Fig. 2

Interval change in SIS and SSS lesions in children. Legend: Coronal oblique STIR images of the sacroiliac joints in a 12-year-old girl (a) and at age 16 years (b). a There is marrow edema along both the iliac and sacral aspects of the sacroiliac joints (arrows), left greater than right. Findings are consistent with active sacroiliitis. On follow-up imaging 4 years later (b), the marrow edema has resolved. Coronal oblique T1-weighted images of the sacroiliac joints in a 15-year-old boy (c) and at age 19 years (d). c There are erosions along both aspects of the iliac bones. In addition, there is high T1-weighted signal within both joints consistent with backfill (arrows). On follow-up imaging 4 years later (d), there is ankylosis of both joints with fat metaplasia, as demonstrated by fatty marrow signal along the fused sacroiliac joints (arrows). Note the normal appearance of the marrow within the upper sacrum demonstrated low signal (*)