Transrectal ultrasonography: interpretation of normal intestinal wall structure for the preoperative staging of rectal cancer.

An in vitro ultrasonographic study was carried out to four polyethylene membranes of different thicknesses and on four normal surgical specimens from the rectal wall, to assess its various layers and their anatomical structure. Three different techniques were used, and an electronic equipment measured the thickness of the membranes and of the surgical specimens. Polyethylene membranes less than 1000 microns thick gave linear echoes; the 1000-microns-thick membrane gave a three-layer image--two hyperechogenic layers separated by a hypoechogenic one. The rectal wall was separated into the mucosa-submucosa and the muscularis propria-serosa or muscularis propria (in the extraperitoneal rectum); their thickness ranged from 900 to 1000 microns and from 1500 to 8000 microns, respectively. Even though the mucosa-submucosa, the muscularis propria-serosa, and the muscularis propria each gave a three-layer ultrasonographic image (hyper-hypo-hyper), the resulting image of the entire rectal wall was of five layers (hyper-hypo-hyper-hypo-hyper). Transrectal scan carried out on 20 control patients confirmed the results obtained in vitro, even though at times it was difficult to detect clearly the fifth layer. Interpretation of the five layers was as follows: the second and fourth corresponded to the mucosa-submucosa and muscularis propria, respectively; the first, third, and fifth were considered to reflect interfaces originating from different acoustic impedance structures. The fourth layer was always thicker than the second one. There was no difference between intra- and extra-peritoneal images. A further study carried out on nine patients with rectal cancer by transrectal ultrasonography proved the reliability of this method in assessing the cancer infiltration of the wall and its importance in the preoperative staging of rectal cancer.