Anatomy of the anal canal and perianal structures as defined by phased-array magnetic resonance imaging

Background: The anatomy of the anal canal and perianal structures has been imaged using endoluminal magnetic resonance imaging (MRI). Phased‐array MRI avoids the use of an endoluminal coil that may distort anatomy. The aim of this study was to describe the anatomy of the anal canal and perianal structures using phased‐array MRI. Methods: Imaging was performed in 14 men and 19 nulliparous women. The dimensions of the anal canal, puborectalis, external anal sphincter, perineal body, superficial transverse perineal muscle, bulbospongiosus, ischiocavernosus and anococcygeal body were measured in different planes, and sex differences were calculated. Results: The lateral canal was significantly longer than its anterior and posterior part (P < 0·001). The anterior external anal sphincter was shorter in women than in men (P = 0·01) and occupied, respectively, 30 and 38 per cent of the anal canal length (P = 0·001). The caudal ends of the external anal sphincter formed a double layer. The perineal body was thicker in women than in men (P < 0·001) and easier to define. The superficial transverse muscles had a lateral and caudal extension to the ischiopubic bones. The bulbospongiosus was thicker in men than in women (P < 0·001). The ischiocavernosus and anococcygeal body had the same dimensions in both sexes. Conclusion: Phased‐array MRI is a non‐invasive technique that allows an accurate description of the normal anatomy of the anal canal and perianal structures. © 2001 British Journal of Surgery Society Ltd