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Vol 9, No 3, May/June 2001 187 Magnetic resonance (MR) imaging of the foot and ankle has lagged behind MR imaging of other joints in clinical acceptance and utility, because of the complex anatomy of the foot and ankle and the need for small-field-of-view, high-resolution images. Recent advances in both hardware and software, however, have made possible the acquisition of high-resolution images. This feature, combined with the degree of soft-tissue contrast that can be achieved with MR imaging and the ability to obtain images in multiple planes, has led to the increasing importance of this modality in imaging of the foot and ankle for both diagnosis and surgical planning. It is important that physi- cians understand the common clinical MR imaging techniques and their role in evaluating disorders of the foot and ankle to most effectively utilize this diagnostic modality. Technique Because of the complex anatomy of the foot and ankle and the small size of many of its structures, the acquisition of high-resolution images is necessary. To obtain such images, the foot and ankle should be imaged separately by using a surface coil. When a comparison view of the unaffected foot is needed to assess tendon or ligament sym- metry, this should be accomplished by scanning each foot separately in a surface coil. Attempting to scan both feet together in a body coil or a head coil saves time but at the cost of having to use a large field of view, which results in low-resolution images that are often nondiagnostic. The optimal field of view of the images should be no larger than 16 cm 2 , and the matrix should range from 192 x 256 to 256 x 512. Section thickness depends on the pulse sequence used but should be 3 to 4 mm when using spin-echo (SE) sequences and 1 to 2 mm when using gradient-echo (GRE) sequences. For the purpose of MR imaging, the foot and ankle can be divided into three zones: the ankle and hindfoot, the midfoot, and the forefoot. 1 The midfoot is adequately examined by imaging both the hindfoot and the forefoot; therefore, examination protocols can be further simplified into two zones: ankle-hindfoot and forefoot. To image the hindfoot and ankle, the patient is placed in a supine position with the medial malleolus centered in the coil. The foot is allowed to rest in a relaxed position, generally in 10 to 20 degrees of plantar-flexion and 10 to 30 degrees of external rotation. The foot position may need to be altered when imaging specific ligaments, such as the calcaneofibular ligament. For forefoot examinations, the patient can be either supine or prone with Dr. Recht is Section Head, Outside Imaging, Department of Diagnostic Radiology, Cleve- land Clinic Foundation, Cleveland, Ohio. Dr. Donley is Staff Surgeon, Department of Ortho- paedic Surgery, Cleveland Clinic Foundation. Reprint requests: Dr. Recht, Department of Diagnostic Radiology, Cleveland Clinic Foundation, A21, 9500 Euclid Avenue, Cleveland, OH 44195. Copyright 2001 by the American Academy of Orthopaedic Surgeons. Abstract Magnetic resonance (MR) imaging of the foot and ankle is playing an increasingly important role in the diagnosis of a wide range of foot and ankle abnormalities, as well as in planning for their surgical treatment. For an optimal MR study of the foot and ankle, it is necessary to obtain high-resolution, small-field- of-view images using a variety of pulse sequences. The most common indication for MR imaging of the foot and ankle is for the evaluation of tendon and bone abnormalities, such as osteomyelitis, occult fractures, and partial and complete tears of the Achilles, tibialis posterior, and peroneal tendons. Magnetic resonance imaging has also been shown to be helpful in the diagnosis of several soft- tissue abnormalities that are unique to the foot and ankle, such as plantar fasciitis, plantar fibromatosis, interdigital neuromas, and tarsal tunnel syndrome. J Am Acad Orthop Surg 2001;9:187-199 Magnetic Resonance Imaging of the Foot and Ankle Michael P. Recht, MD, and Brian G. Donley, MD the toes centered in the coil. It is important to image in all three planes (transaxial, sagittal, and coronal) for all indications. However, for tendon and ligament disorders as well as for soft-tissue masses, the transaxial plane of imaging is the most useful; most sequences should be acquired in this plane. For bone abnormalities, particularly those of the talar dome, the sagittal and coronal planes provide the greatest amount of