Early Detection of Sacroiliitis on Magnetic Resonance Imaging and Subsequent Development of Sacroiliitis on Plain Radiography. A Prospective, Longitudinal Study

Objective: To investigate the diagnostic value of magnetic resonance imaging (MRI) in the detection of early sacroiliitis.

Methods: Twenty-five consecutive HLA-B27 positive patients with inflammatory low back pain and < or = grade 2 unilateral sacroiliitis on conventional radiography (modified New York criteria) were studied. Erythrocyte sedimentation rate, C-reactive protein, plain radiography (PR), and MRI of the sacroiliac (SI) joints were obtained at study entry and PR of the SI joints after 3 years. Each radiograph and MR image set was interpreted independently. SI joints were scored according to the modified New York Criteria for radiological sacroiliitis. MRI scans were also scored for the presence of subchondral marrow edema. The relationship between > or = grade 2 sacroiliitis (by modified New York criteria for radiological sacroiliitis) shown on MRI and the subsequent development of > or = grade 2 sacroiliitis on PR after 3 years was investigated.

Results: At study entry > or = grade 2 sacroiliitis was found on MRI in 36 of 50 SI joints. Edema was found in 20 of 50 SI joints. After 3 years > or = grade 2 sacroiliitis was found on PR in 21 of 44 SI joints. The positive predictive value of > or = grade 2 sacroiliitis on MRI for the development of > or = grade 2 sacroiliitis on PR after 3 years was 60%; sensitivity was 85% and specificity 47%.

Conclusion: Our data suggest that MRI of the SI joints can be used to identify sacroiliitis earlier than PR.