Preoperative MR imaging of anal fistulas: Does it really help the surgeon?

PURPOSE: To evaluate the accuracy of magnetic resonance (MR) imaging with a quadrature phased-array coil for the detection of anal fistulas and to eva luate the additional clinical value of preoperative MR imaging, as compared with surgery alone. MATERIALS AND METHODS: Fifty-six patients with anal fistulas underwent high -spatial-resolution MR imaging. Twenty-four had a primary fistula; 17, a re current fistula; and 15, a fistula associated with Crohn disease. MR imagin g findings were withheld from the surgeon until surgery ended and verified, and surgery continued when required. RESULTS: MR imaging provided important additional information in 12 (21%) o f 56 patients. In patients with Crohn disease, the benefit was 40% (six of 15); in patients with recurrent fistulas, 24% (four of 17); and in patients with primary fistulas, 8% (two of 24). The difference between patients wit h or without Crohn disease and between patients with a simple fistula versu s the rest was significant (P < .05). The sensitivity and specificity for d etecting fistula tracks were 100% and 86%, respectively; abscesses, 96% and 97%, respectively; horseshoe fistulas, 100% and 100%, respectively; and in ternal openings, 96% and 90%, respectively. CONCLUSION: High-spatial-resolution MR imaging is accurate for detecting an al fistulas. It provides important additional information in patients with Crohn disease-related and recurrent anal fistulas and is recommended in the ir preoperative work-up.