible coils. Figures 72 and 73 demonstrate use of the table strap with the flexible coils.

Echelon MRI System

Introduction

The 1.5T Echelon system comes equipped with a variety of trough and table pads, as well as various positioning pads and sponges (Figure 74). These positioning aids can be used to support the position of both the patient and the coil, as well as to keep your patient comfortable and secure. The magnet bore of the Echelon does not permit lateral patient table movement. However, accurate patient and coil positioning, and correct use of the sagittal and axial laser lights (Figure 75) to center the patient’s anatomy ensures that high quality images will be acquired through isocenter scanning.

Coils and Positioning

RAPID Torso/Body Coil

The coil of choice for a study of the femurs is the RAPID Torso/Body coil. Trough and/or table pads are placed on the table as needed, depending on the patient’s size and body habitus. The lower portion of the coil should be positioned in the middle of the table on top of the pads. The exact placement of the coil on the table, and the direction it faces, are based on the patient’s preference to enter the scanner head first or feet first. The patient should be in a supine position on the lower portion of the coil with the midpoint of their femurs aligned with the coil’s horizontal center marks. Their feet should be pointed in the same direction as the coil cables and plugs. The upper portion of the coil is then placed on the base and secured with the Velcro® straps (Figure 77). The midline of the patient’s body should be aligned with the longitudinal center mark on the upper portion of the coil.

Transmit / Receive Body Coil

This integrated coil is the coil of choice for very large patients, as well as high-anxiety or claustrophobic patients. These patients can be scanned without a coil being placed over or around their bodies. However, use of the T/R body coil in this manner is not recommended for routine day-to-day scanning. Specific parameter changes should be made before using the integrated coil. The patient can be positioned on the table either head first or feet first. Trough and/or table pads are placed on the table as needed, depending on the patient’s size and body habitus. The patient’s midline should be centered in the sagittal laser light, and the midpoint of their femurs should be centered in the axial laser light.

Regardless of the coil being used, your radiologist’s preference may be to have you secure the patient’s feet with their toes slightly inverted. This is done to minimize rotation of the femoral heads, so images can be acquired with the femurs in true anatomic position. Sponges, tape, and MR-safe sandbags can be used to secure the patient’s feet in the desired position. The patient’s arms and hands can be placed at their sides, above their head, or on their chest, with no interlocking of their fingers. Their arms and hands should remain outside of the RAPID torso/body coil.

Scan Setups

The following are HMSA suggestions for femur imaging. Always check with your radiologist for his/her imaging preferences. You may be required to scan all femurs bilaterally, or to scan bilaterally in certain planes using specific sequences, with the remainder of the scans performed unilaterally on the affected femur.

Axial Scans

When positioning axial slices for the femurs, a coronal image and a sagittal image can be used to ensure inclusion of all pertinent anatomy. The slices should extend superiorly to include the entire femoral head and acetabulum bilaterally, and inferiorly to include anatomy below the femoral condyles. Axial slices should be aligned perpendicular to the shafts of the femurs, as seen in the coronal image in Figure 78.

For unilateral axial femur slice setup, parameters will have to be altered to maintain adequate signal with the smaller FOV that is required. Again, axial slices should be aligned perpendicular to the shaft of the femur, as seen in both the coronal and sagittal images in Figure 79.

Coronal Scans

Coronal slices of the femurs should include the area from the anterior to the posterior margins of the thighs. The slices are typically aligned along the long axis of the femur, as seen in the sagittal image in Figure 80. Thinner slices may be requested when scanning in the coronal plane.

Sagittal Scans

Sagittal slices of the femurs should extend from the medial to the lateral margins of the thighs. The slices should be aligned along the long axis of the femur, as seen in the coronal image in Figure 81. Two different slice groups are required for bilateral femur coverage.

Knee

MRI may be requested for: Bones and Cartilage of the Knee

The knee joint is the largest, most complicated, and most vulnerable joint in the body, as it does not have a stable bony configuration. It consists of the tibiofemoral and patellofemoral articulations, which include the femur, tibia, and patella. The knee is a synovial joint that is enclosed by a ligament capsule. The capsule contains synovial fluid that keeps the joint lubricated (Figure 82). The knee provides flexible movement, but must also bear large weight and pressure loads. During walking, the knees support 1.5 times your body weight. When climbing stairs, they support 3-4 times your body weight. When squatting, your knees support 8 times your body weight.

The tibiofemoral articulation is a modified hinge joint that allows bending and straightening, but also allows for slight rotation. This articulation consists of the lateral and medial condyles of the femur resting on the lateral and medial aspects of the tibial plateau. The femoral condyles make up the distal portion of the femur, which is expanded in order to assist with weight distribution at the knee joint. The medial femoral condyle is typically larger and rounder. The condyles are united anteriorly to provide the articular surface for the patella, but they are separated posteriorly by the intercondylar notch. This notch, or fossa, is the attachment site for the cruciate ligaments, the ligaments of Humphrey and Wrisberg, and the frenulum of the patellar fat pad. A large part of the posterior distal femur is called the popliteal surface. This area is covered by fat, which separates it from the popliteal artery. The medial and lateral edges of the popliteal surface are attachment sites for muscles. Superior to the femoral condyles are the epicondyles, which are the attachment sites for muscles, tendons, and capsular ligaments. The medial epicondyle is the attachment site for the medial (or tibial) collateral ligament (Figure 83). The lateral femoral epicondyle is the attachment site for the lateral (or fibular) collateral ligament, as well as the tendon of the popliteus muscle, fibers of the iliotibial tract, and the lateral capsular ligament. Superior and posterior to the epicondyles is the most distal extent of the linea aspera, the bony ridge of the femur.

The tibia is the distal portion of the tibiofemoral articulation at the knee. The tibia is the second longest bone in the body, ranked just behind the femur. Its proximal end is flattened and expanded to provide a larger surface for the body weight that is transmitted through the femur. Like the femur, the proximal tibia has medial and lateral condyles. The medial condyle is larger, and somewhat flattened where it contacts the medial meniscus. The lateral condyle has a circular look to its femoral articular surface. The lateral tibial condyle articulates with the head of the fibula posteriorly, which is as close as the fibula comes to any involvement in the knee joint. Both the medial and lateral condyles rise in the center of the superior aspect of the tibia to form the intercondylar eminence. Posterior to this eminence are the attachments sites for the posterior horns of the medial and lateral menisci, which will be discussed with the ligaments of the knee. The medial and lateral tibial condyles, and the area of the intercondylar eminence are often grouped together and referred to as the tibial plateau (Figure 84). This is a critical weight-bearing area, and greatly affects the stability of the knee joint. The tibial tuberosity (or tubercle) is located on the anterior surface of the proximal tibial shaft. It has a smooth upper portion, and a roughened lower portion, which is the insertion site for the patellar tendon. The lateral side of the tibial tuberosity has a ridge for the attachment of fibers from the iliotibial tract. This is the strongest direct attachment site for the iliotibial tract. The IT tract, or band, helps in limiting lateral movement of the knee.

The patella is the third bone involved in the knee joint, specifically in the patellofemoral articulation. Patella means “little plate” in Latin, which describes the look and function of this sesamoid bone. The patella develops in the tendon of the quadriceps femoris muscle (Figure 85). It moves when the leg moves, and protects the knee joint by relieving friction between the bones and muscles when the knee is bent or straightened. The patellofemoral joint is a saddle-type synovial joint, allowing the patella to glide along the bottom front surface of the femur between the femoral condyles in the patellofemoral groove. Ossification of the patella is typically completed in females by age 10, and in males between the ages of 13-16. If the patella has more than one ossification center, and the additional center does not fuse, it is termed a bipartite patella (Figure 86).

Articular, or hyaline, cartilage covers the ends of the bones involved in any joint. In the knee joint, this includes the distal end of the femur, the proximal end of the tibia, and the posterior aspect of the patella (Figure 87). In larger joints, this cartilage is approximately ¼” thick. Articular cartilage is white, shiny, rubbery, and slippery, enabling surfaces to slide against one another without damage.   Articular cartilage is very flexible, due in part to its high water content, which also makes it highly visible on MRI. In contrast to the bones that it covers, articular cartilage has almost no blood vessels, so it is not good at repairing itself. Bones, on the other hand, have numerous blood vessels, and are good at self-repair.

Another type of cartilage is found between the femur and tibia- the fibrous cartilage that makes up the medial and lateral menisci. The menisci, also referred to as “articular disks”, wrap around the round ends of the femur to fill the space between the femur and tibia (Figure 88). Since the menisci are more fibrous in composition, they have tensile strength and can resist pressure. They can help spread the force from our body weight over a larger area. By helping with weight distribution, the menisci protect the articular cartilage on the ends of the bones from excessive forces. The menisci are fashioned to be thicker on their outsides, creating a shallow socket on the tibial surface. They act like a wedge on the rounded distal portion of the femur, improving the overall stability of the knee joint by preventing any “rolling” of the femur. Despite how strong they sound, the menisci can crack or tear when the knee is forcefully rotated or bent.  The medial meniscus is fused with the medial collateral ligament, so it is less mobile than the lateral meniscus. It is often injured when the anterior or posterior cruciate ligaments are injured. The inner 2/3 of the medial meniscus receives a limited blood supply, so the entire meniscus is usually slow to heal. The lateral meniscus suffers from fewer injuries than the medial meniscus. Meniscal tears are one of the most common causes of knee pain, with suspected meniscal tears the most common indication for an MRI of the knee joint.

Symptoms that might indicate a problem with the bones of the knee joint include locking of the joint, the knee giving way, crackling or grinding felt in the joint, and pain and swelling. Locking of the joint can be indicative of a “loose body” (bone, cartilage, or foreign object) in the joint space, which can often be removed through arthroscopy (Figure 89). A knee that gives way can indicate that the patella is out of the patellofemoral groove, which leaves the knee unstable. Crackling and grinding at the joint can result from degenerative arthritis or osteoarthritis, as well as from a dislocating patella. An increase in pain with activity can occur due to a stress fracture or bone fracture. One of the pathologic conditions that can affect the bones of the knee joint is osteochondritis dissecans, which can affect the distal femur, and was discussed previously with the femur anatomy. Various types of arthritis manifest in the bones of the knee joint, including osteoarthritis, infectious arthritis, and rheumatoid arthritis. Chondromalacia patella, also known as patellofemoral syndrome or “runner’s knee” results from an irritation of the undersurface of the patella (Figure 91). If the patella is not tracking correctly in the patellofemoral groove, the articular cartilage may rub against the knee joint (Figure 90). The cartilage degenerates, and becomes irritated and painful. This condition is most common amongst young, healthy athletes, especially females and runners that are flat-footed. Treatment is typically rest and physical therapy to stretch and strengthen the quads and hamstrings. If surgery is required, it may be to perform a “lateral release”, as the abnormal tracking of the patella can cause a tightening of the lateral tissues of the knee. The lateral release procedure cuts the tight tissues, so the patella can return to its normal position and tracking. Osgood-Schlatter disease involves the anteriorly located tibial tuberosity, and the patellar tendon that inserts on that tuberosity (Figures 92, 93). This condition affects children during their growth spurts, and is typically found more in boys. During growth spurts, contractions of the quad muscle put additional stress on the patellar tendon at its attachment site on the tibial tuberosity. This can result in multiple subacute avulsion fractures and inflammation of the tendon. Excess bone growth occurs on the tuberosity, and a lump on the tuberosity can be seen and felt. This lump can become irritated and swollen, causing knee and leg pain. This condition is typically worsened with running, jumping, and climbing stairs. Osgood-Schlatter usually resolves with rest, ice, compression and elevation, as well as maturity of the youngster’s skeleton.

Ligaments of the Knee

Ligaments are the tough bands of tissue that connect bones. They are considered to be “viscoelastic”, meaning they can gradually lengthen under tension, but return to their original shape when the tension is removed. However, if they are stretched for a prolonged period of time, or past a certain point, the ligaments cannot retain their original shape, and may eventually tear or snap. This is one of the reasons that a dislocated joint should be re-located as quickly as possible. If the ligaments lengthen, they leave the joint weakened and prone to future dislocations. Controlled stretching exercises to lengthen ligaments, and make the joints more supple, are part of the daily routines of athletes, gymnasts, dancers, etc. Damaged ligaments can lead to unstable joints, wearing of the cartilage, and eventually osteoarthritis. The numerous ligaments of the knee joint are the most important structures in controlling stability of the knee. Many of these ligaments were mentioned in the femur anatomy section, as they have attachments on the distal femur. The more important ligaments will be reviewed here in greater detail, in regards to their functions in the knee joint. The main intracapsular ligaments are the anterior and posterior cruciates (Figures 94, 95).  Intracapsular ligaments are not very common in synovial joints. They provide stability, but permit a larger range of motion as compared to capsular or extracapsular ligaments. The anterior cruciate ligament (ACL) stretches from the lateral femoral condyle to the anterior intercondylar area of the tibia, preventing the tibia from being pushed too far anterior relative to the femur. It is the more commonly injured of the cruciate ligaments, and can be torn during twisting and bending of the knee. Women are at higher risk for ACL ruptures due to the facts that the maximum diameter of the intercondylar fossa is in its posterior aspect (the ACL attaches anteriorly), and the overall width of the intercondylar fossa is smaller in females. The posterior cruciate ligament (PCL) stretches from the medial femoral condyle to the posterior intercondylar area of the tibia, preventing posterior displacement of the tibia relative to the femur. It is the stronger of the two cruciate ligaments, and is injured less frequently; however, it can be injured from direct force or trauma. The menisci are also considered to be intracapsular structures, with connections to ligaments inside and outside the joint capsule. Two of their intracapsular ligaments are the anterior and posterior transverse meniscomeniscal ligaments. They attach the medial and lateral menisci to each other at their anterior and posterior aspects. Posterior transverse meniscal ligaments are very rare- only 1-4% of knees will have them. Two additional intermeniscal ligaments are the medial and lateral oblique meniscomeniscal ligaments (Figure 96). Their names describe their anterior horn attachment sites; they attach on the posterior horn of the opposite meniscus (i.e. medial oblique meniscomeniscal attaches to the anterior horn of the medial meniscus and posterior horn of the lateral meniscus). The oblique meniscomeniscal ligaments both traverse the intercondylar notch, and pass between the anterior and posterior cruciate ligaments (Figure 97).

