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■Specialized Pulse Sequences and Imaging Protocols

Indiscriminate use of MRI can confuse the clinical situation by identifying findings that are not necessarily pathologic or that are poorly correlated with clinical symptoms; these findings in turn can lead to unnecessary and costly interventions and can delay proper diagnosis and treatment.1 MRI of the foot and ankle has improved with the development of high-field systems and enhanced surface coil technology. Most centers use a standard screening examination, consisting of an array of specified imaging planes and pulse sequences. For the ankle, the patient is imaged in a supine, feet-first position with the malleoli several centimeters inferior to the center of the coil; images are obtained in the axial, coronal, and sagittal planes relative to the tabletop. Imaging of the foot is performed in so-called oblique planes, which are parallel and perpendicular to the long axis of the metatarsals, and with minimal plantarflexion to accentuate the fat plane between the peroneal tendons, improve visualization of the calcaneofibular ligament, and decrease the magic angle e ect.2

In addition to standard T1-weighted and T2-weighted sequences, fat-suppressed T2-weighted and STIR sequences are extremely sensitive for detecting marrow and soft-tissue pathology by accentuating the increased signal of fluid and edema relative to the surrounding structures. A protondensity pulse sequence is occasionally used, often with fat suppression; it combines characteristics of T1-weighted and T2-weighted images. These sequences are acquired with an FSE technique that produces the desired contrast in a fraction of the time needed for conventional T2-weighted sequences, minimizing patient motion, improving throughput, and providing great benefit for claustrophobic patients. Novel developments such as parallel imaging techniques,3 combined with increased signal-to-noise ratio at higher field strengths, may continue to reduce scan times.

Imaging parameters can also be refined for optimal cartilage visualization (see Chapter 14). Gradient-echo sequences are often used to evaluate ligaments and articular cartilage (see Chapter 14), particularly in the coronal plane, allowing for optimal visualization of the articular cartilage within the mortise joint. However, early reports comparing cartilage

imaging within the ankle at 3.0 T and 1.5 T have suggested that coronal intermediate-weighted FSE sequences result in better images than do fat-suppressed gradient-echo–based sequences at 3.0 T, primarily because of increased artifact at higher field strength on gradient-echo sequences.4 In addition, gradient-echo sequences should be avoided in the presence of indwelling metal hardware because of the technique’s inherent increased artifact (see Chapter 1).

MR arthrography can be performed as a direct or indirect method. Direct MR arthrography is performed by injecting approximately 10 mL of diluted gadolinium contrast agent into the ankle joint from an anterior approach, entering just medial to the tibialis anterior tendon with slight cranial angulation; care should be taken to avoid the course of the dorsalis pedis artery.5 Postinjection imaging sequences typically include fat-suppressed T1-weighted and T2-weighted images. Indirect MR arthrography is performed by injecting gadolinium intravenously and imaging the joint after contrast has been taken up by the synovium and then di used into the joint.

Direct MR arthrography of the ankle can improve visualization of acute and chronic ligamentous injuries, particularly those involving the LCL complex, although arthrography can also be useful for visualizing syndesmotic and deltoid ligament tears. MR arthrography has also been shown to be an accurate technique for visualizing intraarticular pathology in the various ankle impingement syndromes, in di erentiating stage II from stage III osteochondral talar lesions, and in identifying loose bodies and synovial disorders.5 Indirect MR arthrography is limited by the lack of joint distention and by inferior contrast relative to direct arthrography, although it can be a useful adjunct to MRI for the assessment of the following5:

•Subtle cartilage defects

•Osteochondral talar lesions

•Ligamentous injuries

•Synovitis

•Sinus tarsi syndrome

However, with the advent of high-field, high-resolution imaging and novel noncontrast MR techniques, direct and indirect MR arthrography techniques may be rendered obsolete in the future.

Fig. 9.1 A sagittal T2-weighted image with fat suppression showing a talar contusion. Note the focus of high signal within the talus (arrow), which signifies edema within the bone marrow. The cortex is intact.

The diagnosis of ankle sprain is usually accomplished with a thorough history and physical examination. Conventional radiographs may reveal any associated avulsion fractures or osteochondral injuries, and stress views can be obtained to evaluate structural instability. Occasionally, MRI is used to confirm the diagnosis or for the evaluation of associated injuries. Ligament injuries are best evaluated on T2-weighted and fat-suppressed T2-weighted MR images. These images usually show increased signal intensity that represents the edema and fluid collection secondary to trauma.6,8 Discontinuity of the ligament on T1-weighted and T2-weighted images signifies disruption.6,8 The anterior talofibular ligament is the weakest and the most vulnerable of all ankle ligaments. Tears in this ligament are best diagnosed on axial images (Fig. 9.2).

