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

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2296 VII Imaging of the Musculoskeletal System A C B D Figure 47-101. Navicular stress reaction in a 36-year-old avid runner who had recently begun marathon training. The markers (m) indicate the proximal and distal extents of the patient’s pain. A, Sagittal T1-weighted MRI shows abnormally dark bone marrow in the dorsal half of the navicular (arrow). B, Sagittal inversion recovery (IR) image, being more sensitive for edema, shows abnormally bright signal throughout the navicular (arrows). C, Long-axis, oblique axial T1-weighted image shows abnormally dark bone marrow in the central third of the navicular, emanating from the proximal articular surface adjacent to the head of the talus (arrow). D, Fat-suppressed T2-weighted image in the same plane, being more sensitive for edema, shows abnormally bright signal throughout the navicular (arrows). E, Short-axis, oblique coronal T1-weighted image just distal to the talonavicular joint shows abnormally dark bone marrow in the dorsal central portion of the navicular (arrow). F, Fat-suppressed T2-weighted image at that same location, being more sensitive for edema, shows abnormally bright signal throughout the navicular (arrows). None of these MRIs demonstrates a discrete fracture line, and we call this a stress reaction rather than a stress fracture. E F Ch047-A05375.indd 2296 9/9/2008 5:35:55 PM
47 Ankle and Foot 2297 47 Figure 47-102. Navicular fatigue (stress) fracture in a 16-year-old who developed midfoot pain while cross-country skiing. A, Anteroposterior radiograph of the foot reveals a subtle nondisplaced fracture in the middle third of the navicular (arrowhead in magnified dashed box). MRI was obtained 4 days later. Axial oblique T1- (B) and fat-suppressed T2-weighted (C) images reveal a discrete fracture in the middle third of the navicular (arrowhead) as well as diffuse bone marrow edema. Continued A B C developed a specific CT protocol that reformats the images in thin, 1-mm slices using a small, 6-cm FOV centered on the navicular (see Fig. 47-47D). Whether seen by CT or MRI, navicular fatigue injuries begin at the dorsal, central, proximal navicular where it articulates with the head of the talus. This is illustrated by the black arrows pointing to the dark regions of bone marrow on the T1-weighted images in the stress reaction in Figure 47-101. More fluid-sensitive fat-suppressed T2- weighted or inversion recovery images show bone marrow edema emanating from this dorsal/central/proximal site. This is illustrated by the white arrows in Figure 47-101. When stress reactions progress to stress fractures, the cortical disruption starts at the dorsal/central/proximal site on the navicular and propagates in a plantar direction vertically in the sagittal plane (see Fig. 47-102) or in an oblique sagittal plane (see Fig. 47-103). Because of the primarily sagittal orientation of these fractures, they may be difficult to appreciate on sagittal CT images and are better seen on oblique coronal (see Fig. 47-102F) and oblique axial (see Fig. 47-102G) images. Because they tend to be nondisplaced incomplete fractures, they are best seen on images that are reformatted into a small FOV with thin slices. Because these patients may undergo serial CT scans to follow the progress of fracture healing, it is useful to have a standard protocol (as in Fig. 47-47D) to help retain uniform reformatting parameters from one scan to the next (see Fig. 47-103C to F). Ch047-A05375.indd 2297 9/9/2008 5:35:56 PM
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