information. A variety of pulse sequences can be utilized in the examination of the foot and ankle. T1-weighted (short repetition time [TR]/short echo time [TE]) SE images provide excellent anatomic detail and information about the integrity of the bone marrow. T2-weighted (long TR/long TE) SE images allow detection of the increased water content seen with most pathologic processes as abnormal high signal intensity. Fast SE T2-weighted sequences have largely replaced conventional SE T2-weighted sequences because of the ability to obtain images in a shorter time period with higher resolution. Gradient- echo sequences allow the acquisition of thin contiguous sections that can be reformatted in multiple planes. These sequences have been shown to be useful in the detection of cartilage abnormalities. 2,3 Short-tau inversion recovery (STIR) imaging is a method of fat suppression that has proved very sensitive in detecting marrow abnormalities, as well as increased water content in soft tissues. 4 Cur- rently, most STIR sequences also use a variant of the fast SE technique, which allows the images to be acquired in a shorter period of time. Another method of fat suppression is the use of chemical-selective fat suppression, which takes advantage of the differences in resonance frequency between fat and water protons. When evaluating a soft-tissue or osseous mass, T1-weighted chemical- selective fat-suppressed imaging after injection of intravenous contrast material (e.g., gadolinium– diethylenetriaminepenta-acetic acid [DTPA]) improves the conspicuity of the mass and facilitates the differentiation of a solid soft-tissue mass from a fluid-filled cyst. 4 Magnetic resonance arthrography may play a role in the detection of ligament abnormalities and intra-articular bodies and in the staging of osteochondral defects. 5,6 The foot and ankle can be imaged with high-field-strength (>0.5- T) or low-field-strength (≤0.5-T) magnets. A high-field-strength MR Imaging of the Foot and Ankle Journal of the American Academy of Orthopaedic Surgeons 188 Definitions of Radiologic Terms Chemical-selective The use of chemically selective radio-frequency fat suppression pulses to eliminate fat signal by taking advantage of the difference in resonance frequency between fat and water protons Echo time (TE) The time between the middle of the excitation pulse and the middle of the spin echo Gradient-echo (GRE) Describing a sequence in which an echo is produced by a single radio-frequency pulse followed by a gradient reversal Proton-density- An image acquired with a long TR (e.g., 2,000-3,000 weighted image msec) and a short TE (e.g., 20 msec) to emphasize differences in proton density and minimize T1 and T2 differences between tissues Repetition time (TR) The time between successive excitations of a section Short-tau inversion Describing a sequence that suppresses fat signal by recovery (STIR) the use of a 180-degree inversion pulse and a short inversion time Spin-echo (SE) Describing a sequence in which an echo is produced by a 90-degree radio-frequency pulse followed by one or more 180-degree radio-frequency pulses T1 Spin-lattice or longitudinal relaxation time. The time constant for magnetization to return to the longitudinal axis after application of a radio- frequency pulse. T1-weighted image An image acquired with a short TR (e.g., 400-600 msec) and a short TE (e.g., 10-20 msec) to emphasize differences in T1 relaxation rates between tissues. Fat is of high signal intensity, and fluid is of low signal intensity on T1-weighted sequences. T2 Spin-spin relaxation time. The time constant for loss of phase coherence of a group of spins and the resulting loss in the transverse magnetization signal. T2-weighted image An image acquired with a long TR (e.g., 2,000-3,000 msec) and long TE (e.g., 80-100 msec) to emphasize differences in T2 between tissues. Fluid is bright on T2-weighted sequences. magnet, with its higher signal-to- noise ratio, allows the acquisition of high-resolution images in a shorter time period, thus decreasing the potential for patient motion. In addition, chemical-selective fat suppression is not available with low- field-strength magnets. Tendons Ten tendons cross the ankle joint on their path from the lower leg into the foot (Fig. 1): the Achilles tendon posteriorly; the peroneus brevis and longus laterally; the tibialis posterior, flexor digitorum longus, and flexor hallucis longus medially; and the tibialis anterior, extensor digitorum longus, extensor hallucis longus, and peroneus tertius anteri- orly. Tendons are composed primarily of collagen, elastin, and reticulin fibers. Normal tendons appear as homogeneous low signal intensity on MR imaging because of their lack of mobile protons. 