Imaging in Axial Spondyloarthritis

The primary use of imaging focuses on diagnostic evaluation, and pelvic radiography is still the primary modality for evaluation of disease in the sacroiliac joints. Its unreliability and inability to directly assess inflammation has led to increasing use of MRI, which directly assesses inflammatory changes as well as the structural changes associated with inflammation. Both radiography and MRI have therefore been incorporated into new classification criteria designed to capture both early and established spondyloarthritis. Fat metaplasia on T1-weighted MRI is an important intermediary tissue on the pathway from inflammation to ankylosis in both the sacroiliac joints and spine.

Key points

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For radiographic evaluation of the sacroiliac joints a single view is sufficient and additional imaging increases radiation without enhancing diagnostic sensitivity.
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Bone marrow edema on fat-suppressed MRI of the sacroiliac joints occurs before radiographic abnormalities but may occur nonspecifically in about 20% of patients with mechanical back pain and in healthy individuals.
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Erosion on T1-weighted MRI of the sacroiliac joints is more specific but less sensitive than bone marrow edema for spondyloarthritis and may be observed early after symptom onset.
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Fat metaplasia on T1-weighted MRI of the sacroiliac joints is characterized by a distinct border and proximity to subchondral bone but seldom occurs in the absence of other lesions.
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Pelvic MRI is usually sufficient for routine diagnostic evaluation of spondyloarthritis but spinal imaging may be indicated in the setting of localized spinal symptoms.