Pelvic MRI and CT images are interchangeable for measuring peripouch fat.

A total of 27 pouch patients with inflammatory bowel diseases, who underwent pelvic MRI-DIXON and CT scan within one year, were included. Peripouch fat areas were measured at the middle height level of pouch (AreaM) and the highest level of pouch (AreaH). Our results demonstrated that measurements of perianal fat thickness, AreaM and AreaH based on MRI image were accurate and reproducible (correlation efficiency(r): intraobserver: 0.984-0.991; interobserver: 0.969-0.971; all P < 0.001). Bland-Altman analysis showed that more than 92.593% (25/27) of dots fell within the limits of agreement. We also identified strong agreements between CT and MRI image in measuring perianal fat thickness(r = 0.823, P < 0.001), AreaM (r = 0.773, P < 0.001) and AreaH (r = 0.862, P < 0.001). Interchangeable calculating formula to normalize measurements between CT and MRI images were created: Thickness_CT = 0.610 × Thickness_MRI + 0.853; AreaM_CT = 0.865 × AreaM_MRI + 1.392; AreaH_CT = 0.508 × AreaH_MRI + 15.001. In conclusion, pelvic MRI image is a feasible and reproducible method for quantifying peripouch fat. Pelvic MRI and CT images are interchangeable in retrospective measurements of peripouch fat, which will foster future investigation of the role of mesentery fat in colorectal diseases.