OBJECTIVE: To compare the diagnostic performance of two different sets of magnetic resonance imaging (MRI) for the detection of subchondral erosions in the sacroiliac joints regarding the application of fat-water separation techniques when acquiring T1-weighted (T1w) images, using multi-detector computed tomography (MDCT) as the reference standard. METHODS: We retrospectively included 31 consecutive patients having or being suspected for axial spondyloarthritis (SpA) assessed using both MRI and MDCT. Three sets of images were independently assessed for the presence of erosions by two musculoskeletal radiologists (R1, R2): (1) MRI with standard T1w without fat suppression, (2) MRI with both T1w with and without fat suppression, and (3) MDCT. The diagnostic performance of both sets of MRIs was assessed using MDCT as the referent. RESULTS: The assessment of T1w images with fat suppression substantially increased sensitivity (76% vs. 63% R1; 70% vs. 60% R2), specificity (97% vs. 84% R1; 96% vs. 81% R2), positive predictive value (85% vs. 45% R1; 81% vs. 40% R2), and overall accuracy (94% vs. 80% R1; 92% vs. 77% R2) in the detection of erosions when compared to the assessment using T1w images without fat suppression. CONCLUSION: The assessment of T1w images with fat suppression substantially improves the diagnostic performance of MRI in the detection of erosions in the sacroiliac joints. KEY POINTS: • The presence of erosions in the sacroiliac joints may influence the decision on the diagnosis of axial spondyloarthritis. • T1w fat-suppressed MR imaging relatively increases the contrast between the joint space (high signal) and the adjacent subchondral bone (low signal), potentially improving the detection of erosions in the sacroiliac joints. • T1w fat-suppressed images improve the diagnostic performance of MRI in the detection of erosions in the sacroiliac joints compared to T1w without fat suppression, using MDCT as the reference.