The medial (or tibial) collateral ligament is considered a capsular ligament, as it is part of the articular capsule surrounding the synovial knee joint. It acts as mechanical reinforcement for the joint, protecting the knee from valgus force, or being bent open medially due to stress on the lateral side of the knee. The medial collateral ligament (MCL) is one of the most commonly injured of all knee ligaments, occurring in all sports, in all ages, and often times with medial meniscal tears (Figures 98-101). It has both superficial and deep components. Fibers from the superficial portion of the MCL attach to the medial epicondyle of the femur and the medial tibial condyle. Fibers from the deep medial collateral ligament attach to the medial meniscus. Proximal to the attachment point, this ligament is referred to as the meniscofemoral ligament, as it attaches the medial meniscus to the medial aspect of the femur. Distal to the meniscal attachment, the ligament is referred to as the meniscotibial (or coronary) ligament, as it attaches the medial meniscus to the medial aspect of the tibia. The meniscofemoral and meniscotibial are also referred to as the meniscocapsular or medial capsular ligaments, as they play an important role in anchoring peripheral parts of the medial meniscus in the medial side of the knee. The meniscotibial ligament is typically injured more often than the meniscofemoral ligament. The meniscotibial ligament attaches to the tibia several millimeters inferior to the articular cartilage. Its job is to stabilize and maintain the meniscus in its proper position on the tibial plateau. Disruption of the meniscotibial ligament can result in a floating meniscus or meniscal avulsion, while the meniscofemoral ligament may not be affected. The deep medial collateral ligament is short, and tightens quickly with rotation motions. It is often damaged, along with the ACL, when the mechanism of injury involves tibial rotation. Diagnosis and surgical repair of the deep medial collateral ligament can be challenging.

In addition to fibers of the medial collateral ligament, the deep portion of the capsular compartment of the medial knee is the location of the medial knee’s posterior support. The posterior oblique ligament is attached proximally to the medially located adductor tubercle of the femur, and distally to the tibia and the posterior aspect of the knee joint capsule. If the posterior oblique is injured, it is usually torn from its femoral origin. The posterior oblique ligament provides static resistance to valgus loads as the knee moves into full extension, as well as dynamic stabilization to valgus forces (stress from lateral side) as the knee moves into flexion. It acts as an important restraint to posterior tibial translation in cases of posterior cruciate ligament injury. The posterior oblique ligament has three “arms”. Its superior capsular “arm” becomes continuous with the posterior knee capsule, and the proximal portion of the oblique popliteal ligament. The oblique popliteal ligament is also an important posterior stabilizing structure for the knee joint Figure 102). It extends from the posteromedial aspect of the tibia, running obliquely and laterally upward to insert near the lateral epicondyle of the femur.

The lateral (or fibular) collateral ligament is considered an extracapsular ligament. It helps to provide joint stability and protects the lateral side of the knee from varus forces, or inside bending forces that are directed at the medial side of the knee. Injuries to the lateral collateral ligament are less common than injuries to the medial collateral, as the opposite leg can guard against medial forces that can lead to lateral collateral injuries. Injuries can occur in sports such as soccer and rugby, where the knee is extended and unprotected during running. The lateral, or fibular, collateral ligament stretches obliquely downward and backward, from the lateral epicondyle of the femur to the head of the fibula (Figure 103). It is not fused with the capsular ligament or with the lateral meniscus, so it has increased flexibility and decreased incidence of injury when compared to the medial collateral ligament. Similar to the medial meniscus, the lateral meniscus has a meniscotibial, or coronary, ligament. It connects the inferior edges of the lateral meniscus to the periphery of the tibial plateau. The lateral meniscus also has a meniscofemoral ligament that extends from the posterior horn of the lateral meniscus to the lateral aspect of the medial femoral condyle. It is given two distinct names, based on its location in relation to the posterior cruciate ligament (PCL). The ligament of Humphrey passes in front of the posterior cruciate ligament. It is less than 1/3 the diameter of the posterior cruciate ligament, but may be confused for the posterior cruciate during arthroscopy. The ligament of Wrisberg passes behind the posterior cruciate ligament, and is about ½ of the posterior cruciate’s diameter (Figure 104). Its femoral origin often merges with the posterior cruciate ligament. Both ligaments are present in only about 6% of knees. Approximately 70% of people have one or the other of these ligaments, with the majority possessing the more posterior ligament of Wrisberg (Figure 105). MRI is the preferred imaging modality for medial collateral or lateral collateral ligament injuries, as it can detect any associated internal knee derangements, cruciate-collateral ligament injuries, or cartilage deficiencies.

The patellar ligament is the connection between the patella and the tibia, extending from the apex (inferior aspect) of the patella to the tibial tuberosity. Technically, it is connecting two bones, so it is a ligament. However, it is most often referred to as the patellar tendon, because the superficial fibers that cover the front of the patella and extend to the tibia are continuous with the central portion of the common tendon of the quadriceps femoris muscle. The posterior surface of the patellar ligament is separated from the synovial membrane of the knee joint by a large infrapatellar pad of fat. Injuries to the patellar ligament can occur from overuse, such as sports that involve jumping and quick directional changes, as well as running-related sports. This is the ligament that is injured in jumper’s knee (or patellar tendonitis), which begins with inflammation, and can lead to degeneration or rupture of the patellar ligament and the tissue around it (Figure 106). Patients with patellar ligament injuries typically complain of pain in the area below the kneecap, which will increase with walking, running, squatting, etc. They can often be treated in the same manner as other soft tissue injuries- with rest, ice, compression and elevation. The patellar ligament attachment at the tibial tuberosity is the site of Osgood-Schlatter disease, which was discussed previously.

Along the sides of the patella and the patellar ligament are the medial and lateral patellar retinacula (Figure 107). They are fibrous tissue stabilizers for the patella that form from the medial and lateral portions of the quad tendons as they pass down to insert on either side of the tibial tuberosity. The lateral retinaculum is the thicker of the two, but both have superficial and deep layers. Within the deep layers are various ligaments (whose names indicate the structures they connect) that help support the patella in its position, relative to the femur below it. The deep layer of the lateral patellar retinaculum is the location where the lateral patellofemoral ligament meets the iliopatellar band, which is a tract of fibers from the iliotibial (IT) band that connects to the patella. The deep layer of the medial patellar retinaculum has three focal capsular thickenings, referred to as the medial patellofemoral, medial patellomeniscal, and medial patellotibial ligaments. The medial patellofemoral ligament is strong enough to influence patellar tracking, and acts as a major medial restraint. Imbalances in the forces that control patellar tracking during flexion and extension of the knee can lead to patellofemoral pain syndrome (runner’s knee), one of the most common causes of knee pain. This can result from overuse, trauma, muscle dysfunction, patellar hypermobility, and poor quadriceps flexibility. Typical symptoms include pain behind or around the patella that is increased with running, and activities that involve knee flexion. MRI is typically not necessary for this diagnosis. Physical therapy has been found to be effective for the treatment of patellofemoral pain syndrome.

Muscles and Tendons of the Knee

The flexor and extensor muscles of the knee have been discussed previously, as the majority of them are the anterior and posterior muscles of the thigh. We will review the thigh muscles involved in knee movement, and add two muscles of the lower leg that also affect the knee. The quadriceps femoris muscles of the anterior thigh are the main knee extensors (Figure 108). As these muscles contract, the knee joint straightens. The tendons of the vastus medialis, vastus intermedius, vastus lateralis, and rectus femoris join at the superior aspect (base) of the patella to form the patellar tendon. This tendon continues over the patella and attaches it to the tibial tuberosity (since it is connecting bone to bone, it is sometimes called the patellar ligament). The quadriceps, along with the gluteal muscles, are responsible for the thrusting forces necessary for walking, running, and jumping. The quads also help control movement of the patella, as they are attached to it by the quadriceps tendons (Figure 109). The patella increases the force exerted by the quadriceps muscles as the knee is straightened.

The posterior thigh muscles, also known as the hamstrings, are the main knee flexors, with assistance from the sartorius, gracilis, gastrocnemius, and popliteus muscles. The knee bends when the hamstrings contract. The hamstring muscles give the knee joint the strength needed for propulsion in running and jumping. They also help to stabilize the knee by protecting the collateral and cruciate ligaments, especially when the knee twists. The three hamstring muscles have varying attachment sites around the knee joint (Figure 110). The biceps femoris attaches to the head of the fibula and the superolateral aspect of the tibia. The semitendinosus attaches on the anterior aspect of the tibia, medial to the tibial tuberosity, crossing over the medial collateral ligament. The tendon of the semitendinosus muscle is sometimes used for cruciate ligament reconstruction. The semimembranosus attaches at the posteriomedial aspect of the medial tibial condyle. The sartorius muscle is also a knee flexor, although it is an anterior thigh muscle. It inserts on the anterior medical aspect of the tibia. The gracilis muscle of the medial thigh is one of the hip adductors, but also plays a part in knee flexion. Like the semitendinosus tendon, the tendon of the gracilis is sometimes used for cruciate ligament reconstructions. The gracilis attaches to the medial aspect of the proximal tibia.

Additional flexors of the knee joint include some of the posterior muscles of the lower leg. The large superficial gastrocnemius muscle has a medial and a lateral head, which originate from the medial and lateral femoral condyles, respectively. It runs the length of the posterior lower leg, attaching to the calcaneus by the Achilles tendon. The gastrocnemius gives us the ability to flex our knee while our foot is flexed, as it connects to both joints. It is involved in standing, walking, running, and jumping. The popliteus is a deep posterior lower leg muscle that helps with knee flexion, and also rotates the tibia medially, which aids in knee stability. The popliteus originates from the outer margin of the lateral meniscus of the knee joint. It extends posteriorly and inserts on the medial aspect of the tibia, inferior to the medial tibial epicondyle.

The important tendons of the knee include the quadriceps, patellar, and hamstring tendons, and the iliotibial band (Figure 111). Tendons attach muscles to bones. These major knee tendons have all been discussed with either the bones or the muscles that they attach. The quadriceps tendon was mentioned with the quadriceps muscle as the muscle’s attachment to the patella. The quad tendon continues over the patella, then attaches the apex of the patella to the tibial tuberosity. It is then called the patellar tendon (or ligament). Hamstring tendons were discussed with the hamstring muscles, the posterior muscles that are flexors of the knee. Hamstring tendons are sometimes used for cruciate ligament reconstructions. Tendonitis, which is the inflammation of a tendon, is a common knee injury amongst athletes in a variety of sports. The iliotibial band (or IT tract) functions like a tendon, as it attaches the knee to the tensor fasciae latte muscle. The band is actually a fibrous reinforcement of the fascia lata, or deep tissue of the thigh. It runs from the ilium to the tibia. Proximally, it acts as a hip abductor, while distally it acts as lateral stabilization for the knee, and aids with medial rotation of the tibia. The IT band is in constant use during walking and running, which can lead to irritation at the point where it passes over the lateral femoral epicondyle. A ‘tight” IT band can cause inflammation and/or irritation at the femoral epicondyle, or at the point of insertion on the lateral tibial condyle. This condition is called IT band friction syndrome. It is common amongst runners, hikers, and cycling enthusiasts.

Nerves of the Knee

The main nerves to the knee that come from the sacral plexus of nerves are the tibial nerve and the common peroneal nerve (Figure 112). Both are branches of the sciatic nerve, and begin posteriorly, slightly above the actual knee joint. Both of these nerves, or their branches, continue through the lower leg and foot, providing sensation and muscle control. The tibial and common peroneal nerves are also both involved in cutaneous innervation, which is the supply of nerves to the skin of the knee. The tibial nerve remains posterior and more medial, branching at the medial ankle to innervate the foot. The common peroneal nerve begins posterolaterally, moving anteriorly near the neck of the fibula. It then branches into the superficial and deep peroneal nerves, which continue their anterior descent to the foot. The tibial and common peroneal nerves are the most commonly injured nerves when a knee is dislocated. Nerves can grow back, but they do so at a rate of approximately ½ inch per month.

Nerves from the lumbar plexus that affect the knee include the lateral femoral cutaneous, and the saphenous, which is a branch of the femoral nerve (Figure 113). The saphenous nerve travels more medially and gives off infrapatellar branches around the knee joint. Below the knee, the saphenous nerve sends branches to the skin of the anterior and medial lower leg. The lateral femoral cutaneous nerve sends an anterior branch to the skin of the anterior and lateral thigh, down to the area of the knee. Terminal filaments of this nerve communicate with the infrapatellar branch of the saphenous nerve, forming the peripatellar plexus.

Arteries and Veins of the Knee

The popliteal artery, a branch of the superficial femoral artery, is the main arterial supply to the knee joint. It runs along the posterior aspect of the distal femur, behind the knee joint. At the supracondylar ridge, the popliteal artery gives off the blood supply to the knee, which consists of various genicular arteries (Figure 114). Inferior to the knee joint, the popliteal branches into the anterior and posterior tibial arteries, which supply the lower leg. The popliteal artery is a common site for both atherosclerosis and aneurysms, and is listed as the most common site for peripheral arterial aneurysms. Approximately 50% of these aneurysms are bilateral. Although they rarely rupture, popliteal aneurysms may serve as a focus for abrupt thrombotic occlusion of the involved popliteal artery, which can affect the foot on the same side. A thrombus within an aneurysm can also lead to a distal embolism. The genicular arteries are sources of continued blood flow to the knee and lower limb, in case of an obstructed popliteal artery. The descending genicular, also called the highest or supreme genicular, branches from the femoral artery, just superior to the popliteal branch. It supplies the adductor magnus and hamstring muscles, then joins with the network of genicular arteries around the knee joint. The middle genicular pierces the oblique popliteal ligament, and supplies the ligaments and synovial membrane inside the knee articulation (including the ACL and PCL). The sural artery joins the anastomoses of the genicular arteries, and also supplies muscles of the lower leg, including the large gastrocnemius muscle. The anastomotic pattern around the knee joint is supplied by the popliteal artery posteriorly, the descending genicular artery medially, and the descending branch of the lateral circumflex femoral artery laterally. The genicular arteries involved in the anastomosis are labeled as the medial and lateral superior geniculars, and the medial and lateral inferior geniculars.

The major deep veins around the knee joint are the popliteal vein, and the anterior and posterior tibial veins (Figure 115). The popliteal vein begins at the junction of the tibial veins in the posterior aspect of the lower leg, just inferior to the knee joint. It ascends posteriorly, continuing as the femoral vein about halfway up the thigh. As deep veins typically follow the arteries, the genicular veins accompany the genicular arteries around the knee joint, then drain into the popliteal vein. The important superficial veins around the knee joint are the small and great saphenous veins. Superficial veins typically do not follow arteries, but rather travel with cutaneous nerves. The small saphenous ascends the lower leg posteriorly, angling from lateral to medial. It merges with the popliteal vein at a position slightly superior to the knee joint. The great saphenous vein, the longest vein in the body, has a medial and anterior course in the lower leg. It moves to a posterior position, but stays medial along the knee joint, moving alongside the medial epicondyle of the femur. The great saphenous then moves anteriorly again through the thigh.

Varicose and “spider” veins are often seen in the leg in the posterior aspect of the knee joint. As mentioned previously, in the femoral vein discussion, veins have valves to ensure the “one-way” uphill flow of blood back to the heart (Figure 116). Communicating vessels, also called perforating veins, exist between the deep and superficial veins to help compensate for valves that may be incompetent, and are allowing blood reflux. If venous walls are weakened or dilated, the cusps of the valves can no longer close properly, and the valves can become incompetent. This leads to an increase in the weight of the column of blood for the veins that are “downstream” from the bad valve. Blood can pool in these veins, causing them to become varicose, where the veins swell, become tortuous, and even bulge through the skin surface. Reticular veins, which are smaller varicose veins that do not bulge through the skin, as well as very small “spider” veins are both typically less severe conditions, but both still involve the backwards flow of blood. Removal of severe varicose veins will actually help blood flow, as the blood will no longer be stagnant in the pooled areas.

