Summary

Background:

Ultra-short-echo magnetic resonance imaging (MRI) sequences improve visualization of bone cortex and enable the generation of an MRI-based computed tomography (CT)-like (CT-l) images.

Objectives:

To compare the agreement between CT-l images derived from merged fast-field echo (MFFE) sequence with conventional CT (cCT) for detecting sacroiliitis-associated structural lesions.

Methods:

Consecutive MRI and conventional CT examinations (maximum one-year interval) of the sacroiliac joints (SIJ) of patients with suspected sacroiliitis, performed between 2022 and 2023 were retrospectively evaluated by a musculoskeletal radiologist and a third-year resident who evaluated half of the study’s images for the presence of erosions, sclerosis, and ankylosis on semicoronal CT-l and cCT-SIJ images. Cohen’s kappa was used to compare the results between the two modalities. The diagnostic performance of the CT-l was analyzed using cCT as the reference standard. Inter-reader reliability was assessed using intraclass correlation coefficients (ICCs) between the two readers

.

Results:

Sacroiliitis was detected in 11 patients (21%). Erosions, sclerosis, and ankylosis were detected in 44%/31%, 48%/42%, 4%/4% on CT-l/cCT, respectively. Statistical analysis regarding ankylosis was not feasible due to its low prevalence. Cohen's kappa agreement between modalities was substantial for erosions (0.68) and almost perfect for sclerosis (0.92). Sensitivity and specificity of CT-l were 0.90 and 0.69 for erosions and 0.72 and 0.91 for sclerosis, respectively. The ICC on CT-l/cCT for detecting erosions was 0.72/0.68 and 0.84/0.91 for sclerosis.

Conclusions

: CT-l MRI sequence is a promising, radiation-free alternative to conventional CT for detecting structural lesions in SIJs, with high diagnostic performance and inter-reader agreement.