MR arthrography was superior to conventional MRI for detecting labral tears and acetabular cartilage defects and showed a higher interobserver agreement. For femoral cartilage lesions, both modalities yielded comparable results.

Lesion-based criteria for a positive SIJ MRI based on both BMO and/or erosion performed best for classification of axial SpA, reflecting the contextual information provided by T1SE and STIR sequences.

Combined spine and SIJ MRI added little incremental value compared with SIJ MRI alone for diagnosing patients with nr-axSpA and enhancing confidence in this diagnosis.

Using this MRI reference criterion, the cutoff for the number of affected SI joint quadrants needed to reach a predefined specificity of >0.90 was >2 for bone marrow edema (BME) in both cohorts and >1 for erosion in both cohorts, and the BME and/or erosion lesions increased sensitivity without reducing specificity. Conclusion. This data-driven study using 2 inception cohorts and comparing clinical and MRI-based classification supports the case for including both erosion and BME to define a positive SI joint MRI finding for the classification of axial SpA.

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