Virtual colonoscopy (VC) or CT colonography is a method of screening the colon for precancerous polyps. Using a CAT or CT scanner and computer methods of reconstructing the images, the colon can be evaluated without a colonoscope and without sedation.
Virtual colonoscopy takes the information produced by a CT scanner and processes this information to produce an image of the colon’s inner surface. When the colon is properly cleansed and expanded (distended) with room air or carbon dioxide, and when the CT information is processed, health care providers can look at the inner lining or surface to detect polyps.
CT scanners produce their images as the patient lies on a “bed” that is pulled through a short tunnel. Within the tunnel, there is a rotating X-ray tube on one side. On the other side is a set of detectors that receive the transmitted x-rays after they have passed through the body. A continuous volume of information is obtained from this exam. The information from these exams can be processed in virtual reality computers. Because the inside of the colon has been distended, providers can look inside the colon and have a view almost completely the same as the colonoscopists.
It is estimated that several million people in the United States alone have not been properly screened for colorectal polyps. The best method for screening is a regular colonoscopy. There are simply not enough endoscopists to screen such a large number of patients. Other methods, such as sigmoidoscopy, evaluate only a part of the colon. Tests to detect small amounts of blood in the stool can miss polyps and even cancers. The barium enema is not nearly as sensitive as a regular colonoscopy. If a noninvasive test could be developed that could select those patients with a polyp, then fewer and more focused colonoscopies could be performed. The combination of newer, faster CT scanners with computer software that would produce images equivalent to an endoscope seems to be a logical step in the development of such a test.
While the accuracy of CT colonography is equivalent to regular colonoscopy in polyps greater than 7 to 8mm, CT colonography is not as good as regular colonoscopy for smaller polyps. Fortunately, these smaller polyps usually are not cancerous. CT colonography cannot differentiate a non-precancerous polyp from a precancerous polyp. Neither can regular colonoscopy. Only a biopsy can provide this information. Biopsy can only be performed via a regular colonoscopy. If an abnormality is detected on a CT colonography, the patient will need a regular colonoscopy to determine the nature of that polyp.
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