7 However, on T1-weighted and proton-density- weighted (long TR/short TE) images, normal tendons can have intermediate signal intensity because of the “magic angle effect.” 8 This effect occurs with short-TE sequences because the signal intensity of structures with poorly hydrated protons, such as tendons, depends in part on the orientation of the structure in relation to the main magnetic field. When a structure is oriented obliquely in relation to the main magnetic field, its signal intensity is increased on short-TE sequences; this increase is greatest when the structure is oriented at 55 degrees in relation to the main magnetic field. The increase in signal intensity is not seen on long-TE (T2-weighted) sequences. The magic-angle effect is noted in most of the tendons of the foot and ankle as they curve across the ankle to pass into the foot. How- ever, this normal increase in signal intensity can usually be differentiated from pathologic changes within the tendon if one is aware of the characteristic location of the magic- angle effect, the lack of high signal intensity on T2-weighted images, and the normal morphology of intact tendons. The transaxial plane is the most useful for evaluating the integrity of tendons. To obtain a true transaxial image of the tendons that traverse the ankle to pass into the foot, it is necessary to select a plane perpen- dicular to the long axis of the tendons as they curve about the ankle (Fig. 2). A useful protocol for tendon pathology includes transaxial T1- weighted SE and STIR images or fat- suppressed fast SE T2-weighted images (obtained with the same section thickness and positions to allow comparison), coronal T1-weighted, and sagittal fat-suppressed fast SE T2-weighted sequences. A sagittal T1-weighted sequence is added when examining the Achilles tendon. Pathologic changes that can be seen in and about tendons on MR imaging include tenosynovitis, tendinopathy, and tendon tears. Tenosynovitis is best visualized on T2-weighted images as high-signal- intensity fluid surrounding a normal- appearing tendon (Fig. 3). “Ten- dinopathy” or “tendinosis” is the term currently favored to describe tendons that are abnormal but not torn. Although some authors still use the term “tendinitis,” studies have not shown a true inflammatory process in tendons. 9,10 Rather, histo- logic studies have demonstrated hyperplasia, fibrosis, and vacuolar, mucoid, eosinophilic, and fibrillary degeneration. On MR imaging, tendinopathy is characterized by altered tendon morphology, usually thickening. There may also be areas of mildly increased signal intensity on short TE (T1-weighted or proton-density-weighted) sequences, but this signal intensity usually decreases on T2-weighted images unless severe tendinopathy is present. Three MR patterns of tendon rupture have been described. 11 Michael P. Recht, MD, and Brian G. Donley, MD Vol 9, No 3, May/June 2001 189 Figure 1 Transaxial T1-weighted image at the level of the distal tibia (T) and fibula (F). Note the homogeneous low signal intensity of the tendons. A = Achilles tendon; ED = extensor digitorum longus; EH = extensor hallucis longus; FD = flexor digitorum longus; FH = flexor hallucis longus; PB = peroneus brevis; PL = peroneus longus; TA = tibialis anterior; TP = tibialis posterior. TA T F TP FD A FH PL PB EH ED Figure 2 True transaxial images of the peroneal tendons. The sections are graphi- cally prescribed off a sagittal image as the tendons curve around the ankle. Type 1 is characterized by hypertro- phy of the tendon with partial tears oriented primarily longitudinally. The tendon is enlarged (Fig. 4, A), with foci of increased signal intensity on T2-weighted and STIR images (Fig. 4, B). Type 2 is characterized by a partially torn atrophic tendon; the appearance is of a small tendon with foci of increased signal intensity on T2-weighted or STIR images (Fig. 4, C). Type 3 tears are complete tendon ruptures (Fig. 4, D). Although all of the tendons of the foot and ankle can be studied with MR imaging, the tendons most often associated with injury or dis- ease are the Achilles, tibialis posterior, and peroneal tendons. Achilles Tendon The Achilles tendon is the largest and strongest tendon in the body, ranging in length from 10 to 15 cm. The Achilles tendon does not pos- sess