- Magnetic resonance (MR) imaging has proved useful in the evaluation of perianal and perirectal lesions resulting from Crohn disease. On T1-weighted MR images, sinus tracts and fistulas are hypointense due to their fluid content; on T2-weighted images, their signal intensity depends on their fluid content and the degree of surrounding fibrosis. Other pathologic entities, such as abscesses in the ischioanal fossa, may become evident at MR imaging even though they remain hidden at digital examination. Rectal wall thickening and perirectal inflammatory changes are often seen at MR imaging of the pelvis. The multiplanar capability of MR imaging greatly facilitates the detection of fistulous tracts that extend into the supralevator space. MR imaging can be helpful to both the surgeon and the gastroenterologist in the assessment of perianal and perirectal complications arising from Crohn disease and, when necessary, in the planning of surgical intervention. MR imaging also recommends itself to the patient because it is noninvasive and does not cause discomfort.