Response monitoring of perianal fistula in Crohn’s disease on MRI

The assessment of treatment response in perianal fistulising Crohn’s disease (pfCD) has shifted from purely clinical evaluation to a combined clinical and radiological approach. MR imaging is the reference standard for evaluating pfCD and has become essential in monitoring treatment response. However, response monitoring is subjective and heterogeneous amongst radiologists, with no clear guidance or radiological criteria. Recently, a global expert group (TOpCLASS) defined a standardised definition for radiologically ‘healing’ and ‘healed’ fistulas. Criteria for radiologically healing/improving fistula include reduction in T1- and/or T2-hyperintensity, diameter, length, decrease or complete absence of previously present abscesses, and increasingly fibrotic tracts. A radiologically healed fistula is defined as the absence of hyperintensity on all sequences, reflecting a completely fibrotic tract, which correlates with sustained clinical remission. In addition, the timing to assess treatment response monitoring on the follow-up MRI is crucial, since early imaging may not accurately reflect treatment response. These standardised MRI criteria and timing for treatment response monitoring facilitate communication between radiologists and clinicians globally. To further enhance objective assessment, emerging quantitative tools such as 3D volumetric measurements are being explored, although further validation is required before routine clinical implementation. The aim of this review is to provide an overview with practical clinical examples of treatment response monitoring for perianal fistulising Crohn’s disease using MR imaging, addressing potential challenges in their interpretation and practical considerations for clinical practice. Question Treatment response monitoring for perianal fistulising Crohn’s disease on MRI is subjective and heterogeneous amongst radiologists, with no clear guidance or radiological treatment response criteria. Findings Recently defined MRI criteria for ‘healing’ and ‘healed’ fistulas standardize treatment response monitoring in daily practice and facilitates communication between radiologists and clinicians globally. Relevance statement This review discusses the different stages of treatment response assessment for perianal fistulising Crohn’s disease on MR imaging with practical clinical examples.