Characterization of bone marrow lesions in axial spondyloarthritis using quantitative T1 mapping MRI.

OBJECTIVE: Conventional magnetic resonance imaging (MRI) uses T1-weighted and short-tau inversion recovery (STIR) sequences to characterize bone marrow in axial spondyloarthritis. However, quantification is restricted to estimating the extent of lesions because signal intensities are highly variable both within individuals and across patients and MRI scanners. This study evaluates the performance of quantitative T1 mapping for distinguishing different types of bone marrow lesions of the sacroiliac joints. MATERIALS AND METHODS: In this prospective study, 62 patients underwent computed tomography (CT) and MRI of the sacroiliac joints including T1, STIR, and T1 mapping. Bone marrow lesions were characterized by three readers and assigned to one of four groups: sclerosis, osteitis, fat lesions, and mixed marrow lesions. Relaxation times on T1 maps were compared using generalized estimating equations and receiver operating characteristics (ROC) analysis. RESULTS: A total of 119 lesions were selected (sclerosis: 38, osteitis: 27, fat lesions: 40; mixed lesions: 14). T1 maps showed highly significant differences between the lesions with the lowest values for sclerosis (1516±220 ms), followed by osteitis (1909±75 ms), and fat lesions (2391±200 ms); p<0.001. T1 mapping differentiated lesions with areas under the ROC curve of 99% (sclerosis vs. osteitis) and 100% (other comparisons). CONCLUSION: T1 mapping allows accurate characterization of sclerosis, osteitis, and fat lesions at the sacroiliac joint but only for homogeneous, non-mixed lesions. Thus, further sequence development is needed before implementation in clinical routine.

New bone formation at the sacroiliac joint in axial spondyloarthritis: characterization of backfill in MRI and CT.

OBJECTIVE: MRI findings of the SI joint space in axial SpA (axSpA) include inflammation and fat metaplasia inside an erosion; the latter is also termed 'backfill'. We compared such lesions with CT to better characterize whether they represent new bone formation. METHODS: We identified patients with axSpA who underwent both CT and MRI of the SI 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 type B, and 84 type C lesions (maximum 1 lesion per type and joint). The HU values were 73.6 (s.d. 15.0) for cartilage, 188.0 (s.d. 69.9) for spongious bone, 1086.0 (s.d. 100.3) for cortical bone, 341.2 (s.d. 96.7) for type A, 359.3 (s.d. 153.5) for type B and 446.8 (s.d. 123.0) for type C lesions. Lesion HU values were significantly higher than those for cartilage and spongious bone, but lower than those for 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).