Challenges in the Diagnosis of Axial Spondyloarthritis Using Magnetic Resonance Imaging

Conventional radiography of the sacroiliac joints (SIJs) remains the first-line modality in the imaging diagnosis of axial spondyloarthritides including ankylosing spondylitis. However, radiographic SIJ changes occur late in the evolution of disease, delaying the radiographic diagnosis by an average of 5-9 years. Therefore, the lack of sensitivity of radiographs to early disease is a diagnostic gap that can considerably delay the initiation of appropriate treatment. This gap may be closed by magnetic resonance imaging (MRI) of the SIJs a highly sensitive, stateof-the-art imaging modalityforthe detection of early spondyloarthritis (SpA). Suitable MRI systems are nowwidely available, and the classification of SIJ changes is becoming more and more accurate. Therefore, MRI can and should be used routinely everywhere today and is no longer confined to large hospitals. It is essential for radiologists interpreting the MRI findings to be familiarwith the spectrum of abnormalities, which are subdivided into active disease processes and structural lesions. Active disease changes include para-articular osteitis, synovitis, capsulitis, and enthesitis; chronic abnormalities include erosions, subchondral sclerosis, transarticular bone bridging, and periarticularfatty lesions. Structural lesions have typical signal intensities and are found in typical locations, allowing good characterisation of these lesions. Moreover, knowledge of possible anatomic variants and differential diagnoses is relevant. If MRI is inconclusive, supplementary computed tomography may be helpful. The current discussion of new concepts (e.g. backfill of erosions) indicates that the spectrum of diagnostic options will expand further in the future. Interdisciplinary cooperation of rheumatologists and radiologists with optimal use of early diagnostic and therapeutic options will improve patient management.