Anal sphincter complex: Endoanal MR imaging of normal anatomy

PURPOSE: To determine the normal anatomy of the anal sphincter complex on magnetic resonance (MR) images. MATERIALS AND METHODS: Ten healthy volunteers (four men, six women; age range, 21-26 years) underwent MR imaging with an endoanal coil. RESULTS: The lower part of the anal canal contained the internal sphincter, the longitudinal muscle layer, and the external sphincter; the upper part comprised the internal sphincter, the longitudinal layer, and the puborectal muscle. At the upper end, the puborectal muscle was attached to the levator ani muscle. Anteriorly, the external sphincter was connected to the urogenital diaphragm; posteriorly, it was attached to the coccyx with the anococcygeal ligament. All perianal spaces were visible. The morphology of the anterior part of the external sphincter, different in men and women, was well displayed. CONCLUSION: Depiction of the anal sphincter complex on MR images is excellent.