The imaging changes are mostly unilateral and by MRI present as extensive BME and concomitant contrast enhancement. There is usually concomitant thickened distended inflamed capsule, joint fluid accumulation and edema in the surrounding soft tissue, sometimes with avascular fluid collections (abscesses), doi:10.2214/AJR.14.14217; doi:10.1007/S002560000242. In contrast, the typical inflammatory SIJ changes in axSpA do not cross anatomic borders and are limited to the bone and intraarticular space.

MRI, semi-coronal T1, STIR and postcontrast T1FS and semi-axial postcontrast T1FS image. There is suppression of the fat signal around the left SIJ and erosion of the iliac joint facet on the T1 image with pronounced oedema and enhancement on the STIR and T1FS + Gd. image, respectively, involving both the joint region and the surrounding soft tissue structures. The soft tissue involvement is best seen on the semi-axial slice. There are no detectable abscesses.