The second most common pattern of ankle injury is a combined injury to the anterior talofibular ligament and

Fig. 9.2 An axial T1-weighted image of the left ankle at the level of the superior talus. No ligament is seen at the expected location of the attachment of the anterior talofibular ligament (arrow). Instead, there are ill-defined fibers extending anteriorly from the fibula, representing a grade III tear.

the calcaneofibular ligament.10 Isolated tears of the calcaneofibular ligament are extremely uncommon.10 Unlike the anterior talofibular ligament, tears of the calcaneofibular ligament are best seen on the coronal T2-weighted, fatsuppressed T2-weighted, and fat-suppressed STIR images (Fig. 9.3). MRI grading of the ligamentous injury can be categorized as follows6:

•Grade I, intact ligament structure but surrounding edema as seen on T2-weighted images

•Grade II, thickening or partially torn fibers of the ligament and surrounding edema

•Grade III, complete disruption of the ligament and loss of its attachment as seen on T1-weighted and T2weighted images

Medial Ankle Sprains

Isolated injuries of the deltoid ligament are uncommon.10 They often occur in conjunction with a fibular fracture or a purely ligamentous syndesmotic injury. Deltoid ligament injuries are best viewed on coronal T2-weighted and fatsuppressed T2-weighted images (Fig. 9.4). Occasionally, medial edema and deltoid thickening are seen on T2-weighted images secondary to an inversion injury.6 This medial

Fig. 9.3 A coronal STIR image of the left ankle showing edema and fluid (arrowhead) tracking longitudinally along the fibers of the calcaneofibular ligament, representing a grade II tear. There is also edema of the distal fibula.

Fig. 9.4 A coronal fat-suppressed T2-weighted image of the right ankle showing a grade II or grade III tear (arrowhead) of the deltoid ligament.

Fig. 9.5 MRI can reveal direct or indirect evidence of syndesmotic injury. (A) An axial fat-suppressed T2-weighted image of the right ankle shows ill-defined, wavy fibers of a torn anteroinferior tibiofibular ligament (arrowhead), direct evidence of a syndesmotic sprain.

(B) More superiorly on the same patient, there is extension of fluid into the syndesmosis (arrowhead), providing indirect evidence of syndesmotic disruption.

Fig. 9.6 A sagittal fat-suppressed T2-weighted image showing the typical MRI appearance of an OCD, with osseous edema and irregularity of the subchondral bone plate and overlying cartilage (arrowhead).

after a traumatic injury, although some patients do not recall a specific injury. An OCD lesion often is misdiagnosed clinically as an ankle sprain.15,16 Patients typically present with pain located deep within the joint that is worsened with activity or sports; patients may also complain of ankle swelling, giving way, or locking.

OCD lesions can occur anywhere on the surface of the talar dome; the classic literature notes anterolateral and posteromedial lesions as the most common.15,17 A recent MRI study identified the centromedial and centrolateral locations as the most common.18

A spectrum of OCD lesions exists, including the follow-

ing16,19:

•Softening of the cartilage

•Partial detachment of an osteochondral fragment

•Complete detachment of an osteochondral fragment

•Free-floating fragment within the joint

•Subchondral cystic lesion

Stable lesions may respond to nonsurgical treatment, whereas unstable lesions and cystic OCD lesions typically are treated surgically.

MRI is a very useful modality for diagnosing OCD lesions, particularly if the lesion is not visualized on standard radiographs. MRI can also assess other pathologic entities, such as tendon or ligamentous injuries, in the patient with persistent pain after ankle sprain. All three imaging planes

can be useful in assessing OCD lesions. The AP location can be determined on axial and sagittal images, whereas the mediolateral location can be assessed on coronal and axial views. The depth of the lesion can be viewed on sagittal and coronal images. The MRI appearance of an OCD lesion typically shows osseous edema along with irregularity of the subchondral bone plate and overlying cartilage (Fig. 9.6). Specialized cartilage-specific sequences can improve imaging of the chondral surface to identify subtle disruption20 (see the detailed discussion in Chapter 14). In some cases, the surrounding bone edema may cause overestimation of the true size of the lesion. MRI appearance also may assist in predicting the stability of the lesion; fluid or granulation tissue beneath the lesion at the interface with the osseous crater appears bright on T2-weighted imaging and implies instability21–23 (Fig. 9.7). MRI has also been shown to be of use in evaluating healing of the OCD lesion after surgery, with some normalization of signal changes noted.24,25

Stress Fractures

Osseous stress fractures occur from repetitive stress that is below the threshold for acute fracture but substantial enough to cause microtrabecular failure. They often occur

Fig. 9.7 A coronal fat-suppressed T2-weighted image of the right ankle showing an unstable OCD with fluid beneath the lesion at the interface with the osseous crater of the medial talus (arrowhead).

