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

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2244 VII Imaging of the Musculoskeletal System Figure 47-48. Anatomic division and major joints shown on a sagittal CT. The joint between the tibia (Ti) and talus (Ta) is the ankle joint (AJ), shown with the yellow line. The joint between the talus and calcaneus (Ca) is the subtalar joint; the posterior facet (P-STJ) is shown with the red line. The Chopart joint, shown with the curved blue line, separates the hindfoot from midfoot. The Lisfranc joint, shown with the angled green line, separates the midfoot from forefoot. images are reformatted off a sagittal reference image, and oblique sagittal images are reformatted off an axial reference image. • Magnetic Resonance Imaging • Coils and Markers When imaging the ankle or foot with MR, it is vital to understand the clinical question that the scan is being requested to address. No one standardized protocol can answer all possible questions. The imaging planes, sequences, and even the selection of which coil to use will vary depending on the clinical circumstances. Whenever possible, an extremity coil should be used. We use a dedicated foot and ankle coil (Fig. 47-49) that incorporates a chimney-like extension so that the toes can be included in the FOV. This chimney design also helps to hold the patient’s foot and ankle in a neutral position, that is, with the bottom of the foot perpendicular to the tibia, as it would be if the patient were standing. Some designs of knee coils have an open top that allows the toes to protrude (Fig. 47-50A). Custom cushioned inserts (Fig. 47-50B) help to keep the heel immobilized and centered in the coil. Although the neutral positioning shown in Figure 47-50A is fine for imaging the ankle and hindfoot, it would not be appropriate for the phalanges. When it is necessary to image the toes, and a foot coil as in Figure 47-49 is not available, the knee coil can be used with the patient’s foot held in plantar flexion. In these circumstances, the technologist should see if having the patient lie prone makes it more comfortable to maintain plantar Figure 47-49. This dedicated foot and ankle coil incorporates a chimney-like extension (arrow) so that the phalanges can be included in the field of view. flexion. Also, patients tend to feel less claustrophobic when prone. Sometimes we have to be creative in our coil selection to accommodate the patient’s physical limitations (Fig. 47-51). We encourage our technologists to place a marker over the sight of maximal tenderness or near a nonhealing ulcer. Markers can be helpful to draw the attention of both the technologist acquiring the images and the radiologist interpreting the images. Do not be dissuaded when the patient initially points to a wide area, such as across the midfoot or around the malleoli. This is typical. Instead, ask the patient to point to one spot with one finger—which, when encouraged, they usually can do. The marker should be placed there. Such a marker needs to be conspicuous on all imaging sequences, including fat-suppressed sequences, and should be placed on the patient in such a way as not to deform the contour of the skin. Although markers are commercially available,* generic capsules containing vitamin E or docusate sodium (Colace) are often used. • Scanning Technique Imaging Planes We use at least nine standard imaging planes in our foot and ankle MRI protocols (Fig. 47-52). The exact slice thickness, interslice gap, and FOV should be optimized to take advantage of the characteristics of the MRI scanner and coil being used. The parameters used at the UW for our GE 1.5-T scanners are spelled out in detail on our MRI scanning parameters and protocols sheets, the most up to date of which can be found at our website (http://www. Radiology.Wisc.Edu/MSKprotocols). *IZI Multi-Modality Radiographic Markers, IZI Medical Products, 7020 Tudsbury Road, Baltimore, MD 21244; (410) 594-9403; http://www.izimed.com. Beekley MR-Spots, Beekley Corporation, 150 Dolphin Road, Bristol, CT 06010; (860) 583-4700; http://www.beekley.com. Ch047-A05375.indd 2244 9/9/2008 5:34:27 PM
47 Ankle and Foot 2245 47 A Figure 47-50. A knee coil can be used to scan the ankle. A, The open top on this knee coil allows the toes to extend through the coil while keeping the foot in neutral position. B, A customized foam pad (white arrow) helps immobilize the hindfoot being scanned. A second pad next to the coil (black arrow) give the contralateral foot a place to rest. B A B C D Figure 47-51. Example of a patient who could not be positioned in the foot or knee coil. This is a 41-year-old with paraplegia from spina bifida who is essentially frozen in the fetal position. The clinical concern was for infection in and around the ankle joint. Because the patient was physically unable to straighten her legs, we could not use the foot coil. Instead, we used a torso coil and covered the leg and ankle. A, Lateral radiograph shows soft tissue swelling. B, T1-weighted, large field-of-view image covering the entire leg as well as the ankle. Because of difficulty positioning the patient, a portion of the other leg is also within the coil. C, Inversion recovery image shows no bone marrow edema but diffuse edema of the subcutaneous fat. D, T1-weighted image with fat suppression after the administration of intravenous gadolinium shows diffuse enhancement of the subcutaneous fat, indicative of cellulitis. The lack of enhancement and edema in the bone marrow exclude osteomyelitis. The gray arrow points to inadequate fat suppression at the edge of the coil, a common occurrence with large fields of view. The straight sagittal plane (see Fig. 47-52A) is our survey plane, and it is usually the first plane acquired in all of our ankle and foot MRI protocols. Most of the other imaging planes are acquired relative to a straight sagittal reference image. The straight sagittal slices are set up off an axial scout image and are oriented parallel to the long axis of the foot. At least two axial orientations are typically used. Straight axial slices (see Fig. 47-52B) are set up off a straight sagittal reference image, either a sagittal scout image or one of the midsagittal slices from the preceding acquisition. The straight axial slices should be roughly perpendicular to the long axis of the tibia and if the ankle is held in the neutral position will be roughly parallel to the bottom of the foot. The slices should begin well proximal to the level of the malleoli and extend distal to the calcaneus. This is our primary plane for imaging the Ch047-A05375.indd 2245 9/9/2008 5:34:31 PM
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Ankle and Foot 47 - Department of Radiology - University of ...

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