MRI-based synthetic CT improved specificity for erosion detection and sensitivity for sclerosis and ankylosis detection compared with T1-weighted MRI in individuals suspected of having sacroiliitis. Background Evaluation of structural lesions in the sacroiliac (SI) joints can improve the accuracy for diagnosis of spondyloarthritis. However, structural lesions, such as erosions, are difficult t...

Background A range of sacroiliac joint (SIJ) MRI protocols are used in clinical practice but not all were specifically designed for diagnostic ascertainment. This can be confusing and no standard diagnostic SIJ MRI protocol is currently accepted worldwide. Objective To develop a standardised MRI image acquisition protocol (IAP) for diagnostic ascertainment of sacroiliitis. Methods 13 radiologist members of Assessment of SpondyloArthritis International Society (ASAS) and the SpondyloArthritis Research and Treatment Network (SPARTAN) plus two rheumatologists participated in a consensus exercise. A draft IAP was circulated with background information and online examples. Feedback on all issues was tabulated and recirculated. The remaining points of contention were resolved and the revised IAP was presented to the entire ASAS membership. Results A minimum four-sequence IAP is recommended for diagnostic ascertainment of sacroiliitis and its differential diagnoses meeting the following requirements. Three semicoronal sequences, parallel to the dorsal cortex of the S2 vertebral body, should include sequences sensitive for detection of (1) changes in fat signal and structural damage with T1-weighting; (2) active inflammation, being T2-weighted with fat suppression; (3) bone erosion optimally depicting the bone–cartilage interface of the articular surface and (4) a semiaxial sequence sensitive for detection of inflammation. The IAP was approved at the 2022 ASAS annual meeting with 91% of the membership in favour. Conclusion A standardised IAP for SIJ MRI for diagnostic ascertainment of sacroiliitis is recommended and should be composed of at least four sequences that include imaging in two planes and optimally visualise inflammation, structural damage and the bone–cartilage interface.

Objectives MRI is the gold standard for soft tissue evaluation in the hip joint. However, CT is superior to MRI in providing clear visualization of bony morphology. The aim of this study is to test the equivalency of MRI-based synthetic CT to conventional CT in quantitatively assessing bony morphology of the hip. Materials and methods A prospective study was performed. Adult patients who underwent MRI and CT of the hips were included. Synthetic CT images were generated from MRI using a deep learning–based image synthesis method. Two readers independently performed clinically relevant measurements for hip morphology, including anterior and posterior acetabular sector angle, acetabular version angle, joint space width, lateral center–edge angle, sharp angle, alpha angle, and femoral head-neck offset on synthetic CT and CT. Inter-method, inter-reader, and intra-reader reliability and agreement were assessed using calculations of intraclass correlation coefficient, standard error of measurement, and smallest detectable change. The equivalency among CT and synthetic CT was evaluated using equivalency statistical testing. Results Fifty-four hips from twenty-seven participants were included. There was no reported hip pathology in the subjects. The observed agreement based on reliability and agreement parameters indicated a strong degree of concordance between CT and synthetic CT. Equivalence statistical testing showed that all synthetic CT measurements are equivalent to the CT measurements at the considered margins. Conclusion In healthy individuals, we demonstrated equivalency of MRI-based synthetic CT to conventional CT for the quantitative evaluation of osseous hip morphology, thus obviating the radiation exposure of a pelvic CT examination. Key Points •MRI-based synthetic CT images can be generated from MRI using a deep learning–based image synthesis method. •MRI-based synthetic CT is equivalent to CT in the quantitative assessment of bony hip morphology in healthy individuals. •MRI-based synthetic CT is promising for use in preoperative diagnosis and surgery planning.