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Background
Axial spondyloarthritis (axSpA) is a chronic inflammatory disease primarily affecting the spine and sacroiliac (SI) joints. Over the last 60 years, there have been significant changes in the terminology, classification criteria, and diagnostic tools used in the evaluation of patients with suspected axSpA. The introduction of highly effective biologics that can relieve symptomatology and mitigate progression with early intervention has emphasized the need for early diagnosis. However, current clinical tools that suggest the diagnosis of axSpA (clinical symptomatology, acute phase reactants, Human Leukocyte Antigen-B27) lack specificity. Radiography remains the most widely used imaging technique for diagnostic evaluation, but it lacks sensitivity and has poor interobserver reliability. To avoid delays in appropriate treatment, the Assessment of Spondyloarthritis International Society (ASAS) classification criteria were developed, which clearly defines sacroiliitis on magnetic resonance imaging (MRI).

Educational Goals / Teaching Points
This exhibit will present an overview of axSpA and demonstrate how imaging plays a key role in its diagnosis and classification. The difference between diagnosis and classification criteria will be emphasized. We will discuss the evolution of the classification criteria focusing on the imaging arm of the current ASAS classification criteria and what defines a positive MRI. We will review the ongoing research by the ASAS MRI Working Group, which encompasses the reappraisal and validation of ASAS MRI lesion definitions. Early results show an increase in sensitivity and specificity of the findings for a positive MRI. Finally, we will review the role of computed tomography (CT).

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
Historically, radiography was the main imaging modality to aid in the diagnosis of the axSpA. However, radiographic signs can be delayed, which limits radiography’s role due to its low sensitivity. MRI has been shown to mitigate these limitations by aiding in the earlier detection due to its ability to demonstrate active and structural lesions of the SI joints, including bone marrow edema (BME), erosions, and fat metaplasia. BME is the most important finding. Two or greater subchondral BME lesions on a single MRI slice or one or more subchondral BME lesions on two consecutive slices results in a positive MRI. Recent research suggests that CT may play an increasing role in the evaluation of patients with axSpA. New techniques, specifically low-dose CT and dual energy CT have shown promising results. CT has advantages of better demonstrating the complex anatomy of the SI joints and allows for better evaluation of erosions with dual energy CT able to delineate BME. These new techniques provide an alternative to MRI for patients who have limited access or contraindications to MRI.

Conclusion
As radiology plays an increasingly vital role in the early diagnosis of axSpA, understanding the current imaging findings will help standardize reporting for radiologists. Being familiar with the ongoing research and emerging technology will help radiologists stay up to date in the evolving realm of axSpA.