MRI Anatomy of the Anal Region

Abstract

The aim of this study was to identify the normal anatomy of the anal region on magnetic resonance images. T1-weighted turbo spin-echo images of anal sagittal sections, anal coronal sections, and oblique anal transverse planes were obtained with a body coil in 60 normal volunteers (30 women and 30 men, aged 19-25 years) at rest in the supine position. T1-weighted images showed fat spaces and muscles simultaneously, allowing visualization of 7 image layers, including the mucosa, submucosa, anal smooth muscle, inner (intersphincteric) space, vertical levator, outer (intersphincteric) space, and external anal sphincter. The anal smooth muscle was derived from the rectal smooth muscle, and the inner space originated from the perirectal space. The outer space lay between the vertical levator and the external sphincters. The puborectalis did not have a longitudinal portion. The deep, superficial, and SC sphincters were 3 separate muscle bundles. The perianal spaces had a complex interconnection. Multiplanar body-coil MRI studies can show anorectal fat spaces and musculature simultaneously, allowing fat spaces and musculature to serve as mutual referents. The results of imaging of the anal region with this method are different from previous imaging descriptions and may provide a more accurate and systemic description of the anal region structures than was previously available.