Bowel Preparation

There are several techniques that may be used for bowel preparation, and there remains controversy as to how to optimize patient preparation. The goal is to have a well-prepared, well-distended colon that will facilitate polyp detection and minimize false positive findings. A clean, well-distended colon facilitates detection of col-orectal abnormalities whether two dimensional (2D) or 3D techniques are used for data interpretation. More importantly, it often maximizes our ability to differentiate polyps, folds, and residual fecal matter within the colon.

There are three commercially available bowel preparations in the U.S, which include cathartics such as magnesium citrate and phospho-soda, and colonic lavage solutions such as polyethylene glycol. In our experience, the polyethylene glycol preparation frequently leaves a large amount of residual fluid in the colon. While this preparation is adequate for colonoscopy, large amounts of residual fluid will limit the effectiveness of CTC. With conventional colonoscopy, residual fluid can be endoscopically aspirated from the colon. With CTC, the examination is typically limited to only two acquisitions, supine and prone. While supine and prone imaging allows for fluid redistribution, this does not ensure full mucosal evaluation if large amounts of fluid are present. Regarding bowel preparation, polyethylene glycol should be utilized in all patients with substantial cardiac or renal insufficiencies. The poly-ethylene-glycol preparation results in no fluid shifts and no electrolyte imbalances. Therefore, it is safe to use in these patients.

Some investigators have found that the use of intravenous (IV) contrast material may facilitate colorectal polyp detection when large amounts of fluid are present. Occasionally polyps are obscured by residual fluid. After IV contrast administration, a polyp will enhance, and it may become visible despite being submerged in the fluid. The downsides of the routine administration of contrast are the costs, the need for IV

access, and the risk of allergy from the iodinated contrast material.

Given the limitations of bowel preparation, including poor patient compliance and reluctance, as well as residual fecal material that can make interpretation difficult, the possibility of fecal and fluid tagging for CTC is being investigated. Fecal tagging is obtained by having the patient ingest small amounts of barium or iodine with their meals prior to imaging. The high attenuation contrast material will be incorporated within residual fecal matter, facilitating differentiation from polyps. Some researchers have advocated fecal tagging, since tagged fecal matter should allow the improved differentiation of residual stool from colorectal polyps.

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