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

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2242 VII Imaging of the Musculoskeletal System Figure 47-46. Anteroposterior (A) and lateral (B) scout CT views. The scanning field should cover both ankles and should extend from above the syndesmosis to below the calcaneus (white rectangles). In cases of pilon fractures, more of the distal tibia is covered (dashed rectangle). A B We can achieve the highest resolution on the reformatted images by reconstructing the source images at a width equal to the width of the narrowest detector. The reconstruction interval between source images is equal to onehalf the detector width, and this allows for a 50% overlap between slices. We reconstruct two sets of source images; one uses an edge-enhanced bone algorithm for the twodimensional multiplanar reformatted images, the other a smoothing standard algorithm for reformatting in threedimensions. Both sets of source images are sent to the PACS for archival storage and can be accessed any time in the future if additional two- or three-dimensional reformatted images are desired. Because these thin overlapping slices yield hundreds of source images, we store these source images on the PACS in their own folder—a folder separate from where we store the reformatted images, which are images most of us primarily view. Reformatting Technique At the UW we have identified at least 15 different ways that we are commonly asked to create two-dimensional reformatted images of the ankle and foot. These are delineated on our downloadable protocol sheets, portions of which are shown in Figure 47-47. The reformatting protocols are centered on the anatomic divisions illustrated in Figures 47-5 and 47-48. Ankle/Distal Tibia Protocol. Our ankle/distal tibia protocol (see Fig. 47-47A) is centered on the ankle joint. This protocol is used for scanning fractures of the distal tibia (e.g., pilon, malleoli, triplane, juvenile Tillaux) or of the talar dome (e.g., osteochondral lesions). Using a midsagittal reference image, straight axial images are created in a plane parallel to the bottom of the foot. Then, using an axial reference image through the top of the ankle mortise, mortise coronal and mortise sagittal images are created parallel and perpendicular to an imaginary line through the anterior cortex of the medial and lateral malleoli. For distal tibial fractures, we find that creating reformatted images that are 3 mm thick at 3-mm intervals (no gap or overlap between reformatted slices) yields crisp images that do not appear noisy. However, for osteochondral lesions of the talar dome, our surgeons prefer that the mortise coronal and mortise sagittal images be reformatted at 1 mm, yielding images of higher resolution but also with more noise. Hindfoot/Midfoot Protocol. Our hindfoot/midfoot protocol (see Fig. 47-47B) is centered on the Chopart joint and is used to evaluate hindfoot fractures (e.g., calcaneus, talar body) and the subtalar joint (e.g., tarsal coalitions). Using an axial reference image, straight sagittal images are reformatted along a plane parallel to the long axis of the foot. The other three planes are reformatted off a midsagittal reference image. Straight axial images are reformatted in a plane parallel to the bottom of the foot. Oblique coronal and oblique axial images are reformatted in planes both perpendicular and parallel to the posterior facet of the subtalar joint. Forefoot/Midfoot Protocol. Our forefoot/midfoot protocol (see Fig. 47-47C) is primarily used to assess the alignment of the Lisfranc joint and the integrity of the adjacent bones. We find that it works best to create reformatted images in three planes relative to the first metatarsal shaft. A sagittal reference image that best delineates the entire length of the first metatarsal is selected. Long-axis and short-axis planes are reformatted both parallel and perpendicular to the sagittal length of the first metatarsal. The third plane, sagittal to the first metatarsal, is best obtained off an axial reference image that has been obliqued to include the entire length of the first metatarsal. Navicular Protocol. Our dedicated navicular protocol (see Fig. 47-47D) is used to assess the healing of a known navicular fatigue fracture that has perhaps been previously diagnosed by MRI. Because these navicular fatigue fractures tend to be incomplete hairline cracks, we increase the resolution by creating thin (1 mm) reformatted images in a small (6 cm) FOV. Oblique coronal and oblique axial Ch047-A05375.indd 2242 9/9/2008 5:34:23 PM
47 Ankle and Foot 2243 47 A Ankle/distal tibia (centered on ankle joint) 1. Straight axial (off a sagittal) Standard: 3 × 3 mm For OLT: 3 × 3 mm 2. Mortise coronal (off an axial) 3 × 3 mm 1 × 1 mm 3. Mortise sagittal (off an axial) 3 × 3 mm 1 × 1 mm B Hindfoot/midfoot (centered on Chopart joint) 1. Straight sagittal (off an axial) Standard: 3 × 3 mm 2. Oblique coronal (off a sagittal) 3 × 3 mm 3. Straight axial (off a sagittal) 3 × 3 mm 4. Oblique axial (off a sagittal) 3 × 3 mm C Forefoot/midfoot (centered on Lisfranc joint) All planes are relative to 1st metatarsal 1. Axial (long axis) (off a sagittal) Standard: 3 × 3 mm 2. Short axis (off a sagittal) 3 × 3 mm 3. Sagittal (off an axial; may have to oblique reference image to see 1st metatarsal) 3 × 3 mm D Navicular (stress fracture) Reformat 6 cm FOV Reformat 1 × 1 mm 1 and 2. Coronal and axial (off a sagittal) 3. Sagittal (off an axial) Figure 47-47. These are portions of our University of Wisconsin foot/ankle/distal tibia (F/A/T) protocol sheet. A, Ankle/distal tibia protocol. This protocol is appropriate for distal tibial fractures (pilon, malleoli, triplane, and juvenile Tillaux) and for talar dome fractures (osteochondral lesions of the talus [OLT], osteochondritis dissecans). B, Hindfoot/midfoot protocol. This protocol is appropriate for hindfoot fractures (calcaneus, talar body, and subtalar joint) and for tarsal coalitions. C, Forefoot/midfoot protocol. This protocol is appropriate for forefoot fractures (Lisfranc dislocation, metatarsals). D, Navicular protocol. Ch047-A05375.indd 2243 9/9/2008 5:34:25 PM
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