How I Do It: MRI Approach to Perianal Fistulas.

Perianal fistula is a common, miserable, and often embarrassing condition that tends to recur. Most cases result from chronic infection of the anal glands (cryptoglandular), leading to an internal opening in the anal canal and an external opening that discharges fluid. Pelvic MRI has become essential for its management because it helps accurately classify the type and extent of the fistula, enabling optimized preoperative planning before surgery. It is particularly useful for perianal fistulas in patients with Crohn disease, in whom infections are often extensive. Examination involves imaging in three planes aligned with the anal canal axis, using T2-weighted sequences without fat saturation and at least one T2-weighted sequence with fat saturation; intravenous contrast material can also be used. Easy-to-use imaging criteria have been proposed to assess treatment response and healing in daily practice, while disease activity scores are primarily used for clinical research. Radiologists should be aware of mimics, such as pilonidal sinus and hidradenitis suppurativa, and recognize that cancers can rarely develop in fistulas. Reports should be tailored to the needs of the referring clinician rather than being merely descriptive.