a synovial sheath but rather is invested by loose connective tissue (peritenon). The normal Achilles tendon appears as homogeneously low signal intensity on all pulse sequences 1 (Fig. 5, A and B). It has a flat or concave anterior margin on transaxial images, giving it a crescentic shape. At its insertion onto the calcaneus, the tendon becomes more ovoid, with a flattened anterior margin. Acute peritendinitis is manifested by loss of the sharp interface between the tendon and the pre- Achilles fat, with high signal intensity on T2-weighted and STIR images about the tendon, but with preservation of the low signal intensity of the tendon itself. Chronic Achilles tendinopathy has the appearance of a thickened enlarged tendon (Fig. 5, C and D). There may be increased signal intensity within the tendon on T1-weighted and proton-density-weighted images, but the signal usually decreases in intensity on T2-weighted and STIR images. Although mea- surements of the thickness of the Achilles tendon have been pub- lished (normal, <8 mm), 1 careful assessment of the anterior margin of the tendon on transaxial views may be more useful. Loss of the normal concave margin of the anterior aspect of the tendon is a sign that the tendon is thickened and abnormal. MR Imaging of the Foot and Ankle Journal of the American Academy of Orthopaedic Surgeons 190 Figure 3 Tenosynovitis of the flexor hallucis tendon. Note the high-signal-intensity fluid (arrows) surrounding the normal low-signal-intensity tendon and the metal- lic artifact about the tibia secondary to pre- vious hardware placement. Figure 4 Patterns of tendon rupture. A, Type 1 tear of the tibialis posterior tendon. Transaxial T1-weighted image at the level of the sustentaculum tali demonstrates an enlarged, irregularly shaped tibialis posterior tendon (arrow). B, Transaxial STIR image of the same ankle demonstrates high signal intensity within the enlarged tibialis posterior tendon (arrows). C, Type 2 tear of the tibialis posterior tendon. Transaxial T1-weighted image demonstrates a small atrophic tibialis posterior tendon (white arrows) approximately half the diameter of the flexor digitorum tendon (black arrow). D, Type 3 tear of the Achilles tendon. Sagittal T2-weighted fast SE image demonstrates discontinuity of the Achilles tendon. There is retraction of the torn edge of the tendon (arrows) and high signal intensity in the gap between the tendon edge and the calcaneus. A B C D Ruptures of the Achilles tendon most frequently occur 3 to 4 cm above its insertion onto the calcaneus. 7 The MR findings of a partial rupture include focal areas of high signal intensity on T2-weighted and STIR images within the tendon substance but with preservation of some tendon continuity 4 (Fig. 5, E and F). Acute complete ruptures demonstrate loss of tendon continuity, with the gap in the tendon appearing as an area of high signal intensity on T2-weighted and STIR images, representing blood or edema 4 (Fig. 4, D). In chronic complete ruptures, the gap may be filled with low-signal-intensity fibrotic tissue. Tibialis Posterior Tendon Tibialis posterior tears are most commonly seen in middle-aged women, who present with an acquired, painful flatfoot; these are generally chronic tears. 11,12 The tibialis posterior tendon is the most medial tendon in the posterior com- partment at the level of the ankle. The tendon continues into the foot, where it inserts onto the navicular, Michael P. Recht, MD, and Brian G. Donley, MD Vol 9, No 3, May/June 2001 191 A C E B D F Figure 5 A and B, Normal Achilles tendon. A, On T1-weighted sagittal image, the normal Achilles tendon is of homogeneous low signal intensity and has sharp interfaces with the surrounding soft tissue. B, Transaxial T1-weighted image at the level of the distal Achilles tendon. The concave anterior surface of the tendon gives a crescentic shape to the distal por- tion (arrowheads). C and D, Chronic tendinopathy of the Achilles tendon. T2- weighted sagittal (C) and T1-weighted transaxial (D) images demonstrate a thickened Achilles tendon (arrows), which is of low signal intensity. Note the loss of the normal concave anterior surface of the tendon on the transaxial image. E and F, Partial tear of the distal Achilles tendon. T1-weighted (E) and T2-weighted (F) sagittal images demonstrate an enlarged, thickened distal Achilles tendon. On the T2- weighted image, there is high signal intensity (arrow) within the distal Achilles tendon, representing a partial tear. medial cuneiform, metatarsal bases, and sustentaculum tali. The normal tibialis posterior tendon has homogeneously low signal intensity ex- cept for its most distal segment, which may demonstrate intermediate signal intensity on T1-weighted sequences. It should be twice the diameter of the flexor digitorum longus and flexor hallucis longus tendons distal to the level of the medial malleolus. 