MRI of Perianal Fistulas: Anatomy, Diagnosis, and Perianal Crohn Disease Treatment Monitoring.

MRI is the reference standard modality for evaluation of perianal fistulas, providing high-spatial-resolution mapping of the tract origin, course, and secondary extensions to aid classification and treatment planning. Cryptoglandular fistulas most commonly follow intersphincteric or transsphincteric pathways and may be complicated by formation of abscesses. In comparison, fistulas related to perianal fistulizing Crohn disease (PFCD) often arise near the anorectal junction, frequently extending beyond conventional classifications due to branching, high-origin involvement, and associated proctitis. In patients with PFCD, MRI facilitates serial evaluation, allowing stable tracts to be distinguished from new branching or progressive fibrosis. MRI allows differentiation of fluid-persistent tracts, which carry a higher risk of symptom relapse, from fibrosis-predominant tracts, which reflect a favorable response to therapy and a reduced risk of recurrence. Although most perianal fistulas are benign, chronic tracts can undergo malignant transformation. In PFCD, prolonged inflammation and nonhealing fistulas increase the risk of mucinous adenocarcinoma. The authors present an MRI-based approach to interpretation of fistulas, focusing on tract origin and escape pathways, and using the Parks and St James classification systems to improve radiologic assessment. Core components include defining tract trajectory, identifying internal and external openings, and recognizing secondary extensions such as abscesses or supralevator spreading. Radiologists should also assess for proctitis and monitor changes in fistula composition over time. High-spatial-resolution MRI, combined with mapping of the escape pathway and evaluation of the serial tract, improves characterization of fistulas and supports long-term monitoring in patients with cryptoglandular and PFCD fistulas. © The Author(s) 2026. Published by the Radiological Society of North America under a CC BY 4.0 license. Supplemental material is available for this article.