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Is it true that a CAT Scan of the colon can replace the dreaded colonoscopy?
Two new techniques are under development and evaluation for screening for colon polyps and cancer. They both are referred to as "virtual colonoscopy" since they provide views of the colon that are similar to those obtained at colonoscopy. The difference is that the images of the colon are produced by computerized manipulations rather than direct observation through the colonoscope. One technique uses the x-ray images from a CT (CAT) scan, and the other uses magnetic images from an MRI scan. The CT scan technique is available in many radiology units. The MRI scan technique still is experimental. For both types of studies, the colon is cleaned-out using potent laxatives. In the CT study the colon is filled with air. In the MRI study the colon is filled with liquid. The scans are then performed and the images analyzed and manipulated to form a virtual image of the colon.
There is little question that the CT scans of the colon are good. They can find polyps that occasionally are missed by colonoscopy because the polyps lie behind folds within the colon. One criticism of the CT scans is that they cannot find small polyps (less than 5 mm in size) that are easily seen at colonoscopy. A second criticism is that they are poorer than colonoscopy at finding flat polyps. A third criticism is that small pieces of stool can look like polyps on the CT scan and lead to a diagnosis of polyp when there is none. Perhaps the greatest criticism of virtual colonoscopy is that 30% of people have colonic polyps. If they are found by virtual colonoscopy, then standard colonoscopy must be done to remove the polyps, and, therefore, the individual must undergo two procedures.
Virtual colonoscopy is a new technique. It is undergoing continual improvement. It is a good option for individuals who cannot or will not undergo standard colonoscopy. It is not yet clear, however, if it should be a primary screening tool for individuals at either normal risk or high risk for polyps or cancer.
Jay W. Marks, M.D.
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