Bursae of the Knee

The synovial knee joint is home to a large number of bursae (Figure 117). These are fluid sacs and synovial pockets that surround and sometimes communicate with the joint cavity. They facilitate friction-free movement between the bones and moving structures (tendon, muscle). Fluid or debris can collect in the bursa, or fluid can extend into the bursa from the adjacent joint in situations such as excessive friction, infection or direct trauma. This type of pathological enlargement of the bursa is referred to as bursitis, which can mimic several peripheral joint and muscle abnormalities. Radiologists must be able to accurately identify bursal pathology, especially amongst the numerous knee bursae (14 reported in some literature). We will identify a few of the more common bursa, beginning with the suprapatellar bursa. This bursa lies between a quadriceps tendon and the femur, superior to the patella (Figure 118). Fluid is commonly found here when patients have a joint effusion. Bursitis of the prepatellar bursa is also known as “housemaid’s knee”. It occurs from repetitive trauma from kneeling, as seen with housemaids, wrestlers, and carpet-layers. This bursa is found between the patella and the skin (Figure 119). Inflammation of the superficial infrapatellar bursa may be called “Clergyman’s knee”, another bursitis that can occur from excessive kneeling. This bursa is located between the distal third of the patellar tendon and the overlying skin (Figure 120).

The synovial sac of the knee joint sometimes forms a posterior bulge, known as a Baker’s cyst or popliteal cyst (Figure 121). It typically forms between the tendons of the medial head of the gastrocnemius muscle and the semimembranosus muscle, posterior to the medial femoral condyle. Baker’s cysts are not true cysts, as they typically maintain open communication with the synovial sac. However, they can pinch off, and they can rupture. They are usually asymptomatic, but can be indicative of another problem of the knee, such as arthritis or a meniscal tear. Aspiration of the synovial fluid can be performed if the cyst becomes problematic. Treatment is usually necessary if a Baker’s cyst ruptures, as it can cause acute pain behind the knee, and swelling of the calf muscles. A ruptured cyst can also mimic a DVT or thrombophlebitis. Ultrasound and MRI can both be used for confirmation of a Baker’s cyst (Figure 122).

Open MRI Systems

Introduction

When positioning a patient for a knee exam in an open MRI system, it is important to consider the choices you have as far as coil selection and patient setup. Positioning that will result in both the coil and the anatomy at isocenter in all three planes will have a positive impact on the image quality of your study.

Components of positioning to consider include coil selection, table pads and accessory pads, and proper laser light centering. Coil selection will be influenced by the patient’s size and body habitus. The extensive inventory of table and accessory pads should be used for proper centering, as well as patient comfort and stability (Figure 123). The proper use of trough pads and thick or thin table pads are key elements for achieving coronal centering of both the patient and the coil. When scanning on open MRI systems, it is extremely important to center the anatomy of interest in the laser lights (Figure 124) in all three directions: head-to-foot (axial or transverse plane), right-to-left (sagittal plane), and anterior-to-posterior (coronal plane).

Coils and Positioning

Knee (Extremity) Coil

The coil of choice for a study of the knee is the knee (extremity) coil. Place the base of the coil on the table. Add trough pads and/or table pads under the base of the coil to help center the coil in the coronal plane (the coronal laser light should be aligned with the top of the coil base). The horizontal center of the coil base should be between the marking on the patient table and the end of the table nearest the magnetic field. This will allow for sufficient table travel while still achieving isocenter positioning. The longitudinal center marking on the coil base should be placed as close as possible to the midline of the table; however, the patient’s size, comfort, and the laterality of the affected knee must be taken into account when determining the longitudinal positioning of the coil. The extremity coil may be offset slightly towards the affected knee. Coil centering in the sagittal plane can be performed as a final positioning step using the open magnet’s lateral table movement capabilities. The patient should be positioned on the base portion of the coil with the inferior tip of the patella aligned with the horizontal center mark on the coil. The longitudinal midline of the patient’s affected knee should be positioned in the middle of the coil base. The coil pads and accessory pads can be adjusted to help achieve coronal centering of the patient’s knee in the coil base. The upper portion of the coil is then placed on the base and the coil latches are secured to close the coil. Accessory sponges can be added inside the coil to further secure the knee in place. Pads and sponges should be placed under the upper and lower leg for support and comfort. The large knee bolster cushion can be placed under the unaffected knee for patient comfort. It will also serve to remove the unaffected knee from the scan plane, which should decrease wrap-around artifacts. The right and left arrow buttons located on the gantry can be used to move the table from side to side for proper alignment of the coil’s longitudinal center mark with the sagittal laser light (Figure 129). The patient should now be centered in all three planes- centered on the middle of the knee in the coronal and axial planes (Figure 130), and centered midline in the sagittal plane.

Cervical Coil

If the patient’s knee cannot be accommodated in the extremity coil, the cervical coil can be used. Place the base of the coil on the table. Add trough pads and/or table pads under the base of the coil to help center the coil in the coronal plane (the coronal laser light should be aligned with the middle of the coil base). The horizontal center of the coil base should be between the marking on the patient table and the end of the table nearest the magnetic field. This will allow for sufficient table travel while still achieving isocenter positioning. The longitudinal center marking on the coil base should be placed as close as possible to the midline of the table; however, the patient’s size, comfort, and the laterality of the affected knee must be taken into account when determining the longitudinal positioning of the coil. The cervical coil may be offset slightly towards the affected knee. Coil centering in the sagittal plane can be performed as a final positioning step using the open magnet’s lateral table movement capabilities. The patient should be positioned in the base portion of the coil with the inferior tip of the patella aligned with the horizontal center mark on the coil. The longitudinal midline of the patient’s affected knee should be positioned in the middle of the coil base. The coil pads and accessory pads can be adjusted to help achieve coronal centering of the patient’s knee in the coil base. Align the upper portion of the coil on the base, then press down on the upper portion until it clicks to secure it in place. Accessory sponges can be added inside the coil to further secure the knee in place. Pads and sponges should be placed under the upper and lower leg for support and comfort. The large knee bolster cushion can be placed under the unaffected knee for patient comfort. It will also serve to remove the unaffected knee from the scan plane, which should decrease wrap-around artifacts. The table straps can be used to secure the patient’s anatomy and/or the positioning sponges in place (Figure 131). The right and left arrow buttons located on the gantry can be used to move the table from side to side for proper alignment of the coil’s longitudinal center mark with the sagittal laser light (Figure 132). The patient should now be centered in all three planes- centered on the middle of the knee in the coronal and axial planes, and centered midline in the sagittal plane.

For safety purposes, the patient should be encouraged to keep their arms and hands along their sides, but not in contact with their body.

Echelon MRI System

Introduction

The 1.5T Echelon system comes equipped with a variety of trough and table pads, as well as various positioning pads and sponges (Figure 133). These positioning aids can be used to support the position of both the patient and the coil, as well as to keep your patient comfortable and secure. The magnet bore of the Echelon does not permit lateral patient table movement. However, accurate patient and coil positioning, and correct use of the sagittal and axial laser lights (Figure 134) to center the patient’s anatomy ensure that high quality images will be acquired.

Coils and Positioning

Knee (Extremity) Coil

The coil of choice for a study of the knee is the knee (extremity) coil. The coil should be placed at the gantry end of the table. The bottom portion of the coil should be placed on top of the table pads. The patient should be positioned feet first on the base portion of the coil, with the center of their knee aligned with the horizontal center marks on the coil. The upper portion of the coil is then placed on the base, and the coil locking mechanism is secured (Figure 137). The longitudinal midline of the patient’s affected knee (and leg) should be aligned with the longitudinal center mark on the upper portion of the coil. Accessory sponges can be added inside the coil to further secure the knee in place. Pads and sponges should be placed under the upper and lower leg for support and comfort. A positioning pad or sponge placed under the unaffected knee may increase patient comfort, and also serves to remove that knee from the scan plane, which should decrease wrap-around artifacts. The coil’s longitudinal center mark should be aligned as closely as possible with the sagittal laser light.

Cervical Coil

If the patient’s knee cannot be accommodated in the extremity coil, the cervical coil can be used. Place the base of the coil directly on the table top. The patient should be positioned feet first on the base portion of the coil, with the center of their knee aligned with the horizontal center marks on the coil. Align the upper portion of the coil on the base, then press down on the upper portion until it clicks to secure it in place. The longitudinal midline of the patient’s affected knee (and leg) should be aligned with the longitudinal center mark on the upper portion of the coil. Accessory sponges can be added inside the coil to further secure the knee in place (Figure 138). Pads and sponges should be placed under the upper and lower leg for support and comfort. If space permits, a positioning pad or sponge placed under the unaffected knee may increase patient comfort. It will also serve to remove that knee from the scan plane, which should decrease wrap-around artifacts.

For safety purposes, the patient should be encouraged to keep their arms and hands along their sides, but not in contact with their body.

Scan Setups

The following are HMSA suggestions for knee imaging. Knee protocols should be designed to yield diagnostic images of the menisci, bones, articular cartilage, and all ligamentous structures of the knee. While many radiologists may require additional imaging of the ACL, protocols that are designed for optimal imaging of the cartilage and menisci should also produce adequate images of the ACL. Always check with your radiologist for his/her imaging preferences.

Axial Scans

When positioning axial slices for the knee, sagittal and coronal images can be used to insure inclusion of all pertinent anatomy. The slices should extend superiorly to include the entire patella, and inferiorly to include the tibial tuberosity and patellar tendon insertion. A presat can be placed over the unaffected lower extremity to reduce the possibility of wrap-around artifact, as seen in the coronal image in Figure 139.

Coronal Scans

Coronal slices of the knee should include the anatomy from the posterior femoral condyles to the anterior portion of the patella. Visualize a line connecting the lateral and medial condyles of the femur. Typically, the coronal slices are angled so that they are parallel to that line, as seen in the axial image in Figure 140.

Sagittal Scans

Sagittal slices should include the anatomy from the medial condyle to the lateral condyle. The slice group may be angled per your radiologist’s preference, but should remain perpendicular to the coronal slices. Typically, the slice group is angled so that it is parallel to the medial border of the femoral condyle, as seen in the axial image in Figure 141.

In addition to routine oblique sagittal images, some radiologists prefer an additional sagittal scan of the ACL with thin slices and high spatial resolution. Axial and coronal images can be used for slice setup. Referenced literature recommends that the angle of the slice group should not exceed 10° from a line drawn perpendicular to the bicondylar line (line that connects the posterior femoral condyles), as seen in Figure 142.

Lower Leg

MRI may be requested for: Bones of the Lower Leg

The two bones of the lower leg are the tibia and fibula. They are connected by a flexible interosseous membrane, and have proximal and distal articulations with each other. The tibia is the larger and more medial of the two bones. It is the second largest bone in the skeleton, with only the femur being larger. The tibia is the strongest weight-bearing bone of the body. It is vertical in males, but slightly oblique in females to accommodate the greater obliquity of their femurs, and their wider pelvis. The proximal bony landmarks of the tibia were discussed with the knee joint. They included the lateral and medial condyles, the tibial plateau, the intercondylar eminence, and the tibial tuberosity. Proximal articulations of the tibia include the tibiofemoral articulation at the knee joint, and the superior (or proximal) tibiofibular articulation (Figure 143). The latter is a syndesmotic joint between the lateral condyle of the tibia and the head of the fibula which allows little movement. Anteriorly on the proximal tibia is the tibial tuberosity, which is the site of attachment for the patellar tendon. The anterior crest of the tibia is the “sharp” bony ridge that can be felt along the anterior aspect of the lower leg. The boney process that extends inferiorly from the medial tibia is the medial malleolus. It is one of the stabilizers for the synovial ankle joint, which is the articulation of the distal end of the tibia with the talus. The lateral aspect of the distal tibia articulates with the fibula to form the inferior (or distal) tibiofibular articulation.

The tibia is commonly called the shin bone, and is the bone associated with a condition known as “shin splints”. This is not a specific diagnosis, but rather pain over the anterior tibia that can be caused by multiple conditions. The most common cause of shin splints is medial tibial stress syndrome. This syndrome involves irritation of the tendons and the attachments of the tendons to the bone, typically from overuse. Patients may complain of a dull, aching pain along the medial aspect of the tibia, which may be accompanied by tenderness and swelling. It is commonly seen in athletes who have sudden increases in the duration or intensity of their training. Tibial pain can also result from stress fractures of the shin bone. Also known as “fatigue fractures”, stress fractures can occur from repeated low forces on a bone over an extended period of time. They are commonly seen in athletes who run and jump on hard surfaces, such as distance runners, basketball players, and ballet dancers. Stress fracture pain is typically localized to the fracture site. Bone scans and MRI may be used for stress fracture diagnosis, while bone that is attempting to heal around a stress fracture may be seen on x-rays.

The fibula is the smaller and more lateral of the bones of the lower leg. While the tibia bears the body’s weight, the fibula acts as more of a stabilizer. The head of the fibula articulates with the tibia at the superior tibiofibular joint.  The inferior end of the fibula is called the lateral malleolus, a boney process that is a lateral stabilizer for the ankle joint (Figure 144). The lateral malleolus extends more distally than the medial malleolus. The inferior tibiofibular joint involves the articulation of the medial aspect of the fibula with the lateral aspect of the tibia. This is also a syndesmotic joint, as it consists of contiguous bony surfaces united by interosseous ligaments, and is only slightly movable. If the bones of the inferior tibiofibular joint are fractured, they may be repaired with a syndesmotic screw, which temporarily replaces the syndesmosis. The screw inhibits normal bone movement, thereby also inhibiting the joint. The screws are typically removed when the natural articulation is healed.

