Can erosions on MRI of the sacroiliac joints be reliably detected in patients with ankylosing spondylitis? - A cross-sectional study

Four readers independently assessed T1-weighted and short tau inversion recovery sequence (STIR) images of the SIJ from 30 AS patients and 30 controls (15 patients with non-specific back pain and 15 healthy volunteers) ≤45 years old. Erosions, EE, and BF were recorded according to standardized definitions. Reproducibility was assessed by percentage concordance among six possible reader pairs, kappa statistics (erosion as binary variable) and intraclass correlation coefficient (ICC) (erosion as sum score) for all readers jointly.SIJ erosions were detected in all AS patients and six controls by ≥2 readers. The median number of SIJ quadrants affected by erosion recorded by four readers in 30 AS patients was 8.6 in the iliac and 2.1 in the sacral joint portion (P < 0.0001). For all 60 subjects and for all four readers, the kappa value for erosion was 0.72, 0.73 for EE, and 0.63 for BF. ICC for erosion was 0.79, 0.72 for EE, and 0.55 for BF, respectively. For comparison, the kappa and ICC values for bone marrow edema were 0.61 and 0.93, respectively.Erosions can be detected on MRI to a comparable degree of reliability as bone marrow edema despite the significant heterogeneity of their appearance on MRI.Erosions of the sacroiliac joints (SIJ) on pelvic radiographs of patients with ankylosing spondylitis (AS) are an important feature of the modified New York classification criteria [1]. However, SIJ erosions are often difficult to identify on pelvic radiographs and training to recognize radiographic structural changes of the SIJ did not improve the performance of radiologists and rheumatologists in detecting radiographic sacroiliitis [2]. A comparison of SIJ radiographs with computed tomography (CT) scans showed a higher sensitivity of CT scans to detect structural changes indicative of sacroiliitis (86% versus 72%), but the same specificity (84%) [2]. However, assessment of SIJ erosions by CT in clinical practice is limited given two recent reports consistently indicating