A radiology trainee's guide to perianal fistulas

Perianal fistulas are abnormal epithelialized connections between the anal canal to the perineal skin. Magnetic resonance imaging (MRI) has become the gold standard for both pre- and postoperative evaluation, offering excellent soft tissue contrast and superior delineation of fistulous tracts, sphincteric involvement, tract extensions, abscesses, and associated complications. Despite its diagnostic value, interpretation of perianal fistula MRI can be challenging, particularly for radiology trainees, as it requires detailed understanding of complex perianal anatomy, as well as familiarity with optimized MRI acquisition planes and interpretation techniques. This pictorial review aims to provide a structured and practical approach to perianal fistulous disease on MRI. It highlights key imaging features of fistulas, illustrates essential anatomic landmarks of the anal canal and surrounding spaces, and reviews current pathogenesis theories of fistula formation to serve as foundational knowledge. The Parks classification system will be systematically reviewed with case examples and illustrations. Finally, key reporting considerations are presented using a stepwise, trainee-oriented approach, providing a structured MRI interpretation framework to promote accurate and consistent assessment, improve communication with surgeons, and support standardized reporting in clinical practice.