Ligaments of the Lower Leg

Important ligaments of the lower leg include the superior and inferior tibiofibular ligaments, and the middle tibiofibular ligament, or interosseous membrane (Figure 145). Additional ligaments that attach to the superior aspect of the tibia have been previously discussed with the knee anatomy. The superior tibiofibular joint, or syndesmosis, is held in place by the anterior and posterior superior tibiofibular ligaments. They maintain proximal integrity between the tibia and fibula. The interosseous membrane, or middle tibiofibular ligament, helps to stabilize the relationship between the tibia and fibula, and also serves to separate the anterior and posterior muscles of the lower leg. Its upper margin does not quite reach the superior tibiofibular articulation. Distally, it is continuous with the interosseous ligament of the inferior tibiofibular articulation, where it presents perforations for the passage of small vessels. The interosseous membrane may have an important role in the transfer of forces to the fibula. It keeps the fibula active during weight-bearing, and helps to stabilize it to prevent bowing of the bone. The inferior tibiofibular syndesmosis is stabilized by syndesmotic ligaments that include the anterior and posterior inferior tibiofibular ligaments, the interosseous ligament, and the transverse ligament (Figure 146). The anterior inferior tibiofibular ligament (AITFL) crosses just above the front of the ankle to connect the tibia and fibula. The posterior inferior tibiofibular ligament (PITFL) and the transverse ligament both attach across the back of the tibia and fibula. The interosseous ligament is found between the tibia and fibula. These syndesmotic ligaments serve to hold the inferior ends of the tibia and fibula in place, creating the upper surface of the ankle joint. One or more of these inferior ligaments are involved in the injury known as a “high ankle sprain” (Figure 147). These sprains involve stretching and/or tearing of the ligaments above the ankle joint. Tears can be classified as partial, where only some strands of the ligament are torn, or complete, where all ligamentous strands are torn. A mild sprain usually involves stretching or a slight tear in only one of the inferior syndesmotic ligaments. As the sprains (and tears) become more severe in nature, the ankle becomes more unstable, and can result in diastasis, a condition where the ends of the tibia and fibula actually spread apart. This injury can be diagnosed by physical exam by a physician, as well as by patient reports of severe pain that lingers after a “routine” ankle sprain. Even a mild high ankle sprain can involve twice the recovery time of a “routine” ankle sprain. High ankle sprains are one of the most severe sprains of the ankle or foot, and can cause numerous problems for those trying to return to normal activities, let alone athletes trying to resume intense running, cutting, or jumping.

Muscles and Tendons of the Lower Leg

The muscles of the lower leg are divided into the anterior, lateral, deep posterior and superficial posterior fascial compartments (Figure 148). The muscles are enclosed within tight tissue called fascia, and are separated by tough connective tissue septa. These fascial compartments are not very expandable, but have enough room to allow for normal muscle function. Muscle swelling due to vascular insufficiency can result in serious muscle compression and/or muscle death (called compartment syndrome) without fascial decompression. Exercise-induced or exertional compartment syndrome may exhibit symptoms similar to other conditions that cause “shin splints”, with pain over the anterior aspect of the tibia. After activity or exercise, blood flow to muscles increases, and the size of the muscle increases. In patients with exercise-induced compartment syndrome, the fascia constricts the expanding muscle and interrupts blood flow to the muscle. This lack of blood flow causes ischemia, and leg pain. Resting the muscles typically relieves the pain, although numbness and tingling in the leg or foot may be noticed due to the lack of blood flow. The area over the muscles on the anterior aspect of the lower leg may even feel tight. Persistent symptoms of exercise-induced compartment syndrome may require a fasciotomy, which is the surgical release of tight fascia.

The anterior compartment consists of the tibialis anterior, the extensor digitorum longus, the extensor hallucis longus, and the peroneus tertius (Figure 149). All of the anterior compartment muscles are dorsiflexors of the ankle, meaning that they pull the foot and toes upward. The tibialis anterior muscle is also involved in inversion of the foot, where the foot rolls inwards so the sole of the foot increasingly faces the opposite foot (Figure 150). The two extensor muscles extend the toes, with extensor hallucis longus extending the great toe, and extensor digitorum longus extending the four remaining toes.

The lateral compartment muscles include the peroneus longus and peroneus brevis. They are evertors of the foot, meaning that they roll the foot outwards so the sole of the foot increasingly faces away from the opposite foot (Figure 151). They are also active in plantar flexion, such as walking on the toes, or pushing off with the great toe.

The deep posterior muscle compartment includes the tibialis posterior, flexor hallucis longus, flexor digitorum longus, and popliteus (Figure 152). The tibialis posterior and both flexors are involved in plantarflexion of the ankle joint (movement that points toes downward by straightening the ankle). In addition, the tibialis posterior inverts the foot, while the flexor muscles flex the toes (flexor halluces longus flexes the great toe, flexor digitorum longus flexes the four remaining toes). The popliteus muscle works at the superior aspect of the lower leg to help flex the knee, and to rotate the tibia medially.

The superficial posterior muscle compartment includes the plantaris, soleus, and gastrocnemius muscles (Figure 152). The plantaris muscle acts with the gastrocnemius muscle, but is not very significant as either a knee flexor or ankle plantarflexor. It is located between the popliteus muscle and the lateral head of the gastrocnemius. It has a long, thin tendon that descends along the medial head of the gastrocnemius and the soleus which is often mistaken for a nerve on dissection, earning itself the nickname of the “freshman’s nerve”. The plantaris tendon continues inferiorly along the medial aspect of the Achilles tendon, and may insert on the calcaneus with the Achilles, or independently (Figure 153). The plantaris tendon may remain intact when the Achilles tendon ruptures. Injuries to the plantaris muscle and tendon have been termed “tennis leg”. These injuries may be accompanied by tears to the gastrocnemius and soleus muscles, or may occur solely to the plantaris muscle (Figure 154). Tennis leg typically occurs during running or jumping, when extra loading is placed on the ankle with the knee in an extended position. MRI and ultrasonography have been used as the primary imaging techniques for evaluating patients with “non-specific” posterior leg pain. Findings related to tennis leg can mimic those for DVT in the calf, so early evaluation is important. The soleus muscle, which lies under the gastrocnemius, is a plantarflexor for the ankle joint. It originates just below the knee, and joins with the gastrocnemius inferiorly to attach to the calcaneus via the Achilles tendon. The soleus muscle plays an important role in walking, and in maintaining standing posture. Without its constant pull, the body would fall forward. It is also referred to as the skeletal-muscle pump, as it is responsible for pumping venous blood back to the heart from the periphery when the body is in upright posture. The major and most visible calf muscle is the gastrocnemius, which flexes the knee and plantarflexes the ankle joint. It has a medial and a lateral head, with each originating from its respective femoral condyle. Inferiorly, the gastrocnemius forms a common tendon (calcaneal or Achilles tendon) with the soleus muscle, which inserts on the posterior surface of the calcaneus bone (Figure 155). The Achilles tendon is the thickest and strongest tendon in the body, working with the leg muscles, which are the most powerful muscle group in the body. Contraction of the calf muscles pulls the Achilles tendon, which pushes the foot downward, enabling one to stand on their toes, walk, run, and jump. Each Achilles tendon is subject to a person’s entire body weight with each step. During a sprint or push off, this tendon may be subjected to 3-12 times a person’s body weight, depending on speed, stride, terrain, and any additional weight being carried or pushed. Overuse, misalignment, and improper footwear are a few of the causes of Achilles tendon injuries. Sudden increases in training, whether in distance or speed, as well as tightness or weakness of the leg, and uneven leg lengths can all cause undue strain on this tendon. The most common Achilles tendon injuries are tendinosis and tendon rupture. Tendinosis (or tendinitis) is a common injury among middle and long distance runners, presenting as soreness and stiffness that worsens until treated. It can result from repetitive stress to the tendon. Damaged tendon fibers can calcify, and bone spurs can form at the point of the tendon’s insertion. An Achilles tendon rupture is a partial or complete tear of the tendon, which comes on suddenly and is often debilitating. Partial tears are common among middle and long distance runners, especially amongst middle age athletes who undergo little or no training. A partial tear and tendinosis may have similar symptoms. Partial and full tendon ruptures typically occur in sports that require sudden eccentric stretching, such as sprinting and racquet sports. MRI is not always necessary to diagnose Achilles tendinitis, but it is helpful in surgical planning to assess damage to this important tendon (Figure 156).

Nerves of the Lower Leg

The three muscular compartments of the lower leg have separate nerve supplies from their neighbors. Within the compartments, all muscles are typically supplied by the same nerve. The common peroneal nerve splits into the deep and superficial peroneal nerves just inferior to the head of the fibula (Figure 157). The deep peroneal nerve supplies the anterior muscle compartment of the lower leg. The superficial peroneal nerve innervates the lateral muscular compartment. Both the superficial and deep groups of muscles of the posterior compartment are supplied by the tibial nerve. Cutaneous nerves of the lower leg do not follow the muscular compartments for innervation. The saphenous nerve, which is the largest cutaneous branch of the femoral nerve, passes down the medial aspect of the lower leg and into the medial side of the foot. It also sends branches to the anterior aspect of the lower leg. The sural nerve is the sensory supply for the posterolateral side of the lower leg, and continues to the dorsal lateral aspect of the foot, giving rise to lateral calcaneal branches. It passes to the foot through the interval between the lateral malleolus and the calcaneus. The sural nerve is formed from the joining of the medial and lateral sural cutaneous nerves. The medial sural nerve branches off from the tibial nerve, which innervates the posterior muscle compartment. The lateral sural nerve branches off from the common peroneal nerve, whose branches innervate the anterior and lateral muscle compartments.

Arteries and Veins of the Lower Leg

The anterior tibial artery is the major anterior artery of the lower leg. It branches from the popliteal artery and exits the posterior leg compartment through the interosseous membrane. It descends through the lower leg on the membrane’s anterior surface, and supplies the anterior muscle compartment. The main posterior arteries are the posterior tibial and the peroneal, or fibular, artery (Figure 158). Both of the posterior arteries run in a fascial compartment deep to the soleus and gastrocnemius muscles. The posterior tibial artery is a terminal branch of the popliteal artery. It supplies the posterior compartment of the lower leg, then branches into the medial and lateral plantar arteries that supply the plantar surface of the foot. The posterior tibial artery runs toward the medial aspect of the lower leg. Its pulse can be palpated posterior and inferior to the medial malleolus. Absence of the posterior tibial artery pulse can be an indicator of peripheral vascular disease. The peroneal, or fibular, artery is a branch of the posterior tibial artery. It is medial to the fibula, in the deep posterior compartment. The fibular artery sends perforating branches to the anterior compartment, and a nutrient artery to the fibula. It ends as the lateral calcaneal artery.

The venous system in the lower legs consists of both deep and superficial veins, although blood return occurs mainly through the deep veins. As discussed previously, the veins have valves that help maintain the uni-directional flow of blood back to the heart. The deep veins are greatly affected by contractions of the leg muscles, which act as musculovenous pumps. The lower leg muscles work with the deep veins to propel blood up the leg to the larger veins, such as the femoral vein, and on to the heart. When the muscles contract, blood within the veins is squeezed upward, and the valves in the veins open. When the muscles are at rest, the valves close to help prevent the backward flow of blood. The efficient working of the musculovenous pump decreases the likelihood of formations of deep vein thromboses, which often occur near a venous valve. The deep veins of the lower leg include the posterior tibial, the peroneal (or fibular), and the anterior tibial (Figure 159). The posterior tibial ascends from the posteromedial aspect of the lower leg. It joins with the peroneal vein, which is located behind the fibula on the lateral aspect of the lower leg. Together, they form the tibial/peroneal trunk. The anterior tibial vein ascends along the anterior aspect of the interosseous membrane, then moves posteriorly in the superior aspect of the lower leg. The anterior tibial then joins the tibial/peroneal trunk, and these vessels become the popliteal vein.

The major superficial veins of the lower leg are the great and lesser (or small) saphenous veins. The great saphenous vein runs along the anteromedial aspect of the lower leg and thigh, emptying into the femoral vein. The lesser saphenous ascends the posterior aspect of the lower leg, and empties into the popliteal vein, just superior to its juncture with the deep veins. The great and lesser saphenous veins can freely communicate with each other via smaller collateral channels. There is also communication between the superficial and deep veins through perforator veins. This is important in maintaining blood flow from the legs back to the heart, as there is not as much influence from the musculovenous pump on the superficial veins. The majority of the valves in the great saphenous vein are found in the portion of this vein that is in the lower leg. The valves are typically inferior to the communications with the perforator veins, helping to maintain uni-directional flow, and pushing blood into the deep veins. The saphenous veins and their tributaries are often the veins that suffer from valvular insufficiencies. Blood can then pool in the lower parts of the leg, engorging the veins. Varicosities can result, leading to permanent deformities and/or dysfunction.

Open MRI Systems

Introduction

When positioning a patient for a lower leg exam in an open MRI system, it is important to consider the choices you have as far as coil selection and patient setup. Positioning that will result in both the coil and the anatomy at isocenter in all three planes will have a positive impact on the image quality of your study.

Components of positioning to consider include coil selection, table pads and accessory pads, and proper laser light centering. Coil selection will be influenced by the patient’s size and body habitus. The extensive inventory of table and accessory pads should be used for proper centering, as well as patient comfort and stability (Figure 160). The use of trough pads and thick or thin table pads are keys to achieving coronal centering of both the patient and the coil. When scanning on open MRI systems, it is extremely important to center the anatomy of interest in the laser lights (Figure 161) in all three directions: head-to-foot (axial or transverse plane), right-to-left (sagittal plane), and anterior-to-posterior (coronal plane).

Coils and Positioning

RAPID Body Coil

The coil of choice for a study of the lower legs is the RAPID body coil. Fit the base of the coil in the table trough. The longitudinal center mark on the coil will be centered at the midline of the table. The horizontal center mark on the coil should be between the marking on the patient table and the end of the table nearest the magnetic field. This will allow for sufficient table travel while still achieving isocenter positioning. The patient should be positioned on the base portion of the coil with the midpoint of their lower legs (in the head to foot direction) aligned with the horizontal center mark on the coil. The coil pads, accessory pads, and additional table pads can be adjusted to help achieve coronal centering of the lower legs. The upper portion of the coil is then placed on the base and pushed firmly into place to lock the coil. For bilateral lower leg scanning, the midline of the patient’s body should be aligned with the longitudinal center mark on the upper portion of the coil (Figure 168). The patient should now be centered in all three planes- centered on the middle of the lower legs in the coronal and axial planes, and centered midline in the sagittal plane. If a unilateral lower leg scan is to be performed, the table can be moved to the right or left to achieve the best possible sagittal plane centering of the affected lower leg.

Flexible Body Coil

The flexible body coils can be used for lower leg scanning to accommodate larger patients. However, they do not have RAPID capabilities, and should not be used with protocols that are labeled as “RAPID”. Trough and/or table pads should be placed on the table to help center the flexible coil in the coronal plane. The horizontal center of the flexible body coil should be between the marking on the patient table and the end of the table nearest the magnetic field. This will allow for sufficient table travel while still achieving isocenter positioning. The longitudinal center mark on the bottom of the coil should be aligned with the sagittal laser light. Position the patient on the flexible coil with the midpoint of their lower legs aligned with the horizontal center mark on the coil (Figure 169). Depending on the patient’s body habitus, table and/or accessory pads may have to be adjusted to maintain coronal centering. Close the flexible body coil around the patient and secure the latches. Accessory pads can be placed between the flex coil and the patient to maintain the anterior portion of the coil in a level position, which will minimize stress on the latches. For bilateral lower leg scanning, the midline of the patient’s body should be aligned with the longitudinal center mark or stitching found on the anterior aspect of the coil (Figure 170). The patient should now be centered in all three planes- centered on the lower legs in the coronal and axial planes, and centered midline in the sagittal plane. If a unilateral lower leg scan is to be performed, the table can be moved to the right or left to achieve the best possible sagittal plane centering of the affected lower leg.

