Retrospective Case-Control Study for Developing an Artificial Intelligence (AI) Tool for Lesion Detection Using Magnetic Resonance Imaging (MRI) and Clinical Variables for Early Diagnosis of Axial Spondyloarthritis (axSpA)

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• Another known pathology in the SIJ which may influence the MRI interpretation.
• Unreadable MRI images or with insufficient diagnostic quality.

• Fulfillment of the ASAS criteria for axSpA (i.e. MRI and/or clinical arm, Rudwaleit M, et al., Ann Rheum Dis. 2009 Oct;68(10):1520-7).
• axSpA diagnosis according to the physician.
• Fulfillment of the following MRI criteria (a-c must be fulfilled):
• a) Availability of MRI images of the sacroiliac joint (SIJ) including T1- weighted and STIR and/or T2 Fat-Sat (FS) sequences in coronal-oblique planes.
• b) Fulfillment of ASAS criteria for positive MRI:
• b.1)Bone marrow oedema (BMO) on a T2-weighted sequence sensitive for free water (such as short tau inversion recovery [STIR] or T2FS) or bone marrow contrast enhancement on a T1-weighted sequence (such as T1FS post-gadolinium).
• b.2) Inflammation must be clearly present and located in a typical anatomical area (subchondral bone).
• b.3) MRI appearance must be highly suggestive of SpA.
• b.4) Other findings can be present but are not required for a positive MRI (erosion and/or fat metaplasia and/or ankylosis).
• b.5) Fulfillment of ASAS criteria for positive MRI.
• c) MRI expert opinion of having axial SpA.
• Buttock and/or low back pain due to known trauma, stress, disc herniation with an MRI of SIJ showing absence of bone marrow edema, fatty infiltration and bone erosions and not fulfilling ASAS axSpA classification criteria.
• Availability of MRI images of SIJ including T1-weighted and STIR and/or T2 Fat-Sat sequences in coronal-oblique planes.