Figure 9b. Rheumatoid arthritis of the ankle in a 61-year-old woman. (a) Anteroposterior radiograph of the ankle demonstrates a large lytic lesion in the talus (arrow). The lesion was initially thought to be a bone tumor, but the erosions of the fibula (arrowheads) indicate that the primary disease is in the joint. (b) Sagittal T1-weighted MR image demonstrates a low-signal-intensity lesion (*) in the talus with low-signal-intensity pannus (arrows) in the joint space. (c) Sagittal STIR MR image demonstrates intermediate-signal-intensity pannus (arrows) and high signal intensity in the talus representing a large subchondral geode (*).