Diffusion‐Weighted Imaging for the Evaluation of the Sacroiliac Joint in Pediatric Patients

Background Maturational signal in the sacroiliac joint (SIJ) of skeletally immature youth is often misinterpreted as inflammation. Diagnostic tools that improve specificity are greatly needed. Apparent diffusion coefficient (ADC) values from diffusion‐weighted imaging (DWI), when used with standard imaging, may enhance diagnostic accuracy. We aimed to define normative pediatric ADC values and establish thresholds to distinguish normal from inflammatory SIJ signals. Methods ADC values were measured using circular regions of interest (ROI) on the anterior, central, and posterior slices of the cartilaginous SIJs (36 total ROIs). Mean ADCs were analyzed by age group, bone (iliac or sacral), and joint height (superior, mid, inferior), accounting for within‐subject clustering. In sacroiliitis cases, ROIs were placed on DWI at sites of increased signal on fluid‐sensitive sequences. Thresholds differentiating normal and inflammatory signals were derived by age, bone, and joint height (ilium only), and assessed by area under the receiver operating characteristic (AUROC) and specificity. Results The reference group included 86 youth. Inferior ilium ADC values were higher than mid/superior regions in all immature age groups (all p<0.0001) and decreased with age (p=0.0001). Sacral ADCs also declined with age (p=0.0001). No age trend was observed in the superior/mid ilium (p=0.14). ADC thresholds distinguished normal from inflammatory signals with AUROC ≥0.90 in most iliac regions, except the peri‐pubertal inferior ilium (AUROC 0.78). Sacral thresholds performed acceptably (AUROC ≥0.77), though were lower in the pre‐pubertal group (AUROC 0.68). Conclusion Age‐ and bone‐specific ADC reference values were established and effectively differentiated normal from inflammatory SIJ signals.