Fig. 9.8 Calcaneal stress fracture. (A) A sagittal T1-weighted image showing an oblique fracture line in the posterior calcaneus (arrow-

head). (B) A sagittal fat-suppressed T2-weighted image shows marrow edema and edema tracking along the fracture line (arrowhead).

Fig. 9.9 Navicular stress fracture. (A) A sagittal fat-suppressed T2weighted image showing marrow edema (arrowhead) within the navicular. (B) A sagittal T1-weighted image showing a hypointense

en-pointe position and in female athletes, both of whom subject the central metatarsals to increased mechanical load. The second metatarsal is the most frequent location for fracture (more than half of all metatarsal stress fractures), followed by the third and fourth metatarsals.28 Stress fractures of the central metatarsals typically occur in the diaphysis, whereas a stress fracture of the first metatarsal occurs in the proximal metaphysis and a stress fracture of the fifth metatarsal occurs at the diaphyseal-metaphyseal junction.28 Patients typically complain of aching pain related to weight-bearing activities, running, or sports. Conventional radiographs often are diagnostic, although they may be negative early in the process. MRI can be useful in such cases and to rule out other causes of pain such as synovitis. The metatarsal shows the characteristic linear density on T1-weighted images with high signal on corresponding T2-weighted and fat-suppressed T2-weighted images (Fig. 9.10).

■ Degenerative Conditions

Tendon Disorders

region within the dorsal aspect of the navicular with a faint horizontally oriented fracture line (arrowhead).

disorders and is best achieved by obtaining images perpendicular to the tendon’s course.29–33 At the ankle and hindfoot levels, axial views are ideal for tendon imaging, whereas coronal images are best for visualizing more distal structures in

Tendon disorders can represent traumatic injuries, inflammatory conditions, or degenerative tendinosis. Regardless of the etiology, MRI is the ideal method of evaluating tendon

Fig. 9.10 A sagittal fat-suppressed T2-weighted image showing edema of the head of the second metatarsal (arrowhead) in a patient with a stress fracture.

Fig. 9.11 Adventitial bursitis as seen on a sagittal fat-suppressed T2weighted image showing soft-tissue inflammation posterior to the calcaneus (arrowhead).

the midfoot. The magic angle e ect is a phenomenon seen in tissues with well-ordered collagen fibers, such as a tendon.34 This phenomenon occurs when the images are obtained with the magnetic field at an oblique angle (55 degrees) relative to the fibers of the tendon, and it shows abnormal signal intensity on T1-weighted images. T2-weighted images are unaffected by this phenomenon, so corresponding T2-weighted images are scrutinized in the a ected areas to determine if the signal abnormality seen on T1-weighted images represents true pathology or simply artifact. The magic angle e ect is seen most notably in the posterior tibialis and peroneal tendons as they course around the malleoli. Knowledge that this magic angle e ect exists, combined with assessment of the T2-weighted images, assists the radiologist or surgeon in accurately identifying pathology of the tendons about the ankle.

Achilles Tendon Disorders

Achilles tendon disorders represent a spectrum of disease, including the following:

•Retrocalcaneal bursitis

•Paratenonitis

Isolated retrocalcaneal bursitis can be seen secondary to systemic inflammatory disease or localized soft-tissue irritation from impingement by the posterosuperior process of the calcaneus (Haglund process). Sagittal MR images show decreased signal intensity on T1-weighted images and increased intensity on T2-weighted and fat-suppressed T2-weighted images, consistent with soft-tissue inflammation6 (Fig. 9.11). Paratenonitis represents a di use inflammation of the investing paratenon layer that surrounds the Achilles tendon. Sagittal T2-weighted and fat-suppressed T2-weighted images show loss of the normal sharp interface between the Achilles and surrounding soft tissues but normal intrasubstance appearance and morphology of the tendon itself (Fig. 9.12).

The term Achilles tendinitis now is believed to be a misnomer because the tendon typically does not show an inflammatory component. Instead, tendinosis is the preferred term, which more accurately reflects the disorganized collagen fibers, myxoid degeneration, and microscopic intrasubstance tearing seen histologically.35 Achilles tendinosis

Fig. 9.12 The MRI appearance of Achilles paratenonitis is defined by di use inflammation of the investing paratenon layer that surrounds the Achilles tendon. This sagittal fat-suppressed T2-weighted image shows inflammation of the soft tissues surrounding the Achilles (including Kager’s fat pad) (arrowhead) but normal intrasubstance appearance and morphology of the tendon itself. An ankle joint effusion is also seen.