On MRI, active sacroiliitis is subchondral bone marrow oedema on STIR in the typical location; structural damage is erosions, sclerosis, fat metaplasia and ankylosis on T1 — report both, because both enter the classification.
Orient first
- Axial spondyloarthritis affects the sacroiliac joints first; MRI shows it years before radiographs do.
- The ASAS MRI definition of active sacroiliitis requires periarticular bone marrow oedema highly suggestive of the disease — verify the current criteria and wording in use.
- Mimics: mechanical stress (postpartum, athletes), degenerative change, infection (unilateral, with joint effusion and soft-tissue oedema) and osteitis condensans ilii.
Acquire the study
- Oblique coronal T1 and STIR through the sacroiliac joints; oblique axial STIR as an adjunct.
The manoeuvre
- Oblique coronal STIR: subchondral marrow oedema on both sides of the joint, its extent and how many slices it spans.
- Oblique coronal T1: erosions (breaks in the dark subchondral line), sclerosis, fat metaplasia and bone bridges.
- Symmetry: bilateral disease favours spondyloarthritis; unilateral with effusion and surrounding oedema raises infection.
- The lower lumbar spine on the included sections: corner inflammatory lesions and enthesitis.
What confirms it
- Typical subchondral bone marrow oedema, with or without structural lesions, in the right clinical setting.
What licenses you to exclude it
- A normal MRI of the sacroiliac joints makes active axial spondyloarthritis unlikely at that time; a normal radiograph does not.
The classic misread
- Calling a small oedema focus in a postpartum or athletic patient sacroiliitis — mechanical stress causes it too.