The sacroiliac joints are involved in most cases of axial spondyloarthropathy, the first manifestation usually being sacroiliitis. A finding of sacroiliitis at radiography is the classic diagnostic hallmark of axial spondyloarthropathy. However, radiographic changes reflect structural damage rather than active inflammation, which may delay the diagnosis by several years. In the past decade, the field of spondyloarthropathy has undergone major changes, largely driven by the development of new drugs for the treatment of ankylosing spondylitis. In recent years, the Assessment of SpondyloArthritis international Society has focused on the reassessment of existing classification criteria and the development and validation of diagnostic tools to facilitate early diagnosis and assessment of treatment response. Magnetic resonance (MR) imaging is the most recent innovation and the important change with respect to the previously established classification criteria. This modality has become an integral part of managing patients with sacroiliitis. MR imaging can serve as a biomarker of disease activity, allows monitoring, and can provide guidance for the treatment of affected patients, and it will likely become even more central to the care of these patients. Familiarity with the anatomy, anatomic variants, and physiologic changes of the sacroiliac joints is important for correctly interpreting findings and avoiding misdiagnosis.

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