The table straps may be used to further secure both the RAPID and flexible coils. For safety purposes, the patient should be encouraged to keep their arms and hands along their sides, but not in direct contact with their body.

Cervical Spine Coil

If a unilateral lower leg exam is required, the coil of choice is the cervical spine coil. On the Oasis system, the base of the cervical coil fits in the table trough. The longitudinal center mark on the coil will be centered at the midline of the table (Figure 171). On the AIRIS and Altaire systems, the base of the coil should be placed on trough and table pads to help center the coil in the coronal plane. On any of the open systems, the horizontal center mark on the coil should be between the marking on the patient table and the end of the table nearest the magnetic field. This will allow for sufficient table travel while still achieving isocenter positioning. The patient should be positioned on the base portion of the coil with the midpoint of their lower leg aligned with the horizontal center mark on the coil. The coil pads, accessory pads, and additional table pads can be adjusted to help achieve coronal centering of the lower leg. The upper portion of the coil is then placed on the base and pushed firmly into place to lock the coil. The midline of the affected lower leg should be aligned with the longitudinal center mark on the upper portion of the coil (Figure 172). The table can be moved to the right or left to achieve the best possible sagittal plane centering of the affected lower leg. The patient should now be centered in all three planes- centered on the affected lower leg in the coronal and axial planes, and centered midline in the sagittal plane. The large knee bolster cushion can be placed under the unaffected lower leg for patient comfort. It will also serve to remove the unaffected lower leg from the scan plane, which should decrease wrap-around artifacts.

Echelon MRI System

Introduction

The 1.5T Echelon system comes equipped with a variety of trough and table pads, as well as various positioning pads and sponges (Figure 173). These positioning aids can be used to support the position of both the patient and the coil, as well as to keep your patient comfortable and secure. The magnet bore of the Echelon does not permit lateral patient table movement. However, accurate patient and coil positioning, and correct use of the sagittal and axial laser lights (Figure 174) to center the patient’s anatomy ensure that high quality images will be acquired through isocenter scanning.

Coils and Positioning

RAPID Torso/Body Coil

The coil of choice for a study of the lower legs is the RAPID Torso/Body coil. Trough and/or table pads are placed on the table as needed, depending on the patient’s size and body habitus. The lower portion of the coil should be positioned in the middle of the table on top of the pads. The exact placement of the coil on the table, and the direction it faces, are based on the patient’s preference to enter the scanner head first or feet first. The patient should be in a supine position on the lower portion of the coil with the midpoint of their lower legs aligned with the coil’s horizontal center marks. The upper portion of the coil is then placed on the base and secured with the Velcro® straps (Figure 176). The longitudinal center mark on the upper portion of the coil should be at a position midway between the lower legs.

Scan Setups

The following are HMSA suggestions for lower leg imaging. Always check with your radiologist for his/her imaging preferences. You may be required to scan the lower legs bilaterally in certain planes using specific sequences, with different sequences performed only on the affected lower leg (i.e. T2 Fatsat on affected side only).

Axial Scans

When positioning axial slices for the lower legs, a coronal image and a sagittal image can be used to ensure inclusion of all pertinent anatomy. The slices typically extend superiorly to include the knee joint, and inferiorly to include the ankle joint. Depending on your radiologist’s preference, slice coverage may only need to include the joint closest to the affected area of the anatomy. Axial slices should be aligned perpendicular to the shafts of the tibia/fibula, as seen in both the coronal and sagittal images in Figure 177.

For unilateral axial lower leg slice setup, parameters will have to be altered to maintain adequate signal with the smaller FOV that is required. Again, axial slices should be aligned perpendicular to the shafts of the tibia/fibula, as seen in both the coronal and sagittal images in Figure 178.

Coronal Scans

Coronal slices of the lower leg should include the area from the anterior to the posterior margin of the lower leg. The slices are typically aligned along the long axis of the tibia/fibula, as seen in the sagittal image in Figure 179. Thinner slices may be requested when scanning in the coronal plane.

Sagittal Scans

Sagittal slices of the lower legs should extend from the medial to the lateral margin of the lower leg. The slices should be aligned along the long axis of the tibia, as seen in the coronal image in Figure 180. Two different slice groups are required for bilateral lower leg coverage.

Ankle

MRI may be requested for: Bones of the Ankle

The ankle joint is a synovial hinge joint that connects the distal ends of the tibia and fibula of the lower leg with the proximal end of the talus bone of the foot (Figure 181). There is articular cartilage over the ends of all the bones involved in the ankle joint. The articular surface of the tibia forms a bony arch over the talus, which is termed the ankle mortise. This articulation bears more weight than the articulation between the fibula and talus. The medial malleolus of the tibia, the lateral malleolus of the fibula, and the strong ankle ligaments help to stabilize the talus under the tibia. This “true” ankle joint, or talocrural joint, permits only flexion and extension. Inversion and eversion movements occur through subtalar and tarsal articulations.

The distal tibia and fibula were discussed previously in the anatomy of the lower leg. The remaining bone of the ankle joint, the talus, is one of the seven tarsal bones. It has a rather odd “humped” shape, and fits between the tibia superiorly, the fibula laterally, the calcaneus inferiorly, and the navicular anteriorly. The superior articular surface of the talus is called the trochlea. It is wider anteriorly versus posteriorly, but the syndesmotic articulation between the tibia and fibula adapts to this change. The talus is the second largest of the tarsal bones, and has a high percentage of its surface area covered by articular cartilage. The talus is important for its role in helping to transfer weight and pressure forces across the ankle joint and into the foot. It can be injured in auto accidents, falls from heights, and more recently, in snowboarding, as they do not wear rigid boots that might help prevent ankle injuries. Talus fractures from snowboarding may be mistaken for ankle sprains, as they exhibit similar symptoms of lateral tenderness and severe bruising.

Ligaments of the Ankle

The ankle ligaments can be divided into three main groups: the lateral ligaments, the deltoid ligament, and the syndesmotic ligaments. The ligaments are strong, fibrous bands, but they are still susceptible to injury due in part to the excessive movement of the subtalar joint during activity. Injuries to the ligaments of the ankle are the most frequent causes of acute ankle pain. Chronic ankle pain can result from laxity of just one ligament. Injuries to the ankle can affect the joint capsule, the medial or lateral collateral ligaments, and/or the tibiofibular ligaments. The most common mechanism of injury to the ankle is inversion of the foot, which affects the lateral ligaments. Eversion of the foot damages the medial collateral, or deltoid, ligaments.

The three ligaments of the lateral (or lateral collateral) group includes the anterior talofibular, calcaneofibular, and posterior talofibular (Figure 182). The anterior talofibular ligament is typically the first, and sometimes the only ligament to be injured from foot inversion. In approximately 20% of the cases, the calcaneofibular ligament is also injured. If there is involvement of the posterior talofibular ligament, the injury is considered a total rupture of the lateral ligaments. The anterior talofibular ligament begins at the anterior margin of the lateral malleolus, and inserts on the body of the talus. Its job is to limit anterior displacement of the talus, and limit plantar flexion of the ankle. When the ankle is in a neutral position, it is horizontal. However, it is inclined downward with plantar flexion, leaving it more vulnerable to injury and strain. It is reported to appear as two bands that are separated by arterial vessels. The calcaneofibular ligament is inferior to the anterior talofibular ligament. It attaches the lateral malleolus with the posterior region of the lateral calcaneal surface, and is directed inferiorly and posteriorly. It is the only ligament that bridges both the talocrural (ankle) joint, where it allows flexion and extension, as well as the subtalar joint, where it permits subtalar movement. The calcaneofibular ligament is in a horizontal position during extension, and vertical in flexion, remaining tense through the arc of motion. However, isolated rupture of this ligament is rare. The calcaneofibular ligament is separate from the ankle joint capsule, and the majority of it is covered by peroneal tendons and sheaths that cross it superficially. The third ligament of the lateral group is the posterior talofibular ligament. It originates from the malleolar fossa on the medial surface of the lateral malleolus, and travels almost horizontally to insert in the posterolateral aspect of the talus. The posterior talofibular ligament has fibers that insert in various areas, including the posterior talar surface, and the lateral talar process (or os trigonum), if it is present. Some of this ligament’s fibers may contribute to the formation of a tunnel for the flexor halluces longus tendon. A group of these fibers may also fuse with the posterior intermalleolar ligament, where they can become involved with posterior soft tissue impingement syndrome of the ankle. The posterior intermalleolar ligament is reported as a “normal variant’, best seen on T1 and T2 coronal MR images. It is located between the transverse ligament and the posterior talofibular ligament, and runs obliquely from lateral to medial, and from inferior to superior. The posterior intermalleolar ligament tenses when the ankle is in dorsiflexion, and relaxes during plantar flexion. Trauma with forced dorsiflexion may cause injury or rupture of this ligament. In addition, it may extend into the ankle joint when it relaxes during plantar flexion, becoming susceptible to entrapment between the tibia and the talus, and contributing to posterior impingement syndrome.

The ligaments of the medial collateral, or deltoid, group have both superficial and deep components. They originate from the medial malleolus, and insert in the talus, calcaneus, and navicular bones (Figure 183). Many of these ligaments are covered by tendon sheaths from posterior tibial muscles, similar to the peroneal tendon sheath that covers much of the calcaneofibular ligament laterally. Multiple bands or components of the deltoid ligaments can be identified; however, many of the bands are continuous with each other, and therefore, difficult to differentiate. Three ligaments that are always present are the tibiospring, the tibionavicular, and the deep posterior tibiotalar. Three ligaments that vary in their presence include the superficial posterior tibiotalar, the tibiocalcaneal, and the deep anterior tibiotalar.

The ligaments of the syndesmotic ligament complex ensure stability between the distal tibia and fibula, and resist forces that try to separate the two bones. These ligaments were mentioned previously in the lower leg anatomy, as they are involved in an injury known as a “high ankle sprain”. This group consists of the anterior (or anteroinferior) and posterior (or posteroinferior) tibiofibular ligaments, and the interosseous tibiofibular ligament (Figure 184). The anteroinferior tibiofibular ligament runs from the anterior tibial tubercle distally and laterally to insert on the anterior margin of the lateral malleolus. This ligament is divided into several fascicles, or bundles of fibers, allowing for perforating branches from blood vessels. The most distal fascicle may be independent, and have a deeper location. Pathology to this fascicle may be responsible for anterolateral soft tissue impingement. Excision of this fascicle can be performed to end a patient’s pain without compromising the stability of the ankle. The posteroinferior tibiofibular ligament has both superficial and deep components. The superficial component originates at the posterior edge of the lateral malleolus and travels proximally and medially to insert on the posterior tibial tubercle. The deep component is somewhat cone-shaped. It originates in the proximal area of the malleolar fossa, and moves posteriorly to the edge of the tibia. The deep posteroinferior tibiofibular ligament inserts posterior to the cartilaginous covering of the inferior tibial articular surface. Some of its fibers may reach the medial malleolus. This deep component is also called the transverse ligament. It forms a true labrum to provide talocrural joint stability, and prevent posterior talar translation. The interosseous tibiofibular ligament can be considered a distal continuation of the interosseous membrane at the level of the tibiofibular syndesmosis. It spans from the tibia to the fibula, and is located deep to the posteroinferior tibiofibular ligament. This interosseous ligament is a dense mass of short fibers, containing adipose tissue and small branches of blood vessels. Some experts in the field report that this ligament is insignificant, while others report that it is the primary bond between the tibia and fibula, and has an important role in ankle stability.

Muscles and Tendons of the Ankle

The majority of the motion of the ankle is caused by the strong muscles in the lower leg, as their tendons pass by the ankle and connect in the foot. Contraction of the muscles of the lower leg moves the ankle for walking, running, and jumping. The muscles involved in ankle movement were discussed in the anatomy of the lower leg, but the important muscles and their effects on the ankle will be reviewed here. The peroneals (longus and brevis) are the lateral muscles of the lower leg and ankle that evert the foot and are active in plantar flexion (Figure 185). The anterior leg muscles (tibialis anterior, extensor digitorum longus, extensor hallucis longus, and peroneus tertius) are all dorsiflexors of the ankle (pull toes up towards leg). Both the deep and superficial muscles of the posterior leg are plantar flexors for the ankle joint (Figure 186). As these calf muscles tighten, they bend the ankle down.

In addition to the ligaments that surround the ankle, the joint’s stability is greatly dependent on the tendons of the strong muscles that cross the ankle. One of the best known tendons is the Achilles tendon, which connects the gastrocnemius and soleus muscles to the calcaneus. The Achilles is one of the most important tendons for walking, running, and jumping, as well as plantar flexion. The tendons of the tibialis anterior muscle attach anterior to the medial malleolus. Tendons of the tibialis posterior and the flexor tendons (digitorum longus and hallucis longus) attach posterior to the medial malleolus. The tendon of the extensor hallucis longus, the tendons of the extensor digitorum longus, and the peroneus tertius tendon (which becomes part of the extensor digitorum longus) all attach anteriorly to the ankle joint (Figure 187). Tendons of the lateral muscles, the peroneus longus and brevis, attach behind the lateral malleolus (Figure 188).

Mechanical strength to prevent the bowstringing of tendons is provided by the five retinacula that surround the ankle joint. Retinaculum is a localized thickening of a strong, flat sheet of fibrous connective tissue that serves as a tendon to attach muscle to bone, or as fascia to bind muscles together. Average thickness of retinaculum is 1 mm. In the ankle joint, the various retinacula help in maintaining the approximation of the tendons to the underlying bone. The superior and inferior extensor retinacula and the flexor retinaculum keep the flexor and extensor tendons close to the bone, while the superior and inferior peroneal retinacula keep the tendons of the peroneus longus and brevis close to the lateral malleolus. The superior extensor retinaculum is located above the tibiotalar joint (Figure 189). It attaches to the medial and lateral malleoli. Structures that pass deep to the superior extensor retinaculum include the tendons of the tibialis anterior, extensor digitorum and hallucis longus, and peroneus tertius, anterior tibial blood vessels, and the deep peroneal nerve. The inferior extensor retinaculum is located on the anterior aspect of the tarsus and ankle joint, and has a Y-shape configuration. It consists of a stem and three oblique bands. One band attaches to the anterior medial malleolus, and another band attaches at the medial border of the foot at the cuneonavicular joint. The stem acts as a sling to retain the tendons of the extensor digitorum longus and peroneal tertius muscles against the talus and calcaneus bones. There are three roots in the stem that help to make up this sling, and these roots can be best appreciated on coronal MR images. The flexor retinaculum is more medial, and encloses the tarsal tunnel. It covers the area from the medial malleolus to the posterosuperior aspect of the calcaneus. The flexor and superior peroneal retinacula, along with the aponeurosis of the distal leg, form a basket around the Achilles tendon. The superior and inferior peroneal retinacula are found on the lateral aspect of the ankle, with the peroneal muscles and tendons that make up the lateral muscle compartment (Figure 190). The superior peroneal retinaculum is the primary restraint that prevents subluxation and dislocation of the peroneal tendons as they pass behind the retromalleolar groove and through the peroneal tunnel. It is spread from the tip of the lateral malleolus and the lateral border of the retromalleolar groove to the posterior segment of the distal leg. Injuries to the superior peroneal retinaculum may allow a subluxation of the peroneal tendons over the sharp edge of the fibula, or can lead to increased friction of the tendons as they slide in and out of the peroneal groove. These problems can be mistaken for other causes of lateral ankle pain. The inferior peroneal retinaculum has an oblique downward and posterior course from the posterolateral rim of the sinus tarsi to the trochlear process of the talus. The sinus tarsi is the depression on the lateral side of the tarsus distal to and on the same level as the lateral malleolus. Abnormal retinaculum may have consequences for, or occur in association with, nearby tendon abnormalities. It is difficult to determine if thickening of the retinaculum is the source of tendon abnormalities, or is caused by abnormal underlying tendons. The retinacula typically display with low signal intensity on T1 and intermediate weighted MR sequences. Non-fat suppressed T1 sequences offer the benefit of high signal from the fat that surrounds the low signal retinacula. The flexor retinaculum is best displayed on coronal images, while the superior and inferior extensors and peroneals are best seen on axial images.

