CLINICAL AND EPIDEMIOLOGICAL RESEARCH

Extended report

Defining active sacroiliitis on magnetic resonance imaging (MRI) for classification of axial spondyloarthritis: a consensual approach by the ASAS/OMERACT MRI group

M Rudwaleit¹, A G Jurik², K-G A Hermann³, R Landewé⁴, D van der Heijde⁵, X Baraliakos⁶, H Marzo-Ortega⁷, M Østergaard⁸, J Braun⁶, J Sieper¹

¹ Department of Rheumatology, Charité University Medicine, Campus Benjamin Franklin, Berlin, Germany
² Department of Radiology, Aarhus University Hospital, Aarhus, Denmark
³ Department of Radiology, Charité University Medicine, Berlin, Germany
⁴ Department of Rheumatology, University of Maastricht, Maastricht, The Netherlands
⁵ Department of Rheumatology, Leiden University Medical Centre, Leiden, the Netherlands
⁶ Rheumazentrum Ruhrgebiet, Herne, Germany
⁷ Section of Musculoskeletal Disease, Leeds Institute of Molecular Medicine, University of Leeds, Leeds, UK
⁸ Departments of Rheumatology, Copenhagen University Hospitals at Hvidovre and Gentofte, Copenhagen, Denmark

^{Correspondence to} Dr M Rudwaleit, Charité - Universitätsmedizin Berlin, Campus Benjamin Franklin, Rheumatologie, Med Klinik I, Hindenburgdamm 30, 12203 Berlin, Germany; martin.rudwaleit{at}charite.de

Background: Magnetic resonance imaging (MRI) of sacroiliac joints has evolved as the most relevant imaging modality for diagnosis and classification of early axial spondyloarthritis (SpA) including early ankylosing spondylitis.

Objectives: To identify and describe MRI findings in sacroiliitis and to reach consensus on which MRI findings are essential for the definition of sacroiliitis.

Methods: Ten doctors (two radiologists and eight rheumatologists) from the ASAS/OMERACT MRI working group reviewed and discussed in three workshops MR images depicting sacroiliitis associated with SpA and other conditions which may mimic SpA. Descriptions of the pathological findings and technical requirements for the appropriate acquisition were formulated. In a consensual approach MRI findings considered to be essential for sacroiliitis were defined.

Results: Active inflammatory lesions such as bone marrow oedema (BMO)/osteitis, synovitis, enthesitis and capsulitis associated with SpA can be detected by MRI. Among these, the clear presence of BMO/osteitis was considered essential for defining active sacroiliitis. Structural damage lesions such as sclerosis, erosions, fat deposition and ankylosis can also be detected by MRI. At present, however, the exact place of structural damage lesions for diagnosis and classification is less clear, particularly if these findings are minor. The ASAS group formally approved these proposals by voting at the annual assembly.

Conclusions: For the first time, MRI findings relevant for sacroiliitis have been defined by consensus by a group of rheumatologists and radiologists. These definitions should help in applying correctly the imaging feature "active sacroiliitis by MRI" in the new ASAS classification criteria for axial SpA.

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