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Colonoscopy (cont.)

Medical Author:
Jay W. Marks, MD

Jay W. Marks, MD, is a board-certified internist and gastroenterologist. He graduated from Yale University School of Medicine and trained in internal medicine and gastroenterology at UCLA/Cedars-Sinai Medical Center in Los Angeles.

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Medical Editor:
Bhupinder Anand, MD

What is virtual colonoscopy?

Another alternative to colonoscopy is virtual colonoscopy. Virtual colonoscopy is a technique that uses computerized tomography (CT) scanning to obtain images of the colon that are similar to the views of the colon obtained by direct observation through colonoscopy. The images are constructed using the CT images so they do not represent true images. They are virtual images.

In preparation for virtual colonoscopy, the day before the examination, the colon is cleaned-out using laxatives. During the examination a tube is inserted into the anus and is used to inject air into the colon. The CT scans are then performed with the colon inflated, and the scans are analyzed and manipulated to form a virtual image of the colon. When properly performed, virtual colonoscopy can be effective. It can even find polyps "hiding" behind folds that occasionally are missed by colonoscopy.

Nevertheless, virtual colonoscopy has several limitations.

  • Virtual colonoscopy has difficulty identifying small polyps (less than 5 mm in size) that are easily seen at colonoscopy.
  • Virtual colonoscopy has great difficulty identifying flat cancers or premalignant lesions that are not protruding, that is, are not polyp-like.
  • Virtual colonoscopy does not allow removal of polyps that are found. Thirty to forty percent of people have colon polyps. If polyps are found by virtual colonoscopy, then colonoscopy must be done to remove the polyps. Therefore, many individuals having virtual colonoscopy will have to undergo a second procedure, colonoscopy.
  • Virtual colonoscopy exposes individuals to a moderate amount of radiation.
  • Virtual colonoscopy does not allow the use of the newer techniques that are being developed to differentiate between abnormal lesions that need to be biopsied or removed and those that don't. (See section "What's new in colonoscopy?" that follows.)

Because of these limitations, virtual colonoscopy has not replaced colonoscopy as the primary screening tool for individuals at increased risk for polyps or colon cancer. It is currently an option for individuals at normal risk for polyps and colon cancer who cannot or will not undergo colonoscopy.


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