Nerves of the Ankle

The nerves of the ankle joint are continuations from the lower leg that pass on to the foot. The most frequent mechanisms of injury to nerves around the ankle are compression and tension. A compressive neuropathy can be secondary to trauma, and can result from a bone fragment, hematoma, soft tissue edema, or the nerve becoming encased in scar tissue. Nerve compression can result in lesions in the fibro-osseous tunnels, causing ganglion cysts, varicosities, bone and joint abnormalities, and tenosynovitis. Tension neuropathy can be caused by inappropriate footwear, ankle sprains, internal foot derangement, and the wearing of high-heels. The tibial nerve travels posteriorly down the lower leg, and is posterior to the medial malleolus in the ankle region (Figure 191). It passes through the tarsal tunnel on its way to its terminal branches in the foot, which include the medial and lateral plantar nerves. Entrapment of the medial plantar nerve can result in a condition known as jogger’s foot. The nerve can become trapped between the abductor halluces muscle of the foot, and the anatomic crossover point between the flexor hallucis and digitorum tendons (also called the master Knot of Henry). The medial calcaneal nerve may be seen as another terminal branch of the tibial nerve, or as a branch of the lateral plantar nerve. The medial calcaneal penetrates the flexor retinaculum, and supplies the superficial sensory branches to the skin over the medial aspect of the Achilles tendon, and the posteromedial aspect of the heel. The sural nerve, discussed previously in the lower leg anatomy, is the sensory innervation to the lateral heel, and gives sensation to the lateral ankle (Figure 192). The deep peroneal nerve passes under the extensor retinaculum, between the tendons of the extensor muscles, and lateral to the anterior tibial artery. It divides above the ankle joint into the medial sensory branch and the lateral motor branch, which supplies the extensor digitorum brevis muscls of the foot. The deep peroneal nerve can be compressed by the inferior extensor retinacumlum at the point where the extensor hallucis longus tendon crosses over it. The superficial peroneal nerve is located near the tip of the lateral malleolus. It supplies motor branches to the peroneus brevis and longus muscles, and sensory innervation to the dorsolateral ankle. This nerve can be overstretched in inversion or plantar flexion injuries.

Arteries and Veins of the Ankle

The major arteries of the ankle area are the anterior and posterior tibial arteries. Both have numerous branches, and both contribute to the ankle anastomoses that form networks of arteries around the medial and lateral malleoli. The anterior tibial artery becomes the dorsalis pedis artery on the front of the ankle joint (Figure 193). It crosses from the lateral aspect to the medial aspect near the tendon of the extensor hallucis longus. The anterior tibial artery further branches into the anterior medial and anterior lateral malleolar arteries. The anterior medial malleolar is approximately 5 cm. above the ankle joint. On the medial side of the ankle, it forms an anastomosis with branches of the posterior tibial (including the medial calcaneus), and the medial plantar arteries. The anterior lateral malleolar artery supplies the lateral ankle, and forms an anastomosis with the perforating branches of the peroneal artery, and ascending portions of the lateral tarsal artery. The posterior tibial artery is behind the tibia and at the back of the ankle joint, running parallel with the medial border of the Achilles tendon. The lower portion is typically midway between the medial malleolus and the medial process of the calcaneal tuberosity. It divides into the medial and lateral plantar arteries to supply the foot. The posterior tibial artery is typically accompanied by two veins and the tibial nerve, which crosses from the medial to the lateral side of the artery. The peroneal artery is one of the larger branches of the posterior tibial artery. It is located on the back of the fibular (lateral) side of the leg. Behind the tibiofibular syndesmosis, the peroneal artery divides into the lateral calcaneal branches, which supply the lateral and posterior surfaces of the calcaneus. The lateral calcaneal branches are the terminal branches of the peroneal artery, and communicate with the lateral malleolar network. Additional branches of the posterior tibial include the posterior medial malleolar artery, which winds around the medial malleolus; the communicating branch, which lies transverse across the back of the tibia; and the medial calcaneal artery, which is behind the Achilles tendon, travels around the heel to muscles on the medial aspect, then forms an anastomosis with the peroneal and medial malleolar arteries, and the lateral calcaneal arteries on the back of the heel.

As in the lower leg, there are both deep and superficial veins in the ankle region that work to return blood to the heart. The dorsal vein of the foot is an anterior deep vein that returns blood to the anterior tibial vein. Posteriorly, the medial and lateral plantar veins return blood from their respective sides of the foot to the posterior tibial vein, which rises along the medial aspect of the tibia (Figure 194). The superficial veins in the ankle region include the lateral marginal vein of the foot, which meets the small saphenous vein at the lateral malleolus. The small saphenous vein travels in a posterior position behind the fibula as it ascends the lower leg to join the popliteal vein. Medially, the superficial medial marginal vein in the foot returns blood to the great saphenous vein. The great saphenous vein ascends anteriorly and medially through the lower leg and femur, eventually joining the femoral vein near the hip. The great saphenous vein moves to a posterior position at the knee joint.

The problems of venous insufficiency, varicosities, etc. can also plague the ankle region. Chronic ankle swelling can be a symptom of both venous reflux disease and heart failure. However, heart failure typically includes the patient having shortness of breath with no ankle discoloration, while venous insufficiency typically includes ankle discoloration with no shortness of breath. Ankle discoloration can result from the great saphenous vein’s inability to pump blood to the heart efficiently. Blood can then pool in the calves and ankles, resulting in swelling, red or purple discoloration, pain, and rope-like varicose veins under the skin (Figure 195).

Although both deep and superficial veins have valves, valve failure is most commonly seen in the superficial great saphenous vein. When the valves fail, they allow blood to “leak” toward the ankles. As blood pools in the smaller veins near the skin, the ankles swell. One non-surgical treatment for venous insufficiency is radiofrequency ablation (Figure 196), in which the great saphenous vein is closed off. It is an accessory vein, so blood is then re-routed to one of the deep veins of the lower leg, such as the posterior tibial vein.

Open MRI Systems

Introduction

When positioning a patient for an ankle exam in an open MRI system, it is important to consider the choices you have as far as coil selection and patient setup. Positioning that will result in both the coil and the anatomy at isocenter in all three planes will have a positive impact on the image quality of your study.

Components of positioning to consider include coil selection, table pads and accessory pads, and proper laser light centering. Coil selection will be influenced by the patient’s size and body habitus. The extensive inventory of table and accessory pads should be used for proper centering, as well as patient comfort and stability (Figure 197). The use of trough pads and thick or thin table pads are keys to achieving coronal centering of both the patient and the coil. When scanning on open MRI systems, it is extremely important to center the anatomy of interest in the laser lights (Figure 198) in all three directions: head-to-foot (axial or transverse plane), right-to-left (sagittal plane), and anterior-to-posterior (coronal plane).

Coils and Positioning

The coil of choice for a study of the ankle depends on the patient position that is required by your radiologist. If the ankle does not have to be positioned at a 90° angle, the Extremity coil would be the coil of choice. If the radiologist requires that the ankle be positioned at a 90° angle, the Head coil is recommended.

Knee (Extremity) Coil

Place the base of the coil on the table. Add trough pads and/or table pads under the base of the coil to help center the coil in the coronal plane (the coronal laser light should be aligned with the top of the coil base). The horizontal center of the coil base should be between the marking on the patient table and the end of the table nearest the magnetic field. This will allow for sufficient table travel while still achieving isocenter positioning. The longitudinal center marking on the coil base should be placed as close as possible to the midline of the table; however, the patient’s size, comfort, and the laterality of the affected ankle must be taken into account when determining the longitudinal positioning of the coil. The extremity coil may be offset slightly towards the affected ankle. Coil centering in the sagittal plane can be performed as a final positioning step using the open magnet’s lateral table movement capabilities. The patient should be positioned on the base portion of the coil with the ankle joint aligned with the horizontal center mark on the coil. The longitudinal midline of the patient’s affected ankle should be positioned in the middle of the coil base. The coil pads and accessory pads can be adjusted to help achieve coronal centering of the patient’s ankle in the coil base. The upper portion of the coil is then placed on the base and the coil latches are secured to close the coil. Accessory sponges can be added inside the coil to further secure the ankle in place. Pads and sponges should be placed under the lower leg for support and comfort. The large knee bolster cushion can be placed under the unaffected leg for patient comfort. It will also serve to remove the unaffected leg from the scan plane, which should decrease wrap-around artifacts. The right and left arrow buttons located on the gantry can be used to move the table from side to side for proper alignment of the coil’s longitudinal center mark with the sagittal laser light (Figure 203). The patient should now be centered in all three planes- centered on the middle of the ankle in the coronal and axial planes (Figure 204), and centered midline in the sagittal plane.

Head Coil

The head coil is the coil of choice if the radiologist requires that the ankle be positioned at a 90° angle. Fit the base of the coil in the table trough. The longitudinal center mark on the coil will be centered at the midline of the table. The horizontal center mark on the coil should be between the marking on the patient table and the end of the table nearest the magnetic field. This will allow for sufficient table travel while still achieving isocenter positioning. The patient should be positioned on the base portion of the coil with their ankle joint aligned with the horizontal center mark on the coil. The longitudinal midline of the patient’s affected ankle should be positioned in the middle of the coil base (Figure 205). The coil pads and accessory pads can be adjusted to help achieve coronal centering of the patient’s ankle in the coil base. The upper portion of the coil is then placed on the base and the coil latches are secured to close the coil. For the Oasis head coil, the upper portion of the coil slides over the base portion and is secured by a locking lever. Accessory sponges can be added inside the coil to further secure the ankle in place. Pads and sponges should be placed under the lower leg for support and comfort (Figure 205). The large knee bolster cushion can be placed under the unaffected leg for patient comfort. It will also serve to remove the unaffected leg from the scan plane, which should decrease wrap-around artifacts. The patient should now be centered in all three planes- centered on the middle of the ankle in the coronal and axial planes, and centered midline in the sagittal plane.

You may be able to perform a bilateral ankle exam in the head coil, depending on the patient’s size and body habitus. Place both ankles on the base of the head coil, using a coil strap to help hold the ankles in place, as seen in Figure 206. The ankle joints should be aligned with the horizontal center marks on the coil. The midline between the patient’s ankles should be aligned with the longitudinal center mark on the coil. The coil pads and accessory pads can be adjusted to help achieve coronal centering of the patient’s ankles in the coil base. Secure the upper portion of the coil on the base, or slide the upper portion over the base of the coil. Accessory pads and sponges can be added inside the coil to further secure the ankles in place. Pads and sponges should be placed under the legs as needed for support and comfort. The patient should now be centered in all three planes- centered on the middle of the ankles in the coronal and axial planes, and centered midline between the ankles in the sagittal plane.

Echelon MRI System

Introduction

The 1.5T Echelon system comes equipped with a variety of trough and table pads, as well as various positioning pads and sponges (Figure 207). These positioning aids can be used to support the position of both the patient and the coil, as well as to keep your patient comfortable and secure. The magnet bore of the Echelon does not permit lateral patient table movement. However, accurate patient and coil positioning, and correct use of the sagittal and axial laser lights (Figure 208) to center the patient’s anatomy ensures that high quality images will be acquired.

Coils and Positioning

Foot & Ankle Coil

Place the foot & ankle coil on top of the table pads. Unlatch the anterior elements for easy patient positioning. Once the patient’s foot is positioned in the coil, latch the anterior elements (Figure 211). Use accessory pads and sponges inside the coil to stabilize the foot and ankle. Additional pads can be placed under the affected leg for support and patient comfort. The coil’s longitudinal center mark should be aligned as closely as possible with the sagittal laser light.

Knee (Extremity) Coil

The coil should be placed at the gantry end of the table. The bottom portion of the coil should be placed on top of the table pads. The patient is typically positioned feet first on the base portion of the coil, with the center of their ankle aligned with the horizontal center marks on the coil. The upper portion of the coil is then placed on the base, and the coil locking mechanism is secured (Figure 212). The longitudinal midline of the patient’s affected ankle (and leg) should be aligned with the longitudinal center mark on the upper portion of the coil. Accessory sponges can be added inside the coil to further secure the ankle in place. Pads and sponges should be placed under the lower leg for support and comfort. A positioning pad or sponge placed under the unaffected lower leg may increase patient comfort, and also serves to remove the unaffected ankle from the scan plane, which should decrease wrap-around artifacts. The coil’s longitudinal center mark should be aligned as closely as possible with the sagittal laser light.

Scan Setups

The following are HMSA suggestions for ankle imaging. Always check with your radiologist for his/her imaging preferences.

Axial Scans

When positioning axial slices for the ankle, sagittal and coronal images can be used to insure inclusion of all pertinent anatomy. The slices should extend superiorly to include the distal tibia, and inferiorly to include the plantar fascia, as seen in the sagittal image in Figure 213. If the slice group is angled, it should remain parallel to the axis of the calcaneus.

Coronal Scans

Coronal slices of the ankle should cover the anatomy from the calcaneus (including the Achilles tendon) to the metatarsal bases, as seen in the sagittal image in Figure 214. The coronal slice group should be perpendicular to the axial slices.

Sagittal Scans

Sagittal slices of the ankle should cover the anatomy from the medial malleolus to the lateral malleolus, as seen in the coronal image in Figure 215. If the sagittal slices are angled due to the patient’s position in the coil, the slices should remain parallel to the body of the talus.