12 Type 1 tears of the posterior tibialis tendon, which are the most common type of tear, are manifested by an enlarged tendon, which may be four to five times the size of the flexor digitorum tendon (Fig. 4, A and B). There is increased signal intensity within the tendon on short- TE images, which often remains high on T2-weighted and STIR images. Type 2 tears present as a smaller than normal tendon, often the same size as or smaller than the flexor digitorum longus (Fig. 4, C). Type 3 tears are complete tendon ruptures. Peroneal Tendons The peroneus longus and brevis tendons occupy a common synovial sheath up to the level of the calca- neocuboid joint, beyond which the sheath bifurcates. At the level of the lateral malleolus, the peroneus brevis tendon is anteromedial or anterior to the peroneus longus tendon. The posterior edge of the fibula is normally concave in this region, forming a groove within which the tendons lie. The tendons are kept within this groove by the superior peroneal retinaculum. On MR imaging, normal peroneal tendons are of similar size and homogeneous low signal intensity. The peroneus longus tendon is ovoid, but the peroneus brevis tendon may have a flattened appearance in the retromalleolar groove. The superior peroneal retinaculum is usually identifiable as a discrete structure. Although complete ruptures of the peroneal tendons are uncommon, longitudinal splits of the peroneus brevis tendon have been increasingly recognized. 13-15 Longi- tudinal splits are thought to be caused by either forced dorsiflexion or repetitive peroneal subluxation, which leads to compression of the peroneal brevis against the posterior aspect of the fibula. Interposition of the peroneus longus between the portions of the split peroneus brevis tendon can occur. On MR imaging, a split peroneus brevis appears either as a C-shaped structure at or below the level of the lateral malleolus, which partially wraps around the peroneus longus tendon, or as a completely bisected tendon (Fig. 6, A). There may or may not be increased signal intensity within the tendon. An osseous ridge at the lateral margin of the fibula has been associated with a split peroneus brevis tendon, and is considered to represent changes secondary to repetitive subluxation of the peroneal tendons. 13 Other MR findings that are associated with, and may predispose to, splitting of the peroneus brevis MR Imaging of the Foot and Ankle Journal of the American Academy of Orthopaedic Surgeons 192 A B C Figure 6 Lesions of the peroneal tendons. A, Transaxial T1-weighted image obtained just distal to the lateral malleolus demonstrates a completely bisected peroneus brevis tendon (arrowheads). T1-weighted transaxial (B) and coronal (C) images show subluxation of the peroneal tendons (arrows) so that they lie lateral to the malleolus, rather than posterior to it. tendon include a flat or convex fibular groove, a ligamentous tear, or the presence of a peroneus quartus muscle or the low-lying belly of the peroneus brevis muscle. 14 Traumatic peroneal subluxation or dislocation is associated with disruption of the superior retinaculum or stripping of the periosteum at its attachment onto the fibula. This is not an uncommon injury in athletes and may be misdiagnosed as an ankle sprain. 7 Traumatic dislocation can also be seen with calcaneal fractures. The abnormally positioned peroneal tendons are easily seen on MR images lying lateral (Fig. 6, B and C), or in extreme cases anterior, to the lateral malleolus. Ankle Ligaments Magnetic resonance imaging can demonstrate both intact (Fig. 7) and abnormal (Fig. 8) ankle ligaments. However, its role in the evaluation of ankle ligament injuries remains uncertain, especially in cases of acute ligament injury, which are most often diagnosed on clinical examination. It may play a limited role in defining which ligaments are injured, the extent of such injury in patients with chronic ankle instability, and the presence of osteochondral injuries of the talar dome in patients with chronic ankle pain after ligament injuries. The ankle ligaments can be grouped into three complexes: the lateral complex, consisting of the anterior talofibular, posterior talofibular, and calcaneofibular ligaments; the deltoid ligament, which has several components; and the syndesmotic complex, composed of the interosseous membrane, the anterior and posterior tibiofibular ligaments, and the transverse tibiofibular ligament. To evaluate these ligaments with MR imaging, it is necessary to image them in a plane parallel to their long axes. This plane varies for the different ligaments, but a cadaveric study of the ankle ligaments demonstrated that particular planes were optimal for studying the various ligaments. 