LAUSR

OBJECTIVE MRI findings of the sacroiliac joint space in axial spondyloarthritis (axSpA) include inflammation and fat metaplasia inside an erosion; the latter is also termed "backfill". We compared such lesions with CT to better characterize them whether they represent new bone formation. METHODS We identified patients with axSpA who underwent both CT and MRI of the sacroiliac joints in two prospective studies. MRI datasets were jointly screened by three readers for joint-space-related findings and grouped into three categories: type A: high short tau inversion recovery (STIR) and low T1 signal; type B: high signal in both sequences; type C: low STIR and high T1 signal. Image fusion was used to identify MRI lesions in CT before we measured Hounsfield units (HU) in each lesion and surrounding cartilage and bone. RESULTS Ninety-seven patients with axSpA were identified, and we included 48 type A, 88 B, and 84 C lesions (maximum 1 lesion per type and joint). HU were 73.6 ± 15.0 for cartilage, 188.0 ± 69.9 for spongious bone, 1086.0 ± 100.3 for cortical bone, 341.2 ± 96.7 for type A, 359.3 ± 153.5 for B, and 446.8 ± 123.0 for C lesions. Lesion HU values were significantly higher than those of cartilage and spongious bone but lower than those of cortical bone (p< 0.001). Type A and B lesions showed similar HU values (p= 0.93), whereas type C lesions were denser (p< 0.001). CONCLUSION All joint space lesions show increased density and might contain calcified matrix, suggesting new bone formation, with a gradual increase in the proportion of calcified matrix towards type C lesions (backfill).