The purpose of this study was to evaluate the diagnostic capability of mineralized tissue-magnetic resonance imaging (MT-MRI) to detect erosions, sclerosis, and ankylosis of the sacroiliac joint (SIJ) in patients with axial spondyloarthritis, by comparison with T1-weighted MRI and pelvic computed tomography (CT), using SIJ CT as the standard of reference.

This retrospective study included 100 patients (62 women, 38 men; mean age, 39.9 ± 14.3 [standard deviation] years; age range: 19–79) with suspected axial spondyloarthritis, who underwent SIJ MRI and CT on the same day between August 2023 and March 2025. Two musculoskeletal radiologists independently scored the amount of structural lesions of SIJs. The diagnostic confidence in each imaging modality was also evaluated.

The erosion scores showed no significant difference between SIJ CT and MT-MRI for both readers (P ≥ 0.07) but were lower than those of SIJ CT when using T1-weighted MRI for both readers (P ≤ 0.011). The performance of pelvic CT for depicting erosion was intermediate but closer to that of MT-MRI. The sclerosis scores were 16–20 % lower with T1-weighted MRI than with SIJ CT (P ≤ 0.012) and 3 % lower with MT-MRI (P ≤ 0.04), but 8 % overestimated using pelvic CT (P ≤ 0.001). For both groups, SIJ CT obtained the highest confidence score, which was superior to pelvic CT (P ≤ 0.001), itself superior to T1-weighted MRI and MT-MRI (P ≤ 0.001), with the latter two not differing significantly (P ≥ 0.109).

T1-weighted MRI alone is insufficient for reliably evaluating structural lesions of SIJs due to sacroiliitis in patients with axial spondyloarthritis. MT-MRI emerges as the closest alternative to SIJ CT, demonstrating excellent diagnostic performance while eliminating radiation exposure.