Role of Phased Array MRI in the Diagnosis and Presurgical Stadiation of Perianal Fistulas

Purpose: To assess the value of MRI performed with phased array coil in the diagnosis and presurgical staging of perianal and anal fistulas. Methods and Materials: 34 patients (20 Crohn disease) with clinical evidence or suspicion of anal fistulas underwent pelvic MRI (SIEMENS NUMARIS 1.5 T) performed with phased array coil. Images were obtained in the axial and coronal planes using TSE T2w high resolution sequences (matrix 512x512) with and without FS, T2 HASTE and T1w FLASH sequences with FS, before and after gadolinium enhancement. In 10 cases the cutaneus orificium was evident and fistulas were injected by Gadolinium. The following parameters were considered: presence of a fistula and relationship with the sphincters and elevator ani and presence of abscesses or complications. All patients underwent surgery, considered the gold standard. MRI and surgical findings were assessed using the Parks fistula-in-ano classification and the St. James MR imaging classification of perianal fistulas. Results: MRI documented no evidence of fistula in 3 patients, intersphinteric fistulas in 9 (grade 1 and 2 St. James), transphinteric fistulas in 16 (grade 3 and 4 St. James), transelevator in 2 (grade 5 St. James) and complex ano-rectum-vaginal fistulas in 4. 1 case of transelevator fistula was not found having relationship with the sphincters. Abscesses were found in 12 cases and secondary tracts in 8 cases. Surgery demonstrated intersphinteric fistulas in the 3 negative at MRI (false negatives). Concordance with surgery was 88% with high sensitivity and specificity. Conclusion: MRI is an accurate technique for identifications and classifications of the anal and perianal fistulas and their complications. Phased array coil, in our experience, offers both elevated field of view and spatial resolution, able to demonstrate perianal pathology