The primary use of imaging focuses on diagnostic evaluation, and pelvic radiography is still the primary modality for evaluation of disease in the sacroiliac joints. Its unreliability and inability to directly assess inflammation has led to increasing use of magnetic resonance imaging (MRI), which directly assesses inflammatory changes as well as the structural changes associated with inflammation. Both radiography and MRI have therefore been incorporated into new classification criteria designed to capture both early and established spondyloarthritis. Fat metaplasia on T1-weighted MRI is an important intermediary tissue on the pathway from inflammation to ankylosis in both the sacroiliac joints and spine.