Probing Perianal Fistulas with Diffusion-Weighted Magnetic Resonance Imaging: Insights into Activity and Architecture

Magnetic resonance imaging (MRI) is the imaging modality of choice for evaluation of perianal fistulas. With the emergence of newer treatment modalities, it is imperative to assess fistula “activity,” along with its architecture, for effective management. The objectives of this study were to estimate the cut-off for the mean apparent diffusion coefficient (ADC) value to differentiate between active and inactive fistulas and to assess the role of diffusion-weighted imaging (DWI) in the characterization of perianal fistulas. MRI examination was performed on 54 patients using a 3T MRI system. Combined T2W + DWI and contrast-enhanced MRI (CE-MRI) images were evaluated for the visibility of fistulas. The ADC value was measured using region-of-interest placement within the fistula tract. Fistulas were designated as active or inactive based on surgical and/or clinical findings. Data were analyzed using standard statistical methods. Combined T2W + DWI and CE-MRI showed substantial agreement in visualization of fistulas, their internal openings, secondary tracts, and associated abscesses (p < 0.01). Inactive fistulas had significantly higher ADC values than active fistulas (p < 0.01). A cut-off of ADC value ≤1.170 × 10−3 mm2/s predicted active fistula with a sensitivity of 92.7%, a specificity of 85.7%, and a diagnostic accuracy of 82%. Addition of DWI to T2-weighted imaging is a logistically feasible way of assessing fistula activity, along with optimal evaluation of the fistula architecture. DWI obviates the requirement of gadolinium-based contrast administration.