Fig. 2.

Intra-articular high signal intensity in the sacroiliac joints space on T1-weighted MR images. Coronal oblique T1-weighted MR images acquired in a 25-year-old man with SpA show high intra-articular signal intensity (arrows). This high signal intensity is clearly filling up the eroded iliac bone on the right (i.e., “backfill”). Extensive fatty degeneration of the bone marrow of the sacral side of the sacroiliac joints is present, i.e., post-inflammatory structural changes. Note: b and c are enlargements of the regions outlined in a