An international consensus group has developed a standardized magnetic resonance imaging (MRI) protocol for diagnostic evaluation of the sacroiliac joints in patients with axial spondyloarthritis.

The protocol, approved by the Assessment of SpondyloArthritis International Society (ASAS), outlined a minimum four-sequence MRI acquisition protocol designed to visualize inflammation, structural damage, and the bone-cartilage interface of the sacroiliac joints (SIJ).

The consensus recommendations, published in Annals of the Rheumatic Diseases, included:

• Three semicoronal sequences parallel to the dorsal cortex of the S2 vertebral body: T1-weighted spin echo, T2-weighted with fat suppression, and thin-slice T1-weighted sequence (preferably three-dimensional gradient echo)
• One semiaxial sequence perpendicular to the semicoronal plane: T2-weighted with fat suppression. The protocol emphasizes consistent orientation of sequences and imaging in two planes. Thirteen radiologist members of ASAS and the SpondyloArthritis Research and Treatment Network as well as two rheumatologists participated in the consensus process. Participants included experts from 12 European and 12 North American academic radiology departments.

Among the key findings from the consensus process were:

• Semicoronal sequences should be parallel to the dorsal cortex of the S2 vertebral body, providing a reference point for interpreting SIJ anatomy. Between 40% to 80% of patients may exhibit anatomical variants in the SIJ.
• T1-weighted spin echo shows a sensitivity of 61% to 79% and specificity of 93% to 95% for erosion detection, using computed tomography as the gold standard.
• Three-dimensional gradient echo techniques demonstrate superior sensitivity (82% to 96%) and specificity (93% to 97%) compared with standard T1-weighted spin echo for erosion detection.
• Reader reliability for erosion detection improved with three-dimensional gradient echo sequences (kappa 0.71) compared to standard two-dimensional T1-weighted spin echo (kappa 0.56).

Further, BME positivity for sacroiliitis according to ASAS criteria decreases by 33% to 56% in healthy populations when assessed in two perpendicular planes instead of only the semicoronal plane. Specific sequence parameters recommended included:

• Semicoronal T1-weighted spin echo: TR 530 ms, TE 12 ms, 3-mm thick
• Semicoronal T2-weighted spin echo with fat suppression: TR 5940 ms, TE 88 ms, 3-mm thick
• Semicoronal T1-weighted three-dimensional gradient echo (VIBE) with fat suppression: TR 4.04 ms, TE 1.57 ms, 1-mm thick
• Axial T2-weighted spin echo Dixon water-only: TR 4360 ms, TE 93 ms, 4-mm thick.

The total scan time for the four-sequence protocol is typically around 15 minutes, with potential for shorter durations (as little as 10 minutes) on newer 3T scanners.

An informal survey of the participating academic radiology departments revealed that all sites performed a minimum of the MRI sequences, with 19 performing four to eight sequences for SIJ evaluation.

The consensus group did not recommend routine use of contrast-enhanced sequences. Recent studies have shown that structural lesions can develop early in sacroiliitis and occur as frequently as active inflammatory lesions at the time of diagnosis.

Alternative techniques considered included T2-weighted Dixon sequences, balanced steady-state free precession, Liver Acquisition with Volume Acceleration, susceptibility-weighted imaging, ultrashort echo time, and zero echo time sequences.

The protocol has not been specifically designed for pediatric populations. The majority of these patients demonstrate absence or blurring of the subchondral cortex on T1-weighted spin echo MRI, complicating interpretations.

The recommendations did not include guidance on whether all or part of the spine should be scanned.

The protocol was approved at the 2022 ASAS annual meeting with 91% of the membership in favor.