Diffusion-weighted MR imaging of abdominopelvic abscesses

This study was conducted to determine the incremental value of diffusion-weighted MR imaging (DW-MRI) over T2-weighted imaging diagnosing abdominopelvic abscesses and compare apparent diffusion coefficient (ADC) values of abscesses and non-infected ascites. In this IRB-approved, HIPAA-compliant study, two radiologists retrospectively compared T2-weighted, T2-weighted + DW-MRI and T2-weighted + contrast enhanced MR images of 58 patients (29 with abscess, 29 with ascites) who underwent abdominal MRI for abscess detection. Confidence and sensitivity was compared using McNemar’s test. ADC of abscesses and ascites was compared by t test, and a receiver operating characteristic (ROC) curve was constructed. Detection of abscesses and confidence improved significantly when T2-weighted images were combined with DW-MRI (sensitivity: observer 1—100%, observer 2—96.6%) or contrast enhanced images (sensitivity: both observers—100%) compared to T2-weighted images alone (sensitivity: observer 1—65.5%, observer 2—72.4%). All abscesses showed restricted diffusion. Mean ADC of abscesses (observer 1—1.17 ± 0.42 × 10⁻3 mm2/s, observer 2—1.43 ± 0.48 × 10⁻³ mm2/s) was lower than ascites (observer 1—3.57 ± 0.68 × 10⁻³ mm2/s, observer 2—3.42 ± 0.67 × 10⁻³ mm2/s) (p < 0.01). ROC analysis showed perfect discrimination of abscess from ascites with threshold ADC of 2.0 × 10⁻³ mm2/s (Az value 1.0). DW-MRI is a valuable adjunct to T2-weighted images diagnosing abdominopelvic abscesses. ADC measurements may have the potential to differentiate abdominal abscesses from ascites.