Objective. To develop a new grading method to quantify activity and chronic spondylarthritis (SpA) changes in the sacroiliac (SI) joints identified by magnetic resonance imaging (MRI), taking into account the complex joint anatomy, and to compare the findings with radiographic changes. Methods. A total of 37 patients (15 men, 22 women) fulfilling the European Spondylarthropathy Study Group criteria for SpA were examined by MRI of the SI joint using a semicoronal T1-weighted and T1-weighted fat-saturated (T1FS) sequence, a semiaxial STIR sequence, and postcontrast semicoronal and semiaxial T1FS sequences. The images were assessed independently by 2 radiologists for presence of bone marrow edema and enhancement, fatty marrow deposition, and joint erosion at both the cartilaginous and the ligamentous joint portion. Conventional radiographs were scored in accordance with the modified New York criteria. Results. Inter-and intraobserver agreements for grading activity and chronic changes were good, with kappa values between 0.72 and 0.86 and between 0.83 and 0.90, respectively. Nearly half of the disease activity changes were due to inflammation in the ligamentous joint portion, which was significantly related to ankylosing spondylitis (AS). Patients with AS had significantly higher erosion and fatty marrow deposition scores than patients with nonprimary AS forms of SpA. The degree of erosion by MRI was significantly related to radiographic stages. Conclusion. Separate assessment of the ligamentous joint portion by MRI may be valuable for differentiating AS from nonprimary AS forms of SpA. Erosive changes on MRI were significantly related to radiographic changes, implying a possibility for substituting radiography with MRI.

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Active bone marrow abnormalities were detected nearly equally well with STIR and Gd-enhanced fat-suppressed T1 sequences in patients with SpA, with STIR being most sensitive to visualize active abnormalities in the periphery of chronic changes.

The occurrence of manifest SIJ activity by MRI or chronic changes at baseline was related to progression of chronic changes and the presence of AS at followup.

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