Perianal Imaging in Crohn Disease: Current Status With a Focus on MRI, From the AJR Special Series on Imaging of Inflammation.

Fistulizing perianal disease is a frequent, chronic, and often debilitating manifestation of Crohn disease (CD) in adults and children, which in the past was relatively refractory to treatment. The advent of biologic agents such as anti-tumor necrosis factor-α and cellular therapies, used in conjunction with a range of surgical interventions, has greatly improved disease outcomes, although complete remission can still be elusive. This review considers current perianal imaging options, specifically pelvic MRI and endoanal and transperineal ultrasound, and their roles in diagnosis, management, and assessment of treatment response. Pelvic MRI is the first line technique for imaging of perianal CD given the complexity of fistulas encountered in CD. MRI technical acquisition parameters for adults and children and an approach to MRI interpretation and reporting are provided. Anatomic classification systems for fistulising perianal disease are presented. We also explore the history, current landscape, and future developments of MRI features of perianal disease as imaging biomarkers to quantify activity and severity, and consider CD MRI-based inflammatory activity scores. We discuss the reliability and validation of a number of indices (including PEMPAC, MAGNIFI-CD, mVAI, and Van Assche Index), their potential to quantify treatment response, and possible prognostic capabilities.