The Assessment of Spondyloarthritis International Society (ASAS) magnetic resonance imaging (MRI) working group revised and created new definitions for both inflammatory and structural MRI lesions in spondyloarthritis (SpA), according to research results published in the Annals of the Rheumatic Diseases.

Investigators highlighted the growth of clinical understanding of MRI lesions in the sacroiliac joint in the past decade. As a result, the ASAS MRI working group conducted a consensus update and determined the necessity of new or standardized definitions for MRI lesions in the sacroiliac joint in patients with SpA.

A total of 20 rheumatologists and 5 radiologists in the working group participated in the consensus exercises; they used MRI scans of patients recruited into the ASAS classification cohort for the exercise.

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The working group consensus evaluated the overarching recommendations pertaining to optimal interpretation of MRI lesions in the sacroiliac joint. These definitions were then categorized as either lesions indicating signs of activity or structural lesions.

Lesion Definitions Indicating Signs of Activity

Bone marrow edema: This definition was not revised.

Capsulitis: The wording of this definition was revised to clarify its location.

Joint space enhancement: This new definition, replacing the original definition of synovitis, applies only to contrast-enhanced sequences. Researchers considered this new definition necessary because of the observation that the synovium was only present at the perimeter of the lower third of the cartilaginous portion of the joint.

Inflammation at the site of erosion: This is a new definition that describes inflammation within an erosion.

Enthesitis: This definition was revised; it now excludes the interosseous soft tissues in the ligamentary portion of the sacroiliac joint. The working group made this change because of the difficulty in distinguishing enthesitis from vascular signal.

Joint space fluid: This is a new definition; it describes bright signal in the joint space on a T2-weighted fat suppressed sequence.

Lesion Definitions Indicating Signs of Structural Change

Erosion: This is a revised definition; it now includes wording that describes both a breach in cortical bone and loss of adjacent marrow matrix.

Fat lesion (fat metaplasia): This is a revised definition; it now includes morphologic characteristics indicating the presence of fat metaplasia after the resolution of an inflammatory lesion.

Fat metaplasia in an erosion cavity (‘backfill’): This is a new definition. Researchers considered it necessary, as it defines a “distinctive” structural lesion that may develop after inflammation in an erosion cavity is resolved. Backfill comprises 2 components: a bright signal within the erosion cavity, which indicates a reparative process that resembles the transformation of subchondral bone marrow edema into fat metaplasia in bone marrow, and an irregular band of dark signal indicating sclerosis at the border of the original erosion. Backfill is a composite lesion; it may be seen in the joint cavity after erosions have become confluent.

Sclerosis: This definition was not revised.

Ankylosis: This is a new definition. Researchers considered it necessary to clearly indicate that ankylosis is considered present when there is continuity of bright bone marrow signal across the joint space.

Nonbridging bone bud: This is a new definition, it describes new bone formation in the sacroiliac joint that has not bridged the joint cavity.

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After validation in a multireader exercise, subchondral bone marrow edema was the most frequently detected lesion (≥4 of 7 readers agreed); it was observed in 40.3% of the 278 cases; however, only 31.3% of these cases had subchondral bone marrow edema that met the ASAS definition of a positive MRI. Other types of inflammatory lesions were identified in <10% of cases.

The most frequent structural lesion was erosion, observed in 28.3% of cases, while ankylosis and bone bud were observed in <5% of cases.

Overall, reliability for both types of lesion detection was broadly comparable when all available MRI scans were examined.

“The ASAS MRI-[working group] provides a consensus-based update of MRI lesion definitions… in patients with SpA. Testing of these definitions in scans from the ASAS classification cohort for agreement… demonstrated acceptable readability for most inflammatory and structural lesions. Validation of these lesions for diagnostic, classification, and prognostic utility is warranted,” the researchers concluded.


Maksymowych WP, Lambert RG, Østergaard M, et al. MRI lesions in the sacroiliac joints of patients with spondyloarthritis: an update of definitions and validation by the ASAS MRI working group [published online August 17, 2019]. Ann Rheum Dis. doi:10.1136/annrheumdis-2019-215589