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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., Competing Interests: Competing interests: RGWL: consulting fees from CARE Arthritis and Image Analysis Group. XB: contract with Novartis; Consulting fees from Amgen, Bristol-Myers Squibb, Chugai, Eli Lilly, Galapagos, Janssen, Merck Sharp & Dohme, Novartis, Pfizer, Roche, Sandoz, Sanofi, and UCB; Payment or honoraria from Amgen, Bristol-Myers Squibb, Chugai, Eli Lilly, Galapagos, Janssen, Merck Sharp & Dohme, Novartis, Pfizer, Roche, Sandoz, Sanofi, and UCB; Meeting support from Eli Lilly, Janssen, Novartis, Pfizer and UCB; Participation on a Data Safety Monitoring Board or Advisory Board: Amgen, Bristol-Myers Squibb, Eli Lilly, Galapagos, Janssen, Merck Sharp & Dohme, Novartis, Pfizer, Roche, Sandoz, Sanofi and UCB; Leadership role: Editorial Board Member of Annals of Rheumatic Diseases, ASAS President, EULAR President Elect. SAB: Royalties from Elsevier. JAC: Consulting fees from AstraZeneca and Covera Health; Participation on a Data Safety Monitoring Board or Advisory Board: Carestream, Image Analysis Group, Image Biopsy Lab; Leadership role: RSNA, ACR, IAOAI. TD: Grants or contracts from Berlin Institute of Health (BIH); Payment or honoraria from Berlinflame, Canon Medical Systems, Janssen, MSD, Novartis, UCB. IE: Payment or honoraria from Lilly, Novartis. KGH: Payment or honoraria from MSD, AbbVie Novartis; Cofounder of BerlinFlame. NH: None declared. JJ: Stock in Exo. LJ: None declared. AGJ: None declared. JMDO'N:– None declared. MR: ISS Seed Grant; Consultant for ASAS. MJT: Consulting fees from GE HealthCare; Meeting support from International Skeletal Society; Leadership role—President-elect International Skeletal Society. WPM: Grants or contracts from Abbvie, BMS, Eli-Lilly, Pfizer, UCB; Consulting fees from Abbvie, Celgene, BMS, Eli-Lilly, Galapagos, Pfizer, UCB; Payment or honoraria from Abbvie, Janssen, Pfizer, Novartis; Leadership role—SPARTAN Board of Directors; Chief Medical Officer, CARE Arthritis., (© Author(s) (or their employer(s)) 2024. Re-use permitted under CC BY-NC. No commercial re-use. See rights and permissions. Published by BMJ on behalf of EULAR.)

Objectives: Apply a modified Delphi-based approach and produce a practical, radiology-specific set of definitions for interpretation and standardization of the multiple MRI findings in axial spondyloarthritis (ax-SpA), specifically to aid the general radiologist with a musculoskeletal interest, working with gold standard basic MRI protocols., Materials and Methods: We report the results of a modified Delphi-based consensus of 35 experts from 13 countries in the Arthritis Subcommittee of the European Society of Musculoskeletal Radiology (ESSR). Seventeen definitions were created (i.e., nine for the spine and eight for the sacroiliac joint) and two Delphi rounds were conducted on an electronic database, collated and revised by the project leader with agreement. Group leads were appointed for each definition following the first round. Final definitions included only those that reached a consensus > 80%; if > 50% agreed on exclusion consensus, definitions were excluded. Final results have been shared during the Arthritis meeting at the Annual ESSR Congress., Results: Fourteen definitions, eight for the spine and six for the sacroiliac joint were agreed for standardized reporting. Andersson's, anterior corner sclerotic and costovertebral joint inflammatory lesions of the spine, with active and non-active erosions, and fat metaplasia of the sacroiliac joint reaching the highest consensus (≥ 95%). More than 50% of the experts agreed to exclude joint space inflammation in the sacroiliac joint and tissue backfill. Syndesmophytes reached 76% agreement., Conclusions: Agreed definitions by expert radiologists using a modified Delphi process, should allow standardized actionable radiology reports and clarity in reporting terminology of ax-SpA., Clinical Relevance Statement: The proposed definitions will support reporting from musculoskeletal and general radiologists working with gold-standard basic MRI, improve confidence in lesion assessment, and standardize terminology to provide actionable reports on MRI in patients with ax-SpA., Key Points: Experts applied a modified Delphi method to optimize the definitions of MRI findings of ax-SpA. After two Delphi rounds and one in-person meeting, fourteen definitions reached the agreement threshold. These consensus-based definitions will aid in actionable reporting specifically for the general radiologist with a musculoskeletal interest., (© 2024. Crown.)