MRI Evaluation of Axial Spondyloarthritis: From Early Diagnosis to Therapeutic Response Assessment.

Magnetic resonance imaging (MRI) has reshaped the evaluation of axial spondyloarthritis (axSpA), which comprises radiographic axSpA (historically ankylosing spondylitis) and non-radiographic axSpA, by demonstrating active inflammation and structural lesions before definitive radiographic sacroiliitis. Its clinical value is not the isolated detection of bone marrow edema (BME). MRI is most useful when acquisition is standardized, active and structural domains are reported separately, and findings are interpreted against clinical probability and mimics. This narrative review provides a practical framework for radiologists and rheumatologists. We distinguish classification from diagnosis; separate adult and pediatric pathways; summarize the Assessment of SpondyloArthritis International Society-Spondyloarthritis Research and Treatment Network (ASAS-SPARTAN) four-sequence sacroiliac joint protocol; compare the Spondyloarthritis Research Consortium of Canada (SPARCC), Berlin, and Canada-Denmark (CANDEN) scores; and provide four author-proposed reporting categories: "Typical for axSpA," "Suspicious/equivocal," "Nonspecific," and "Alternative diagnosis favored." Recent TRACE and COAST-V data show that inflammation can decrease markedly within 4 weeks and that erosion reduction, fat metaplasia, and backfill may begin during the same early interval or by 16 weeks; follow-up MRI may therefore depict overlapping suppression of inflammation and early structural remodeling rather than strictly sequential phases. However, MRI improvement is not a validated surrogate for long-term structural or patient-reported outcomes, and current guidelines do not support routine serial imaging. Advanced sequences, synthetic bone imaging, low-dose CT integration, and artificial intelligence remain limited by protocol heterogeneity, gaps in external validation, reader variability, and uncertain clinical utility. MRI should reduce diagnostic delay without assigning disease labels to nonspecific lesions; formal scoring and repeat imaging should be used only when results are likely to change management.