On MRI the upper and lower parts of the anal canal are identified and appear different. The upper part contains the internal anal sphincter, the longitudinal muscle layer and the puborectalis muscle. The lower part contains the internal anal sphincter, the longitudinal muscle layer and the external anal sphincter. The longitudinal muscle layer lies in a slit like space between the internal anal sphincter and the external anal sphincter and puborectalis muscle, the intersphincteric space. On T2W1 all the muscles except the internal sphincter, which has intermediate signal intensity, have low signal intensity (Figure 3.28).
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