Foot/Toes

MRI may be requested for: Bones of the Foot

The foot is a strong and complex weight-bearing structure that allows mobility. It has more than 26 bones, 33 joints (20 of which are actively articulated), and more than 100 muscles, tendons and ligaments. It is typically divided into three areas- the hindfoot, the midfoot, and the forefoot (Figure 216). (These areas may not coincide with the anatomical areas as defined by your radiologist for inclusion on your MR images). The hindfoot includes two of the seven tarsal bones, which are the talus and the calcaneus (Figure 217). The superior aspect of the talus, along with and the distal tibia and fibula, form the ankle joint. The calcaneus, or heel bone, is the largest bone in the foot. It articulates with the talus at the subtalar joint. The midfoot contains the five remaining tarsal bones, which all have somewhat irregular shapes. The navicular and cuboid are proximal, with the navicular articulating with the talus on the medial aspect of the foot, and the cuboid articulating with the calcaneus laterally. The three cuneiforms (medial, intermediate or middle, and lateral) are in a row between the navicular bone and the proximal ends of the first, second and third metatarsals. The lateral aspects of the lateral cuneiform and the navicular articulate with the medial aspect of the cuboid. The bones of the midfoot are connected to the hindfoot and forefoot by various muscles and the plantar fascia. They form the arches of the foot, which serve as shock absorbers. The ankle joint permits only flexion and extension, while the joints of the hindfoot and midfoot permit inversion and eversion movements of the foot. These movements allow the foot to adjust to walking or running on tilted surfaces. Joints involved in these movements include the subtalar (talocalcaneal) and the transverse tarsal joints (talocalcaneonavicular and calcaneocuboid). The forefoot includes the five metatarsal bones, and the fourteen phalanges. Similar to the thumb, the great toe has only two phalanges, while the other four toes have three phalanges. Also in similarity with the hand, the foot has a variety of interphalangeal and metatarsophalangeal joints. The distal first metatarsal is typically the site of two or more sesamoid bones, which are accessory bones.

For football fans, a well-known injury that typically affects the first metatarsophalangeal joint is turf toe. The mechanism of injury is hyperdorsiflexion, which results in a hyperextended great toe (Figure 218). Turf toe can occur if the toe is jammed, or if the patient is repeatedly pushing off on the toe for running or jumping. The sesamoid bones in the tendon of the great toe work like pulleys for the tendon that moves the great toe. They give leverage when you walk or run, and help to absorb some of the weight that presses on the ball of the foot. The first metatarsophalangeal joint may transfer eight times the body’s weight during push off. Turf toe injuries result in compression of the articular surface of the metatarsal head, and separation of the sesamoid components. The joint capsule at the base of the toe may be torn, leading to instability of the joint and accelerated cartilage wear. Turf toe causes an increase in tension on the plantar aspect of the joint, where the plantar plate is located. This plate is fibrocartilaginous tissue that attaches distally to the proximal phalanx, and proximally to the sesamoid bones. Turf toe injuries typically disrupt the plantar plate or its attachments (Figure 219). MRI findings of turf toe that have been reported have described marked soft-tissue edema at the plantar aspect of the affected metatarsophalangeal joint, and a high signal intensity tear in the normally low signal intensity plantar plate (Figure 220). This injury is commonly seen in athletes that play on hard artificial turf, as well as in athletes that wear shoes that offer minimal support to the joints of the forefoot.

Another hyperdorsiflexion injury of the first metatarsophalangeal joint is called “skimboarder’s toe”. Skimboards look like small, flat surfboards, and are ridden in shallow water at the edge of the beach. The top surface of the skimboard is usually waxed to minimize slippage of the feet when the skimboarder jumps onto the board. The toes are used to help grip the skimboard. If the board slips posteriorly, especially if it is waxed, hyperdorsiflexion at the level of the metatarsophalangeal joints may result. Contrary to turf toe injuries, skimboarder’s toe typically involves structures that are dorsal to the metatarsophalangeal joints, rather than plantar structures. Skimboarder’s toe may result in hyperdorsiflexion of the extensor hallucis longus or extensor digitorum longus tendon, causing tearing of the extensor expansion (Figure 221).

The bony anatomy of the foot also includes three arches that are made up of the foot’s interlocking bones, strong ligaments, and “pulling” muscles during activity (Figure 222). The medial longitudinal arch displays a definite curve above the ground. It includes the calcaneus, talus, navicular, medial and intermediate cuneiforms, and the three medial metatarsals. The medial arch transmits the force of the body’s weight to the ground when standing, and to the great toe in locomotion. The medial arch creates a giant lever that gives spring to the gait. The lateral longitudinal arch is very low to the ground, and includes the calcaneus, cuboid and the two lateral metatarsals. It redistributes part of the body’s weight to the calcaneus and the distal end of the fifth metatarsal. These two longitudinal arches act as pillars for the transverse arch, which runs obliquely across the tarsometatarsal joints. The transverse arch includes the three cuneiforms and the cuboid. Excessive strain on the tendons and ligaments of the foot can result in “fallen arches” or flat feet.

Ligaments of the Foot

Many of the important ligaments of the foot have been discussed previously in the ankle anatomy, as their function is to connect the foot and ankle bones. They will be reviewed in more detail regarding their functions in the foot. A ligament not previously discussed is the plantar fascia, which is also termed a “ligament-type structure”. It is the longest ligament of the foot, and is made up of medial, central, and lateral bands (Figure 223). The plantar fascia supports the longitudinal arch, from heel to toes, when walking. It is able to stretch or contract, enabling the arch to curve or flatten as needed. The plantar fascia provides balance, and gives the foot the strength needed to begin the act of walking. In patients with faulty foot mechanics, typically over-pronation, this fascia can become stretched and stressed, and develop microtears. These can lead to inflammation and pain, as well as the development of fasciitis or fasciosis, which is a degenerative condition (Figure 224). The spring ligament, or plantarcalcaneonavicular ligament, was briefly mentioned in the ankle anatomy, but is very important in foot mechanics. It provides the “spring-like” action when walking. It connects the calcaneus and navicular, preventing the head of the talus (anterior aspect) from pushing down between them (Figure 225). The deltoid ligament, or medial ligament of the ankle, connects the medial malleolus to the three bones of the heel, which are the talus, navicular, and calcaneus. The deltoid has four parts which were discussed previously- the anterior and posterior tibiotalar ligaments, the tibionavicular ligament, and the tibiocalcaneal ligament. The lateral ankle has three ligaments that connect the foot and ankle (Figure 226). The anterior and posterior talofibular ligaments connect the lateral malleolus with the talus, and the calcaneofibular ligament connects the lateral malleolus with the calcaneus. There are numerous ligaments connecting the multiple tarsal and metatarsal bones of the foot that will not be discussed individually, but can be identified in Figure 225.

Muscles and Tendons of the Foot

The foot muscles are divided into the extrinsic and intrinsic groups. The extrinsic muscles are those that originate on the anterior or posterior lower leg, while the intrinsic muscles originate on the dorsal or plantar aspect of the foot. The extrinsic muscles have been discussed in the lower leg and/or ankle, and will be reviewed regarding their insertion on the foot. All of the lower leg muscles attach to the bones of the foot except the popliteus. The anterior group of the lower leg muscles, which are all dorsiflexors of the foot, includes the tibialis anterior, and the extensors digitorum longus and hallucis longus (Figure 227). The tibialis anterior inserts near the first tarsometatarsal joint, and is involved in lifting the foot up during the swing phase of walking to avoid striking the toes. The extensor digitorum longus descends along the lateral aspect of the tibia, then inserts on the second through fifth digits, as well as at the base of the fifth metatarsal. It dorsiflexes the above-referenced digits. The extensor hallucis longus crosses from the medial aspect of the fibula to the great toe, where it dorsiflexes that digit. The lateral muscle group includes the peroneus longus and peroneus brevis. Peroneus longus runs along the fibula, with brevis deep to it. Its tendons pass behind the lateral malleolus, then the peroneus longus crosses the plantar surface to insert on the first tarsometatarsal joint. Peroneus brevis remains on the lateral aspect of the foot, inserting on the proximal fifth metatarsal. Both of the peroneal muscles are strong pronators, and aid in plantarflexion. Both also act as braces for the transverse arch. The posterior lower leg muscles are further divided into superficial and deep muscles. The superficial muscles are plantar flexors of the ankle and foot, and include the soleus, gastrocnemius, and plantaris. The soleus and the two heads of the gastrocnemius combine to form what is called the triceps surae, which lifts the heel, and flexes the knee when walking. The Achilles tendon is formed from the tendons of the soleus and gastrocnemius muscles and inserts on the calcaneus. The deep posterior muscles that are involved with the foot include the tibialis posterior and the flexors- digitorum and hallucis longus. The tibialis posterior is a plantar flexor of the ankle joint, and inserts on the plantar surface of the tarsal bones. The flexor digitorum longus runs down the posteromedial aspect of the leg, sending branches to insert on the plantar aspect of the second through fifth digits to flex them. The flexor hallucis longus descends through the posterior leg, moving from lateral to medial, to insert on the plantar surface of the great toe where it is a flexor. Both of the toe flexors pass through the flexor retinaculum, and both are also plantarflexors of the ankle.

The intrinsic muscles of the foot are divided into dorsal and plantar groups. The intrinsic dorsal muscles include two small extensor muscles- the extensor digitorum brevis and the extensor hallucis brevis. Ext. hallucis brevis inserts on the proximal phalanx of the great toe, while the ext. digitorum brevis muscles attach to the middle phalanges of the second through fourth digits. The majority of the extensor function of the toes is carried out by the extrinsic extensor muscles (extensor halluces and digitorum longus). The intrinsic plantar muscles are divided into four layers, moving from deep to superficial (Figure 228). The fourth or deepest layer consists of three plantar interossei muscles, and four dorsal interossei muscles. The plantar interossei muscles are wedged between the third through fifth metatarsals, where they act as adductors for the third through fifth toes. They also flex the third through fifth metatarsophalangeal joints, and contribute to extension of the interphalangeal joints of the third through fifth toes. The four dorsal interossei muscles abduct the third through fifth toes, and facilitate other actions of the plantar interossei muscles. The third intrinsic layer includes the flexor hallucis brevis and adductor halluces, which both act on the great toe, and the flexor digiti minimi brevis, which acts on the fifth or little toe. The quadratus plantae and the four lumbrical muscles make up the second layer of the intrinsic plantar muscles. The quadratus plantae inserts in the lateral border of the flexor digitorum longus, and assists that muscle with toe flexion. The four lumbricals arise from the individual tendons of the flexor digitorum longus, and insert into the medial aspect of the extensor expansion. The lumbricals flex the metatarsophalangeal joints, and extend the interphalangeal joints of the second through fifth toes via the extensor expansion. The superficial or first layer of intrinsic plantar muscle includes the abductor halluces and abductor digiti minimi, which abduct the great toe and fifth toe, respectively, and the flexor digitorum brevis, which flexes the second through fifth digits. The intrinsic plantar muscles are covered by the thickened deep fascia of the sole of the foot, called the plantar aponeurosis, which extends from the calcaneus to the fibrous sheath of the flexor tendons.

Each toe also has an extensor expansion, which is a triangular aponeurotic sheath (meaning it joins muscles or connects muscle to bone) with a hood-like appearance (Figure 229). The expansions function as compound tendinous attachments for the extensor digitorum longus, the lumbricals, and the plantar and dorsal interossei muscles. A hood-like base covers the metatarsophalangeal joint dorsally, then tapers to the apex of its triangular shape to attach to the base of the distal phalanges (Figure 230).

Nerves of the Foot

The main nerves of the foot are branches of the tibial and common peroneal nerves, which both originate from the sciatic nerve. Compression of the L4-S1 nerve roots affects the sciatic distribution, which in turn can affect innervation of the feet. Significant compression can lead to specific leg or foot muscle weakness and sensory loss. Radiculopathy at the S1 position can result in loss of the Achilles (tendocalcaneus) reflex, or “ankle jerk”. Trauma to the common peroneal nerve can result in loss of the ankle and toe extensors, or “foot drop”.

The tibial nerve innervates the deep muscles of the posterior compartment of the leg, including the tibialis posterior, and the flexors digitorum and hallucis longus. These muscles plantarflex the ankle, flex the toes, and invert the foot.  The tibial nerve crosses behind the medial malleolus, where it splits into three branches. The smallest branch is the medial calcaneal nerve, which provides sensory innervation to the medial aspect of the heel (Figure 236). The remaining two branches are the medial and lateral plantar nerves, which are considered the two major nerves on the bottom of the foot (Figure 231). The medial plantar nerve is the larger of the two, providing innervation to the flexor digitorum longus, flexor hallucis brevis, adductor hallucis, and first lumbrical muscles. It also provides sensory innervation to the medial two-thirds of the bottom of the foot, as well as the plantar aspects of the great, second, third, and half of the fourth toes (Figure 232). The lateral plantar nerve innervates all of the plantar and dorsal interossei muscles (deep layer), the flexor digiti minimi brevis (third layer), the quadratus plantae and the second through fourth lumbricals of the second layer, and the superficial abductor digiti minimi muscle. The lateral plantar also provides sensory innervation to the lateral one-third of the bottom of the foot, to the fifth toe, and to half of the fourth toe. The medial and lateral plantar nerves are joined at the third interspace of the foot by a branch from each. This is a primary location for a Morton’s neuroma, a benign neuroma most commonly found between the third and fourth metatarsal heads. More accurately, this is not a tumor, but a mass of scar tissue that develops around a pinched nerve due to chronic nerve irritation; in addition, Morton was not the first person to describe it. Regardless of its misnomer, this condition is characterized by pain and/or numbness, or the feeling of “having a pebble in your shoe”. It typically affects the nerve supplies to the adjacent aspects of the third and fourth toes. MRI may be used to aid in the diagnosis of this condition, as well as for conditions that may be clinically confused with a neuroma, such as synovitis/capsulitis, stress fractures, and plantar plate disruption (Figures 233, 234).

The common peroneal (or fibular) nerve splits into deep and superficial branches (Figure 235). The deep peroneal nerve innervates the muscles of the anterior compartment of the leg, which includes the tibialis anterior, the extensors digitorum and hallucis longus, and the peroneus tertius. Most are involved in dorsiflexion of the foot, and the digitorum and halluces longus are toe extensors. The deep peroneal divides into medial and lateral branches at the anterior aspect of the ankle. The medial branch runs down the foot and gives sensation to the space between the great toe and the second toe. The lateral branch innervates the dorsal intrinsic extensor digitorum brevis muscle, further dividing into branches to the second through fourth deep interosseous muscles. The superficial peroneal nerve innervates the lateral compartment of the leg, which includes the peroneus longus and brevis muscles. These muscles evert the foot, and are involved in plantar flexion. The superficial peroneal also gives sensation to most of the skin over the dorsum of the foot, except the first web space, which receives sensation from the deep peroneal.

Additional sensory nerves of the foot include the sural and saphenous nerves (Figures 236, 237). The sural nerve crosses behind the lateral malleolus, and undergoes a name change in the foot. It is referred to as the lateral dorsal cutaneous nerve, and provides sensation for the lateral edge of the foot, including the small toe. The saphenous nerve crosses behind the medial malleolus, and provides sensation for the medial edge of the foot.

