Proper cleansing of the colon is essential if the radiologist is to identify colonic lesions accurately on CT colonography. Remaining pools of fluid in the colon can hide polyps and cancer both on two-dimensional axial and reformatted images and on the three-dimensional endoluminal views (Figs. 3.3,
3.4). Residual solid stool may be misdiagnosed as a polyp, particularly if homogeneous and non-mobile. Large amounts of residual stool can obscure true colorectal polyps and even cancer. Bowel cleansing for CT colonography is currently similar to that used for other colon tests such as the barium enema and standard colonoscopy. There are two main strategies, the first consisting of maintaining a clear liquid diet starting about 24 h before the CT scan. The b a b a
Fig. 3.4. a Supine axial view of the ascending colon demonstrates greater than 50% of the lumen filled with fluid. b Prone axial view in the same patient demonstrates the large adenomatous polyp that would have been missed if dual-position imaging had not been performed. c Prone three-dimensional cube view shows the same large irregular adenomatous polyp second strategy is to clean the colon by having the patient ingest a cathartic or laxative that promotes emptying of the colon. Polyethylene glycol is an electrolyte lavage solution in a nonabsorbable medium that patients drink in large volumes to bring about colonic evacuation. Sodium phosphate and magnesium citrate are saline cathartics which are highly osmotic agents containing inorganic ions that draw fluid into the bowel lumen to induce peristalsis and elimination of bowel contents.
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