Ankle and Foot 47 - Department of Radiology - University of ...

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2248 VII Imaging of the Musculoskeletal System Figure 47-53. Comparison of T2-weighted, fatsuppressed (A) and inversion recovery (B) MRIs in a 35-year-old with plantar fasciitis. Both of these sagittal images of the calcaneus delineate the plantar aponeurosis (open arrowheads), as well as the edema in the adjacent heel fat pad (white arrowheads) and in the bone marrow at its origin (white arrow). A B Protocols Our ankle/foot MRI protocols call for acquiring images in three of the standard planes and typically take 45 to 60 minutes. We start with straight sagittal images to survey the bones. For an ankle MRI, the FOV needs to include the distal tibia and fibula, all of the tarsal bones, and the bases of the metatarsals. For a foot MRI, the FOV needs to include all of the phalanges, all of the metatarsals, and the midfoot tarsal bones back to the Chopart joint. In cases in which the area of clinical concern is vague, the sagittal FOV can be enlarged to include the ankle and forefoot. When they are available, the radiologist should check these first sets of sagittal survey images, particularly the edema-sensitive sequence. If the radiologist observes edema in the talar dome, images can then be acquired in the mortise coronal and mortise sagittal planes (our OLT protocol). If there is edema elsewhere in the tarsal bones, oblique axial and oblique coronal images are acquired (our tarsal stress fracture protocol). If the marrow edema is in a metatarsal, this is usually best demonstrated with short- and long-axis images (our metatarsal stress fracture protocol). If no bone marrow edema is found, we generally proceed with our tendon protocol (straight axial and oblique coronal) unless some other site is clinically requested. Sequences Because detecting abnormally edematous signal in the bones, tendons, and surrounding soft tissues is the key to all musculoskeletal MRI, we run edema-sensitive sequences in all the planes we image. This can be either a fatsuppressed fast-spin echo T2-weighted sequence or an inversion recovery sequence. Although for the most part these two sequences are equivalent (Fig. 47-53), we find that we get the best images when we use inversion recovery for the straight sagittal survey plane and T2-weighted sequences in all other planes. T1-weighted images, being inherently fat sensitive, well demonstrate the normal fat in yellow bone marrow as well as the subcutaneus fat and the deeper fat between muscles and tendons. We use T1 weighting in all imaging planes whenever the tendons are not the primary site of interest. When the tendons are the site of clinical concern, we use proton-density–weighted images, along with T2- weighted sequences, in the straight axial and oblique coronal planes. The straight axial plane well images all 10 of the ankle tendons in cross section at and above the level of the ankle joint. The oblique coronal plane well images the medial and lateral ankle tendons in cross section as they curve under the malleoli. Tears in the substance of the ankle tendons are usually best seen with proton-density– weighted images (Fig. 47-54). However, because protondensity–weighted images are relatively insensitive for fluid, they should always be read side-by-side with edemasensitive images to look for abnormal amounts of fluid in the tendon sheaths, indicative of active tenosynovitis. • Use of Contrast Although the intravenous administration of a gadoliniumbased contrast agent (IVGd) is not needed for most musculoskeletal MRI, there are particular circumstances in which its use is invaluable.* Whenever possible, we use IVGd when there is a clinical concern for an inflammatory arthropathy or synovitis, such as in rheumatoid arthritis (Fig. 47-55). The IVGd causes T1 signal enhancement of the hypervascularized inflammatory synovium (pannus) but not of the adjacent synovial fluid, a distinction that may not otherwise be seen on noncontrast T1-weighted or T2-weighted images. Likewise, we prefer to use IVGd whenever possible in cases of infection. IVGd can help distinguish an enhancing phlegmon from a nonenhancing pus pocket. Also, IVGd can distinguish cellulitis, which demonstrates contrast enhancement of the edematous skin, from “nonspecific edema from other causes” that does not enhance. *During the year leading up to the publication of this chapter, the U.S. Food and Drug Administration (FDA) has issued warnings describing the risk for nephrogenic systemic fibrosis (NSF) after exposure to gadolinium-containing contrast agents in patients with acute or chronic severe renal insufficiency. Information can be found at http://www.fda.gov/cder/drug/infopage/gcca. Before any contrast agent is administered to any patient, that patient should be screened in accordance to your institution’s policies. Ch047-A05375.indd 2248 9/9/2008 5:34:34 PM
47 Ankle and Foot 2249 47 Figure 47-54. Peroneus brevis tear in a 62-year-old. This is a sideby-side comparison of the same straight axial slice acquired using T1-weighted (A), proton-density (PD)–weighted (B), and T2- weighted (C) images. Although T1 shows the fat best and T2 shows the fluid best, PD shows the tendons best, particularly the abnormally increased signal in the split/abnormally flattened peroneus brevis tendon (white arrowhead). A B C A B Figure 47-55. Comparison of T2 weighting with fat suppression (A) and T1 weighting with fat suppression after intravenous gadolinium (B) in a 65- year-old with rheumatoid arthritis. The bright T2 signal in A in the posterior tibial (white arrow) and flexor digitorum longus (white arrowhead) tendon sheaths is shown to be enhancing pannus in B. In comparison, the bright T2 signal in A adjacent to the extensor digitorum longus (black arrow) and the anterolateral ankle joint (black arrowhead) is shown to be nonenhancing fluid surrounded by a thin rim of enhancing synovium in B. We sometimes use IVGd when we detect a soft tissue mass that is bright on T2-weighted sequences and we wish to confirm whether it is solid (Fig. 47-56) or cystic (Fig. 47-57). IVGd is also useful for the detection of Morton’s neuroma by MRI (Fig. 47-58). At UW, we prefer to image Morton’s neuroma with ultrasonography rather than MRI. 36 The contrast-enhanced tissue can be made all the more conspicuous on T1-weighted images by suppressing the signal from fat, and we use fat suppression on nearly all of our postcontrast images. When there is a concern that the degree of fat suppression may not be uniform throughout the image, fat-suppressed T1-weighted images can be obtained before the administration of IVGd to be compared side-by-side with the postcontrast fat-suppressed T1- weighted images. Ankle and Foot Injuries • Ankle Mortise Fractures • Malleoli/Syndesmosis 12 Fractures of the medial and lateral malleoli are commonly the result of twisting injury of the talus in ankle mortise. Radiographs are usually sufficient for the management of what are typically simple fractures. CT axial images through Ch047-A05375.indd 2249 9/9/2008 5:34:35 PM
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