Comparison of MRI appendix biometrics in children with and without acute appendicitis.

OBJECTIVES: The goal of this study is to improve MRI-specific diagnostic criteria for pediatric appendicitis through comparison of normal and abnormal appendix-related imaging features.

METHODS: A retrospective multireader-multicase design was used, including non-contrast MRI performed for suspected pediatric appendicitis following non-diagnostic US from January 2014 to December 2017. Positive diagnosis was defined by surgical pathology or symptom resolution after antibiotics. Four pediatric radiologists independently graded study biometrics while blinded to clinical data. Balanced complete block design was used to determine performance characteristics.

RESULTS: Global diagnosis of appendicitis (208 studies) had sensitivity 90.6% and specificity 97.7%. Median appendix diameter was 10.4 mm among positive cases and 5.8 mm among negative cases (p < 0.001) with an optimal diagnostic cutoff of 7.5 mm (sensitivity 89.4%, specificity 86.5%). Median appendix wall thickness was 2.6 mm among positive cases and 1.7 mm among negative cases (p < 0.001) with an optimal diagnostic cutoff of 2.3 mm (sensitivity 63.1%, specificity 82.9%). Performance characteristics for qualitative appendix features included distinguishable appendix luminal signal (sensitivity 89.6%, specificity 83.7%), intraluminal fluid-signal intensity (sensitivity 63.6%; specificity 52.3%), intraluminal signal intermediate between fluid and bowel wall (sensitivity 91.0%; specificity 37.1%), appendicolith (sensitivity 34.9%; specificity 100.0%), intraluminal layering (sensitivity 25.9%; specificity 100.0%), hyperintense appendix wall signal (sensitivity 31.7%; specificity 100.0%), periappendiceal fluid (sensitivity 66.8%; specificity 72.5%), periappendiceal fatty edema (sensitivity 91.3%; specificity 94.5%), and free pelvic fluid (sensitivity 88.5%; specificity 26.0).

CONCLUSIONS: This study provides MRI-specific performance of pediatric appendicitis quantitative and qualitative biometrics with peri-appendiceal fatty edema, appendix diameter > 7.5 mm, and distinguishable appendix luminal signal demonstrating the highest overall accuracy.

KEY POINTS: • This retrospective multireader-multicase study characterized magnetic resonance imaging-specific diagnostic accuracy of quantitative and qualitative biometrics for pediatric appendicitis. • The optimal quantitative diagnostic thresholds for an abnormal pediatric appendix at MRI included diameter and wall thickness of 7.5 mm and 2.3 mm, respectively. • Qualitative imaging biometrics with high specificity for pediatric appendicitis on MRI included the presence of distinguishable appendix lumen signal from wall signal, appendicolith, intraluminal fluid-fluid layer, appendix wall hyperintensity, and peri-appendiceal fatty edema.