Backfill, baseline MRI (T1SE sequence). HLA B27 positive, 25-year-old male AS patient. The T1SE sequence of the SIJ displays extended erosion of the iliac joint portion on both sides. The excavated iliac bone is replaced by tissue showing MR signal intensity similar to or higher than normal bone marrow (arrows). HLA-B27, human leucocyte antigen B27; MRI, magnetic resonance imaging; T1SE, T1-weighted spin echo sequence.