Abnormal narrowing, ankylosis, and bone bridging of the synovial portion of both sacroiliac joints. Abnormal high signal on T1 and T2WI series, with signal loss on fat suppressed images, denotes fatty metaplasia. STIR sequences show no bone marrow edema within either iliac or sacral surfaces of the joint.
Similar changes of fatty metaplasia are demonstrated at the left L5-S1 facet joint.
Modic type II changes of the S1 superior endplate.