Fig. 1From: MRI of rectal cancer—relevant anatomy and staging key pointsThe anorectal transition may be defined by the upper limit of the internal sphincter of the anus (between arrows in a), which is generally much thicker than the inner muscular layer of the muscularis propria of the rectum, or by the plane that intersects the superior border of the puborectalis muscle (between arrows in b), which may be asymmetric, as in the example depictedBack to article page