16,17 The transaxial plane with the foot positioned in 10 to 20 degrees of dorsiflexion provides the best visualization of the anterior and posterior talofibular ligaments; the anterior, Michael P. Recht, MD, and Brian G. Donley, MD Vol 9, No 3, May/June 2001 193 A C B D Figure 7 Appearance of normal ankle ligaments. A, The intact anterior talofibular ligament (arrowheads) is of low signal intensity on this T1-weighted transaxial image. Note the elliptical shape of the talus and the presence of the lateral malleolar fossa. B, Intact anterior (arrowheads) and posterior (arrows) tibiofibular ligaments are of uniform low signal intensity. The medial border of the lateral malleolus is flattened, indicating that this is the level of the tibiofibular ligaments. C, Intact tibiotalar component of the deltoid (arrowheads). Note the osteochondral defect of the lateral talar dome. D, Posterior talofibular ligaments (arrowheads) on T1-weighted coronal image. The deltoid and posterior talofibular ligaments have a striated appearance, rather than a homogeneous low-signal- intensity appearance like the anterior talofibular ligament. transverse, and posterior tibiofibular ligaments; and the various components of the deltoid ligament. Coronal images allow visualization of the full length of most of the components of the deltoid ligament. The calcaneofibular ligament is best seen on transaxial images with the foot in 40 to 50 degrees of plantar-flexion. It is difficult to image the foot in all of these positions in a reasonable time period. Most MR examinations of the foot and ankle are done in 10 to 20 degrees of plantar-flexion, which is not ideal for imaging any of the ankle ligaments. Therefore, it is important to clearly communicate in advance which specific ligamentous complexes need to be imaged. The MR sequences used to evaluate the ankle ligaments include T1-weighted SE, T2-weighted fast SE, and STIR sequences. In cases of chronic ankle instability, MR arthrography may also be useful. Normal ligaments are thin and of low signal intensity on all MR pulse sequences. Occasionally, they have a striated appearance, especially the deltoid and posterior talofibular and tibiofibular ligaments. 18 Be- cause of the oblique course of the tibiofibular ligaments, the talus may be seen on images demonstrating their fibular attachments. This can lead to misidentification of the tibiofibular ligaments as the talofibular ligaments. The best way to avoid this mistake is to identify the insertion of the ligaments. The shape of the talus and the fibula in the transaxial plane can also be used to correctly identify the two sets of ligaments. 8 At the level of the tibiofibular ligaments, the talus is rectan- gular, and the medial border of the fibula is flattened. At the level of the talofibular ligaments, the talus is more elongated, the sinus tarsi is usually visible, and there is a deep indentation along the medial border of the lateral malleolus (the malleolar fossa). The MR findings in ligament injuries include complete tear of the ligament, ligament waviness or lax- ity, thickening or irregularity of the ligament, increased signal intensity within the ligaments, edema and hemorrhage about the ligament, and abnormal increased fluid within the joint and surrounding tendons. 7,17 In cases of chronic instability, some of the ancillary findings of ligament injury, such as edema and hemorrhage and joint effusions, may not be present. Magnetic resonance arthrography has been shown to be more sensitive and more accurate than conventional MR imaging in this situation. 