Fig. 1.

Normal perianal anatomy of 47-year-old female volunteer imaged with CT and MR imaging.

Anal sphincter complex is seen as two concentric rings. MR imaging: inner (+) internal sphincter and outer external sphincter (white dovetail arrows) appear relatively hypointense on all T2-weighted images without fat suppression (A-C). Levator ani muscle (B, thick arrows) forms superior boundary of fat-containing ischioanal fossa (^*) on either side of anal canal. CT (D-F) of inner internal sphincter (+), outer external sphincter (white dovetail arrows), levator ani muscle (E, thick arrows), and fat-containing ischioanal fossa (^*).