Arteries and Veins of the Foot

 The major arteries of the foot and toes are branches of the anterior and posterior tibial arteries (Figure 238). The dorsalis pedis, or dorsal artery of the foot, is a continuation of the anterior tibial artery. It branches into the lateral and medial tarsal arteries, the arcuate artery, and the deep plantar artery, with distribution to the foot and toes. The lateral tarsal artery distributes blood to the tarsal bones. The medial tarsal arteries distribute blood to the sole of the foot. The arcuate artery branches into the deep plantar artery and the dorsal metatarsal arteries. The deep plantar supplies the sole of the foot, and helps to form the plantar arch of the foot. The dorsal metatarsal arteries branch into the dorsal digital arteries, and supply the dorsum of the foot, including the toes. The dorsal digital arteries distribute blood to the dorsum of the toes.

The posterior tibial artery branches into the medial and lateral plantar arteries, with blood distribution to the foot. The medial plantar artery has deep and superficial branches that distribute blood to the sole of the foot and the toes. The lateral plantar artery branches into the plantar arch and the plantar metatarsal arteries, with blood distribution also to the sole of the foot and the toes. The plantar arch provides the blood supply to the sole of the foot, stretching across the foot from the first metatarsal to the fifth metatarsal. The arch is formed by the lateral and deep plantar arteries (branch of dorsalis pedis). The lateral plantar turns in a medial direction, and unites with the deep plantar artery at the interval between the bases of the first and second metatarsal bones. The plantar metatarsal arteries, which originate from the lateral plantar artery at the arch, give off perforating branches, as well as the common plantar digital arteries. The common plantar digitals branch into the proper plantar digitals, with both distributing blood to the toes.

The veins in the foot, as in other parts of the body, are classified as deep or superficial. The deep veins typically run with the arteries of the same name, while the superficial veins do not have companion arteries. Both deep and superficial veins have valves, with the deep veins typically possessing more valves, especially in the lower limbs. Since venous flow in the lower limbs is from inferior to superior, a discussion of the deep veins of the foot begins with the plantar digital veins. They arise from various plexuses on the plantar surface of the digits. They send intercapitular veins to join the dorsal digital veins, then unite to form the four metatarsal veins (Figure 239). The metatarsal veins use perforating veins to communicate with the veins on the dorsum of the foot, and unite to form the deep plantar venous arch. This arch lies alongside the plantar arterial arch. The medial and lateral plantar veins take off from the deep plantar venous arch, staying close to their corresponding arteries. The plantars act as blood reservoirs of the plantar venous pump, which helps to move venous blood up the legs against gravity. Normal weight-bearing and muscular contractions also contribute to the pumping actions of veins in the foot. Both of the plantar veins communicate with the great and small saphenous veins, then unite behind the medial malleolus to form the posterior tibial vein. The posterior tibial vein drains the medial aspect of the foot, while the anterior tibial vein drains the dorsal aspect. The anterior tibial vein is the upward continuation of the venae comitantes (“accompanying veins”) of the dorsalis pedis artery. These are paired veins that lay in close proximity on either side of an artery, so the pulsations of the artery can be used to aid venous return. Venae comitantes are typically found with smaller arteries, especially in the extremities.

The superficial veins of the foot include the great and small saphenous veins and their tributaries (Figure 240). On the plantar aspect of the foot, superficial veins form the plantar cutaneous venous arch across the roots of the toes. This arch opens at the medial and lateral aspects of the foot into the marginal veins. Proximal to the arch is the plantar cutaneous venous network, which is especially dense in the fat of the heel. This venous network communicates with the cutaneous venous arch and the deep veins, but the majority of its drainage is into the medial and lateral marginal veins. On the dorsum of the foot, the plantar cutaneous venous arch sends intercapitular veins (in the clefts between the toes) to the dorsal digital veins. The dorsal digitals join to form the common digital veins. The common digitals unite across the distal ends of the metatarsals to form the dorsal venous arch. As in the plantar aspect of the foot, proximal to the dorsal venous arch is an irregular venous network. This network receives tributaries from the deep veins, and is joined at the sides of the foot by the medial and lateral marginal veins. The marginal veins unite the superficial tributaries from the plantar and dorsal aspects of the foot. The medial marginal vein continues as the great saphenous vein just anterior to the medial malleolus. The great saphenous is the longest vein in the body, emptying into the proximal femoral vein. The lateral marginal vein continues as the small saphenous vein, beginning just behind the lateral malleolus. Venous flow from the toes to the common iliac vein can be followed in the diagram in Figure 241.

Open MRI Systems

Introduction

When positioning a patient for a foot exam in an open MRI system, it is important to consider the choices you have as far as coil selection and patient setup. Positioning that will result in both the coil and the anatomy at isocenter in all three planes will have a positive impact on the image quality of your study.

Components of positioning to consider include coil selection, table pads and accessory pads, and proper laser light centering. Coil selection will be influenced by the patient’s size and body habitus. The extensive inventory of table and accessory pads should be used for proper centering, as well as patient comfort and stability (Figure 242). The use of trough pads and thick and/or thin table pads are keys to achieving coronal centering of both the patient and the coil. When scanning on open MRI systems, it is extremely important to center the anatomy of interest in the laser lights (Figure 243) in all three directions: head-to-foot (axial or transverse plane), right-to-left (sagittal plane), and anterior-to-posterior (coronal plane).

Coils and Positioning

The coil of choice for a study of the foot depends on the patient position that is required by your radiologist. If the foot does not have to be positioned at a 90° angle, the Extremity coil would be the coil of choice. If the radiologist requires that the foot be positioned at a 90° angle, the Head coil is recommended.

Knee (Extremity) Coil

Place the base of the coil on the table. Add trough pads and/or table pads under the base of the coil to help center the coil in the coronal plane (the coronal laser light should be aligned with the top of the coil base). The horizontal center of the coil base should be between the marking on the patient table and the end of the table nearest the magnetic field. This will allow for sufficient table travel while still achieving isocenter positioning. The longitudinal center marking on the coil base should be placed as close as possible to the midline of the table; however, the patient’s size, comfort, and the laterality of the affected foot must be taken into account when determining the longitudinal positioning of the coil. The extremity coil may be offset slightly towards the affected foot. Coil centering in the sagittal plane can be performed as a final positioning step using the open magnet’s lateral table movement capabilities. The plantar surface of the patient’s foot should be positioned on the base portion of the coil, and the affected area of the foot (forefoot, midfoot, or hindfoot) aligned with the horizontal center mark on the coil. The longitudinal midline of the patient’s affected foot should be positioned in the middle of the coil base. The coil pads and accessory pads can be adjusted to help achieve coronal centering of the patient’s foot in the coil base. The upper portion of the coil is then placed on the base and the coil latches are secured to close the coil. Accessory sponges can be added inside the coil to further secure the foot in place (Figure 248). Pads and sponges should be placed under the lower leg for support and comfort. The large knee bolster cushion can be placed under the unaffected leg for patient comfort. It will also serve to remove the unaffected leg from the scan plane, which should decrease wrap-around artifacts (Figure 249). The right and left arrow buttons located on the gantry can be used to move the table from side to side for proper alignment of the coil’s longitudinal center mark with the sagittal laser light. The patient should now be centered in all three planes- centered on the middle of the affected area of the foot in the coronal and axial planes, and centered midline in the sagittal plane.

Another option is to position the patient prone on the table, with the dorsal surface of the affected foot positioned on the base of the coil. This will elongate the foot for easier patient positioning and slice set up. Centering of the foot in all three planes should be performed as directed above. Accessory pads can be added inside the coil to further secure the foot in position. Pads and sponges should be placed under the lower leg for support and comfort, as seen in Figure 250.

Head Coil

The head coil is the coil of choice if the radiologist requires that the foot be positioned at a 90° angle. Fit the base of the coil in the table trough. The longitudinal center mark on the coil will be centered at the midline of the table. The horizontal center mark on the coil should be between the marking on the patient table and the end of the table nearest the magnetic field. This will allow for sufficient table travel while still achieving isocenter positioning. The patient’s affected foot and ankle should be positioned on the base portion of the coil with their foot aligned with the horizontal center mark on the coil. The longitudinal midline of the patient’s affected foot should be positioned in the middle of the coil base (Figure 251). The coil pads and accessory pads can be adjusted to help achieve coronal centering of the patient’s foot in the coil base. The upper portion of the coil is then placed on the base and the coil latches are secured to close the coil. If using the Oasis head coil, the upper portion of the coil slides over the base portion and is secured by a locking lever. Depending on the size of the patient’s foot, the patient’s toes may protrude through the openings in the top of the coil. Accessory sponges can be added inside the coil to further secure the foot in place, especially when placed against the plantar surface of the foot. Pads and sponges should be placed under the lower leg for support and comfort. The large knee bolster cushion can be placed under the unaffected leg for patient comfort. It will also serve to remove the unaffected foot from the scan plane, which should decrease wrap-around artifacts. The patient should now be centered in all three planes- centered on the middle of the foot in the coronal and axial planes, and centered midline in the sagittal plane.

You may be able to perform a bilateral foot exam in the head coil, depending on the patient’s size and body habitus. Place both feet on the base of the head coil, using a coil strap to help hold the feet in place, as seen in Figure 252. The midpoint of the feet should be aligned with the horizontal center marks on the coil. The midline between the patient’s feet should be aligned with the longitudinal center mark on the coil. The coil pads and accessory pads can be adjusted to help achieve coronal centering of the patient’s feet in the coil base. Secure the upper portion of the coil on the base, or slide the upper portion over the base of the coil. Accessory pads and sponges can be added inside the coil to further secure the feet in place. Pads and sponges should be placed under the legs as needed for support and comfort. The patient should now be centered in all three planes- centered on the middle of the ankles in the coronal and axial planes, and centered midline between the feet in the sagittal plane.

Wrist Coil (Optional Coil)

The optional wrist coil is ideal for performing imaging of the toes. Place the base of the coil on the table. Add trough pads and/or table pads under the base of the coil to help center the coil in the coronal plane (the coronal laser light should be aligned with the top of the coil base). The horizontal center of the coil base should be between the marking on the patient table and the end of the table nearest the magnetic field. This will allow for sufficient table travel while still achieving isocenter positioning. The longitudinal center marking on the coil base should be placed as close as possible to the midline of the table; however, the patient’s size, comfort, and the laterality of the affected toes must be taken into account when determining the longitudinal positioning of the coil. The wrist coil may be offset slightly towards the affected foot. Coil centering in the sagittal plane can be performed as a final positioning step using the open magnet’s lateral table movement capabilities. The dorsal surface of the patient’s toes should be positioned on the base portion of the coil, aligned with the horizontal center mark on the coil. The longitudinal midline of the patient’s affected foot should be positioned in the middle of the coil base. The coil pads can be adjusted to help achieve coronal centering of the patient’s toes in the coil base. The upper portion of the coil is then placed on the base and the coil latches are secured to close the coil. Pads and sponges should be placed under the lower leg for support and comfort. The right and left arrow buttons located on the gantry can be used to move the table from side to side for proper alignment of the coil’s longitudinal center mark with the sagittal laser light (Figure 253). The patient should now be centered in all three planes- centered on the middle of the affected toes in the coronal and axial planes, and centered midline in the sagittal plane.

If necessary, the patient can be positioned in a decubitus position, with the wrist coil placed on its side to accommodate the patient’s toes (Figure 254). Table pads and sponges should be added as needed to maintain the coronal centering of the coil, as well as to comfortably maintain the patient’s toes in the center of the coil.

Echelon MRI System

Introduction

The 1.5T Echelon system comes equipped with a variety of trough and table pads, as well as various positioning pads and sponges (Figure 255). These positioning aids can be used to support the position of both the patient and the coil, as well as to keep your patient comfortable and secure. The magnet bore of the Echelon does not permit lateral patient table movement. However, accurate patient and coil positioning, and correct use of the sagittal and axial laser lights (Figure 256) to center the patient’s anatomy ensures that high quality images will be acquired.

Coils and Positioning

The Foot & Ankle coil is the coil of choice for coverage of the entire foot. For imaging of only the forefoot, midfoot or hindfoot, the Knee (Extremity) coil can be used.

Foot & Ankle Coil

Place the foot & ankle coil on top of the table pads. Unlatch the anterior elements for easy patient positioning. Once the patient’s foot is positioned in the coil, latch the anterior elements (Figure 259). Use accessory pads and sponges inside the coil to stabilize the foot and ankle. Additional pads can be placed under the lower leg on the affected side for support and patient comfort. The coil’s longitudinal center mark should be aligned as closely as possible with the sagittal laser light.

Knee (Extremity) Coil

The coil should be placed at the gantry end of the table. The bottom portion of the coil should be placed on top of the table pads. The patient is typically positioned feet first on the base portion of the coil, with the center of their foot aligned with the horizontal center marks on the coil. The upper portion of the coil is then placed on the base, and the coil locking mechanism is secured (Figure 260). The longitudinal midline of the patient’s affected foot (and lower leg) should be aligned with the longitudinal center mark on the upper portion of the coil. Accessory sponges can be added inside the coil to further secure the foot in place. Pads and sponges should be placed under the lower leg on the affected side for support and comfort. A positioning pad or sponge placed under the unaffected lower leg may increase patient comfort, and also serves to remove the unaffected foot from the scan plane, which should decrease wrap-around artifacts. The coil’s longitudinal center mark should be aligned as closely as possible with the sagittal laser light.

An option for imaging of the forefoot is to position the patient prone on the table, with the dorsal surface of the affected foot positioned on the base of the coil. This will elongate the foot and toes for easier positioning and slice set up. Centering of the foot or toes should be performed as directed above. Accessory pads can be added inside the coil to further secure the foot and toes in position. Pads and sponges should be placed under the lower leg for support and comfort, as seen in Figure 261.

Scan Setups

The following are HMSA suggestions for imaging of the foot. Referrals to long axis and short axis are based on the concept that the foot is 90° from the table surface when the patient is lying supine on the table in true anatomic position. Always check with your radiologist for his/her imaging preferences, especially concerning inclusion of anatomy for forefoot, midfoot, and hindfoot imaging. Parameters will need to be adjusted, taking into account the smaller FOV that will be used for imaging of the toes or inclusion of only a portion of the foot.

Long Axis Scans

When positioning long axis (axial) slices for the foot, sagittal and coronal images can be used to insure inclusion of all pertinent anatomy. The slices should extend from the phalanges to at least the navicular tarsal bone, with additional anatomy included per the radiologist’s preference (Figure 262). The angle of the slice group is typically parallel to the second or third metatarsal.

Short Axis Scans

Short axis (coronal) slices of the foot typically cover the entire foot, depending on your radiologist’s preferences. The slices should be angled so that they are perpendicular to the first and fifth metatarsal shafts, as in Figure 263.

Sagittal Scans

Sagittal slices of the foot should include all of the bony anatomy of the foot. The slices should be positioned so that they are perpendicular to the short axis (coronal) slice group, as in Figure 264.

This concludes the Lower Extremity module of the Hitachi Medical Systems America’s MRI Anatomy and Positioning Series. You must complete the post-test for this activity in order to receive your continuing education credit.

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