5 This is because the torn, scarred ligament is closely applied to the bone and is better visualized when separated from the bone by the intra-articular injection of contrast material (Gd- DTPA). In addition, contrast extra- vasation through the torn ligament into the surrounding soft tissues serves as convincing evidence of disruption of the ligament. Bones Infection Infection of the bones of the foot and ankle occurs most commonly in diabetic patients, usually due to direct extension of soft-tissue infection. Magnetic resonance images, especially STIR and fat-suppressed T1-weighted images acquired after intravenous contrast administration effectively depict the bone marrow changes that occur with osteomyelitis. 19 However, these changes are not specific for osteomyelitis. The differentiation of bone marrow changes due to infection from those due to edema or neuropathy has proved challenging. Although neuropathic tissue may be visualized as low signal intensity on all pulse sequences, 20 it may have a high-signal-intensity appearance on T2-weighted and STIR sequences. 7 Enhancement after intravenous contrast administration is also not specific, as it can be seen in the presence of any process resulting in increased vascular permeabili- ty. Findings useful in the diagnosis of osteomyelitis include soft-tissue changes that extend to the skin surface adjacent to bone-marrow changes, cortical disruption, and periosteal abnormalities (Fig. 9). 4,7 Nonetheless, it is still often difficult to reliably differentiate neuropathy from infection on MR imaging. A useful protocol for the evaluation of osteomyelitis includes T1-weighted SE, STIR, and fat-suppressed T1- weighted SE images obtained after contrast administration. The images are acquired in at least two planes, which are determined on the basis of the site of the suspected infection. Fractures Magnetic resonance imaging has little role to play in the evaluation of acute traumatic bone injuries, as they are usually easily diagnosed on conventional radiography. How- ever, MR imaging may be useful in MR Imaging of the Foot and Ankle Journal of the American Academy of Orthopaedic Surgeons 194 Figure 8 Chronic tear of the anterior talofibular ligament. This transaxial T2- weighted image demonstrates the absence of the anterior talofibular ligament, with high-signal-intensity fluid (arrows) filling the expected location of the ligament. identifying bone contusions, occult nondisplaced fractures, and stress fractures and stress reactions in the foot and ankle. Metatarsal stress fractures can generally be diagnosed without MR imaging. In contrast, stress fractures of other tarsal bones, such as the navicular, cunei- forms, and calcaneus, often present as foot pain of unknown etiology. If conventional radiographs appear normal, MR imaging is a valuable modality, as it can depict pathologic changes in both bone and soft tissue. Stress fractures are diagnosed most readily on STIR and T1-weighted SE images but can also be seen on T2- weighted images (Fig. 10). On T1- weighted images, stress fractures appear as a linear area of low signal intensity compared with normal bone marrow surrounded by a more diffuse area of slightly higher, though still low, signal intensity. 20-22 On STIR images, the linear component remains of low signal intensity, but the surrounding area is of high signal intensity, consistent with bone marrow edema. In addition to stress fractures, MR imaging is also able to depict stress responses, which represent early changes in bone before the develop- ment of a fracture. 22 Stress responses are characterized by globular areas of low signal intensity on T1-weighted images, which increase in signal intensity on STIR sequences. They can be differentiated from fractures by the lack of a linear component. Stress responses appear similar to bone bruises but can be differentiated from them by the lack of an antecedent acute traumatic event. Osteochondral Injuries of the Talar Dome Osteochondral injuries of the talar dome occur most commonly in the second to fourth decades of life and affect both the medial and lateral aspects of the dome. 23 Most lesions are apparent on conventional radiographs; however, MR imaging can depict lesions too small to be seen on plain films and may be useful in evaluating the extent of the lesion and the stability of the fragment. 23 Increased signal intensity separating the lesion from the underlying bone on T2-weighted or STIR images is the most frequent MR sign of instability, but this appearance has also been reported in stable lesions. 24 Other less frequently seen signs of instability are cartilage fractures, focal cartilage defects, and underlying cysts. 