Magnetic resonance imaging (MRI) is, through its ability to directly visualize inflammatory changes in bone and soft tissues, the most sensitive imaging modality for recognizing early spine and sacroiliac joint changes in inflammatory back pain (IBP) and axial spondyloarthritis. MRI findings indicating active disease in the sacroiliac joints (sacroiliitis) include juxta-articular bone marrow oedema, and enhancement of the bone marrow and the joint space after contrast medium administration, while visible chronic changes include bone erosions, sclerosis, periarticular fatty tissue accumulation, bone spurs and ankylosis. Typical lesions of the spine, which indicate active disease, are spondylitis, spondylodiscitis, and arthritis of the facet, costovertebral and costotransverse joints. Structural changes include bone erosions, focal fat infiltration, bone spurs and/or ankylosis frequently occur. Enthesitis is also common, and may affect the interspinal and supraspinal ligaments and the interosseous ligaments in the retro-articular space of the sacroiliac joints. Some patients also have disease manifestations in peripheral joints and entheses, and these can be visualized by MRI as in other diseases [1,2].
MRI has resulted in a major improvement in the evaluation and management of patients with IBP and suspected SpA. Firstly, it permits earlier diagnosis of SpA [3-5]. Diagnosis was previously dependent on presence of bilateral moderate or unilateral severe radiographic sacroiliitis, as part of the modified New York criteria for AS . This frequently delayed the diagnosis by 7-10 years . The current Assessment of SpondyloArthritis international Society (ASAS) criteria only takes MRI into consideration in their definition . However, recent data indicate that inclusion of structural changes (bone erosion) in the definition of a “positive MRI” would increase the sensitivity without decreasing the specificity [8-9]. Thus, it would be relevant for ASAS to revisit the definition of a “positive MRI”. ASAS has also recently developed a proposal for a positive MRI of the spine in SpA .
MRI is also useful for management of patients with SpA, as MRI can provide objective evidence of currently active inflammation in patients with SpA [1,2]. Until the introduction of MRI, disease activity assessment was restricted to patient-reported outcomes, such as the Bath Ankylosing Spondylitis disease activity index (BASDAI) and functional index (BASFI) , because disease activity could not be assessed in a sensitive manner by biochemical (mainly C- reactive protein (CRP)) or physical evaluation [12,13]. Finally, recent data have documented that vertebral corner inflammation has predictive value with subsequent development of radiographic syndesmophytes [14-16].
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