There has been substantial progress in the characterization of MRI lesions in the sacroiliac joints (SIJ) and spine in axial spondyloarthritis (axSpA) since the last consensus-based reports from ASAS^1,2. In particular, new data have emerged regarding structural lesions and the considerable evolution in their appearance according to the degree of inflammation. There is as yet a lack of international consensus on standardized definitions of all the lesions reported to date. Consequently, the ASAS MRI group was convened to evaluate the literature describing the spectrum of MRI lesions in axSpA and to generate a consensus update on standardized definitions for MRI lesions.

The literature pertaining to MRI lesion definitions in axSpA was discussed at 3 meetings of the ASAS MRI group attended by 26 investigators (21 rheumatologists, 5 radiologists). The group reviewed the literature for MRI lesion definitions and decided by consensus which definitions would be retained, which required modification, and which required a new definition. The group also agreed on a set of reference images, a study design (ASAS MRImagine) for multi-reader assessment of lesion definitions using MRI scans from the ASAS classification cohort, a PowerPoint-based reader calibration module, and a study-specific interactive eCRF incorporating links to reference images for recording MRI data.

For definitions denoting signs of activity in the SIJ, there are no revisions to the most current ASAS definition of a positive MRI and for subchondral bone marrow inflammation¹. Definitions for capsulitis and enthesitis are revised. A new definition, joint space enhancement, denotes increased signal on contrast-enhanced images in the joint space of the cartilaginous portion of the SIJ. This replaces the term ‘synovitis’ and a separate definition describes what constitutes joint space fluid. For signs of structural change in the SIJ, the definition for sclerosis is unchanged. A revised definition for a fatty lesion incorporates characteristics typical of axSpA, and for erosion requires both loss of cortical bone as well as adjacent marrow matrix on a T1W image. A new definition, fat metaplasia in the joint space (‘backfill’), denotes the reparative change on a T1W image at the site of erosion when signs of activity recede. The new definition for ankylosis stresses the continuity of bright marrow signal across the joint space on a T1W image while for bone bud, the signal does not bridge the joint. Spinal lesion definitions are divided into those that occur in defined central and lateral sagittal slices. The revised definition of a vertebral corner inflammatory lesion divides this into a regular (type A) and dimorphic (type B) lesion. A new definition for corner erosion requires both loss of cortical bone as well as adjacent marrow matrix. New definitions for new bone growth require bright signal on T1W images extending from the vertebral corner marrow or endplate, which may (ankylosis) or may not (bone spur) be continuous with the adjacent vertebra.

The ASAS MRI group has generated a consensus-based update on MRI lesions in axSpA.

1.Lambert et al. Ann Rheum Dis 2016; 75:1958-1963

2.Hermann et al. Ann Rheum Dis 2012;71:1278–1288