Virtual Colonoscopy

Virtual colonoscopy (VC) (or more appropriately, CT colonography) is a new method of screening the colon for precancerous polyps. Using a CAT, or CT scanner, and new computer methods of rendering or reconstructing the images, the colon can be evaluated without a colonoscope and without sedation.

What Is It?

Virtual colonoscopy takes the information produced by a CT scanner and processes this information to produce an image of the colon's inner surface. The examination is possible because of new, very fast CT scanners and the refinement of computer hardware and software that have been used to produce modern digital movies, such as "Star Wars" or "The Matrix." When the colon is properly cleansed and distended with room air or carbon dioxide, and when the CT information is processed, we can then look at the inner lining or surface to detect polyps.

How Does It Work?

Modern CT scanners produce their images as the patient lies on a "bed" that is rapidly pulled through a gantry, or short tunnel. Within the gantry there is on one side a rotating x-ray tube and on the other 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. This information can be sliced and diced in infinite ways. The information from these exams can be processed in virtual reality computers, much like digital movies are made. Because the inside of the colon has been distended, we can look inside the colon and have a view almost completely the same as the colonoscopists.

Why do we need another test to detect precancerous colon polyps?

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. This would be equivalent to performing a mammogram on only one breast. 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.

How Is It Done?

As with standard colonoscopy, the patient must undergo a preparation that includes a liquid diet and a cathartic that cleanses the colon. This starts the day before the procedure. In addition, the patient has to take small barium tablets during the day, which tags any stool that may remain in the colon. This aids our ability to differentiate stool from a small polyp.

Can a CT colonography detect abnormalities outside the colon?

The "whole body screening" trend with CT has touted the virtues of detecting undiscovered abnormalities. While the data to date is inconclusive, it is likely that a virtual colonoscopy will discover an abnormality outside the colon in a small percentage of patients. These abnormalities may include a renal cancer, an aortic aneurysm, enlarged lymph nodes, or even a pelvic mass in a woman. Published rates for significant abnormalities are between 5 and 15%. You should also know that the CT may detect a potential abnormality that will require further investigation before a certain diagnosis is reached.

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