Bowel Endometriosis: Systematic Approach to Diagnosis with US and MRI.

Endometriosis involving the bowel is a severe form of the disease, and the bowel is the most common site of extragenital endometriosis. Surface lesions of the bowel are considered peritoneal disease. Bowel endometriosis (BE) is defined as endometriotic tissue infiltrating the muscularis propria layer of the bowel wall. BE is estimated to affect up to 37% of patients with known deep endometriosis, highlighting its coexistence with genital endometriosis. The rectosigmoid colon is the most common segment of the bowel involved, followed by the distal small bowel. US and MRI are the most common imaging modalities used to detect BE. Depending on which bowel segment is imaged, endometriosis protocols for transvaginal US after bowel preparation with a transabdominal component and MRI and MR enterography are most commonly used. The authors provide a systematic approach to the diagnosis of BE using these imaging modalities. Imaging protocols and techniques for optimization of visualization of the bowel are discussed, the normal bowel wall anatomy with both imaging modalities is described, and the varying degrees of bowel wall involvement in endometriosis are illustrated. The imaging features of infiltration of endometriosis in the bowel muscularis propria are described in detail, along with key imaging findings to be conveyed to surgical colleagues to optimize surgical treatment and decrease complications, thereby improving overall patient outcomes. ©RSNA, 2025 Supplemental material is available for this article.