Although virtual colonoscopy is relatively non-invasive, many patients still find that two aspects of the exam may cause some anxiety and possibly discomfort. These are the need for bowel preparation and for the colon to be inflated with gas before the images are taken. Currently, it cannot be stressed enough that the colon needs to be thoroughly cleaned and properly distended in order for the test results to be accurate.
The biggest limitation of virtual colonoscopy is bowel preparation. As with other techniques that attempt to image and visualize the colon surface, the colon needs to be cleansed of residual fecal material. Many patients find this the worst part of the examination. There are several bowel preparations available. At NYU Medical Center, we have found that the best commercial preparation kits for virtual colonoscopy are the 24-hour Fleet 1 Preparation (Fleet Pharmaceuticals, Lynchburg, VA) or the LoSo Preparation (EZ-EM, Westbury, NY). The kits are inexpensive and have easy-to-follow instructions.
The Fleet Kit preparation includes a clear fluid diet the day before the exam, a single 45-ml dose of phospho-soda and 4 bisacodyl tablets the day before the exam, and a bisacodyl suppository the morning of the exam. The LoSo Preparation relies on magnesium citrate and 4 bisacodyl tablets the day before the exam and a bisacodyl suppository the morning of the exam. We prefer these preparations because they provided adequate bowel preparation for the majority of patients.
These commercial preparation kits appear to provide a drier colon than a 4-liter electrolyte solution such as Golytely (37). However, these preparations do on occasion leave more fecal residue than a typical electrolyte preparation or a double dose, that is, two 45 ml doses of phospho-soda. In our experience approximately 5 percent of patients who undergo bowel preparation with these commercial kits will have a poor preparation that limits an accurate interpretation.
Given this limitation, the idea of fecal and fluid tagging for virtual colonoscopy is currently being evaluated (38, 39). Fecal tagging can be performed without bowel cleansing (38) or with bowel cleansing (39). Fecal tagging without bowel cleansing relies on having the patient ingest small amounts of iodine or dilute barium with a low-fat and low-fiber diet beginning several days before the examination. When the CT examination is performed, residual fecal material will appear high attenuation (white). Using computer-generated fecal subtraction techniques, the high attenuation fecal material can be subtracted out of the data set leaving only the mucous membrane of the colon and rectum and any colorectal cancers or polyps. This technique is still experimental and has not been proven to be effective; however, research into this area is very important to the future of virtual colonoscopy. As has been pointed out, if virtual colonoscopy could be effective in detecting colorectal polyps and not require a bowel preparation, it would become the screening test of choice (40).
A recent study evaluated the use of fecal tagging with dilute barium to “tag” or “label” residual fecal material along with a less intense bowel-cleansing agent. When a magnesium citrate preparation with a fluid-restricted, low-fiber, low-fat diet as well as fecal tagging was used, polyps with diameters of 10 mm or more were detected in 100 percent of cases. (39). In this study, the fecal-tagging preparation was compared with that of polyethylene glycol. The specificity of virtual colonoscopy in patients undergoing fecal tagging was improved when compared with those undergoing bowel preparation with a standard polyethylene glycol preparation without fecal tagging. Moreover, in this study patients preferred the fecal-tagging preparation to the polyethylene glycol preparation.