Comparison of Radiography and MRI to Detect Structural Lesions of the Sacroiliac Joints: The SIMACT Study

Introduction: Magnetic resonance imaging (MRI) can detect osteitis and thus has a crucial role in the early diagnosis of axial spondyloarthritis, although its performance in identifying structural lesions of the sacroiliac joints is disputed. Radiography is considered the imaging modality of first choice, although several investigators have reported limited interrater reliability. This study compares the diagnostic performance of X-ray and MRI in the detection of structural lesions using computed tomography as the standard of reference.

Materials and Methods: A total of 110 patients with chronic low back pain were included, and three blinded readers rated radiographs, T1-weighted MRI images, and low-dose computed tomography (ldCT) scans for the presence of chronic structural lesions (erosions, joint space changes, sclerosis, and overall positivity). Positivity for each of these lesions was defined, and a contingency analysis was performed. The ldCT was used as the standard of reference for all types of lesions.

Results: An experienced rheumatologist made the diagnosis of axial spondyloarthritis in 53% of the study population (mean age: 36.1 years; range: 19–57); the remaining 47% had other final diagnoses. T1-weighted MRI had higher sensitivity than X-ray for the detection of erosions (79% vs 42%), joint space changes (75% versus 41%), and overall positivity (85% versus 48%); specificity of both modalities was similar (> 80% for both modalities). MRI was inferior to X-ray only for the detection of sclerosis (sensitivity of 30% versus 70%).

Conclusion: MRI is superior to radiography in the detection of structural lesions in patients with suspected axial spondyloarthritis. Further studies should try and establish reliable parameters for positive MRI findings based on the modified New York criteria.