Value of dedicated small-field-of-view sacroiliac versus large-field-of-view pelvic magnetic resonance imaging for evaluating pediatric sacroiliitis

Background: Optimized MRI parameters can be leveraged to improve signal intensity, accelerate imaging acquisition and increase resolution. Higher-resolution imaging with a small field of view (FOV) has been proposed as standard practice for investigating sacroiliac (SI) joints, but the improvement in disease detection and characterization over pelvic imaging with large FOV has not been established. Objective: The purpose of this study was to compare dedicated MR images of the SI joints with survey imaging (large-FOV pelvic MRI) for detecting sacroiliitis. Materials and methods: Fifty-eight pediatric patients suspected of having sacroiliitis underwent dedicated sacroiliac joint and survey pelvic imaging at the same imaging session. We independently evaluated the small- and large-FOV image data sets for presence or absence of sacroiliitis, e.g., bone marrow edema, erosions and synovitis. We used nonparametric statistical tests to compare lesion scores for severity of inflammation. We created test characteristics for the survey pelvic images (low-resolution images of the sacroiliac joints) using dedicated sacroiliac images (small-FOV, high-resolution images) as the gold standard. Results: Dedicated sacroiliac small-FOV MRI detected more sacroiliitis compared to survey pelvic imaging with large FOV (χ²=6.125, P=0.013). Readers detected significantly more features of inflammation on small- compared to large-FOV images, e.g., erosions (P=0.039), synovitis (P=0.009), sclerosis (P=0.017) and osteitis (P=0.001). Test characteristics for pelvic large-FOV imaging were sensitivity=0.76, specificity=1.00, positive predictive value = 1.00 and negative predictive value = 0.75. Conclusion: This study provides test characteristics for survey pelvic MRI with lower-resolution large-field-of-view images as a screening tool for detecting sacroiliitis. Pelvic screening studies with large FOV have lower sensitivity, and dedicated sacroiliac MRI with small FOV is superior in detecting sacroiliitis when compared to pelvic screening MRI.