24 Magnetic resonance arthrography has been shown to be more accurate in evaluating the stability of the fragment than conventional MR imaging in osteochondral lesions of the knee, 6 because of the ability to see contrast material tra- versing the overlying cartilage defect and encircling the loose fragment. More recently, cartilage-specific sequences, such as fat-suppressed T1-weighted GRE sequences, have been used to evaluate the overlying cartilage (Fig. 11). Although MR imaging is highly accurate for evaluating the cartilage in the knee, 2,3 Michael P. Recht, MD, and Brian G. Donley, MD Vol 9, No 3, May/June 2001 195 A B Figure 9 Osteomyelitis of the calcaneus. A, Sagittal T1-weighted image demonstrates a soft-tissue ulcer (white arrow) on the plantar surface of the foot adjacent to the area of abnormal low signal intensity within the calcaneus (black arrows). B, Sagittal fat- suppressed T1-weighted image shows enhancement (arrows) of the calcaneus and adjacent soft tissues, as well as subtle cortical disruption (lower arrow). Figure 10 Stress fracture of the navicular. Coronal T2-weighted image demonstrates high signal intensity within the navicular surrounding a thin linear area of low signal intensity (arrowheads), which represents the fracture line. evaluation of talar cartilage has been more difficult, even with both sagittal and coronal images, because the talar cartilage is considerably thinner. Bone Tumors Although MR imaging is very sensitive in the detection of bone tumors, it frequently lacks specific- ity. However, MR imaging can be useful in evaluating the extent of the tumor and the presence of an associated soft-tissue mass. The imaging is done in all three planes with a combination of T1-weighted SE, STIR, and T2-weighted fast SE sequences. Occasionally, a fat- suppressed T1-weighted sequence is used after administration of intravenous contrast material, as it increases the conspicuity of the bone and soft-tissue abnormalities and improves the differentiation of necrotic tissue from viable tumor. 25 In addition, some researchers have suggested that dynamic contrast- enhanced MR imaging may be useful in assessing the response of osteosarcoma and Ewing’s sarcoma to chemotherapy. 25,26 Associated Soft-Tissue Conditions Plantar Fasciitis The deep plantar fascia, or plantar aponeurosis, is a multilayered fibrous structure, subcutaneous in location, which extends as a thick, strong, dense tissue from the calcaneus posteriorly to the region of the metatarsal heads and beyond. Plantar fasciitis is one of the causes of painful heel syndrome and may be secondary to mechanical, degen- erative, or systemic conditions. The plantar fascia is best visualized on sagittal and coronal images and normally should be 3 to 4 mm thick and of homogeneously low signal intensity on all pulse sequences. 27 In patients with plantar fasciitis, the plantar fascia is thickened (7 to 8 mm thick) and demonstrates areas of increased signal intensity on T2-weighted and STIR sequences 4,27 (Fig. 12). There frequently is also abnormally increased signal intensity in the adjacent subcutaneous tissue. Increased signal intensity may also be seen in the calcaneus at the insertion site of the plantar fascia, presumably secondary to reactive edema. Plantar fasciitis is a clinical diagnosis and rarely, if ever, warrants MR imaging. However, in patients with a painful heel syndrome, it can occasionally be helpful in excluding other etiologic possibilities, such as calcaneal stress fractures and tarsal tunnel masses. Plantar Fibromatosis Plantar fibromatosis is characterized by fibrous proliferation in the plantar fascia. 4 An association between plantar fibromatosis and other conditions associated with proliferation of fibrous tissue, such MR Imaging of the Foot and Ankle Journal of the American Academy of Orthopaedic Surgeons 196 A B Figure 11 Osteochondral injury of the talar dome. A, T1-weighted coronal image demonstrates deformity of the talar dome, with a focal area of low signal intensity within the bone marrow of the talar dome (arrows). B, Fat-suppressed three-dimensional GRE coronal image depicts articular cartilage as high signal intensity. There is disruption of the articular cartilage (arrows) overlying the signal abnormality within the talar dome. A B Figure 12 Plantar fasciitis. A, T1-weighted sagittal image shows thickened deep plantar fascia at its insertion onto the calcaneus (arrow). There is also abnormal intermediate signal intensity within the deep plantar fascia. B, Sagittal STIR image demonstrates abnormal high signal intensity about the deep plantar fascia (arrow).

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