Colonoscopy is a procedure that enables an examiner (usually a gastroenterologist) to evaluate the appearance of the inside of the colon (large bowel). This is accomplished by inserting a long flexible tube (the colonoscope) that is about the thickness of a finger into the anus and then advancing the colonoscope slowly into the rectum and through the colon. The tip of the colonoscope has a light and a video camera. The inner colon lining can be directly visualized in images transmitted from the camera to a television monitor. This traditional method of examination of the colon is called optical colonoscopy or, more commonly, just colonoscopy.

In preparation for colonoscopy, the day before the examination the colon is emptied using laxatives. Immediately prior to colonoscopy, an intravenous infusion (IV) is started, and the patient is placed on a monitor for continuously monitoring the rhythm of the heart, blood pressure, and the amount of oxygen in the blood. Medication usually is given by IV to make the patient sleepy and relaxed. If needed, the patient can receive additional doses of medication during the procedure. Colonoscopy often gives a feeling of pressure, cramping, and bloating; however, with the aid of the medication, colonoscopy generally is well tolerated and infrequently results in significant pain. The type of anesthesia caused by the intravenous medications is classified as conscious sedation and is safer than general anesthesia. With conscious sedation, patients are sleepy but still arousable, and able to breathe on their own. After the colonoscopy, patients are asked not to drive for the rest of the day if medications for conscious sedation were given.

Should there be polyps (benign growths that can lead to cancer) in the colon, the polyps, almost always can be removed through the colonoscope. Removal of these polyps is an important method of preventing colorectal cancer.

Virtual colonoscopy is a technique that uses a computerized tomographic (CT) scan (a type of three-dimensional x-ray) to construct virtual images of the colon that are similar to the views of the colon obtained by direct observation by optical colonoscopy.

In preparation for virtual colonoscopy, the day before the examination the colon is emptied using laxatives in a manner similar to traditional colonoscopy. During the examination, a small tube is inserted into the anus to inject and fill the colon with air. Unlike with traditional colonoscopy, this tube is not advanced into the colon. The CT scan then is performed, and the scans are manipulated by computer software to form virtual images of the colon. When properly performed, virtual colonoscopy can be as effective as routine colonoscopy. It can even find polyps "hiding" behind folds that occasionally are missed by traditional colonoscopy. The scanning takes only 10 minutes, and usually no conscious sedation is necessary.

In October, 2007, researchers from University of Wisconsin published in the New England Journal of Medicine a study comparing traditional colonoscopy to virtual colonoscopy. More than six thousand patients over age 50 were evenly divided to undergo either optical or virtual colonoscopy. The researchers found that virtual colonoscopy was as effective as optical colonoscopy in detecting polyps larger than 5mm.

• Virtual colonoscopy is less invasive and faster to perform than traditional colonoscopy and does not require conscious sedation.
• Even though virtual colonoscopy is less invasive than traditional colonoscopy, virtual colonoscopy still involves injecting air into the colon, which can be uncomfortable for some patients. On the other hand, with adequate conscious sedation, patients usually experience little or no discomfort with traditional colonoscopy.
• Virtual colonoscopy is not as reliable as routine colonoscopy in detecting small polyps (less than 5mm in size). Even though most experts believe that polyps smaller than 5mm are usually benign, some small polyps can be cancerous or become cancerous if not removed.
• Virtual colonoscopy is not as accurate as traditional colonoscopy in finding flat cancers or polyps that are not protruding, that is, are not polyp-like.
• Virtual colonoscopy cannot remove polyps. If polyps are found by virtual colonoscopy, then traditional colonoscopy must be performed to remove the polyps. Therefore, many individuals having virtual colonoscopy will have to undergo a second procedure,.
• Even though the amount of radiation exposure with virtual colonoscopy is considered safe, the long term radiation effects of mass screening with CT is not known.

• In healthy patients who want all the polyps (including ones smaller than 5mm) to be removed, I recommend traditional colonoscopy
• In patients who do not want to undergo routine colonoscopy and are willing to accept the possibility of leaving small polyps untreated, virtual colonoscopy is an alternative.
• In unhealthy patients with advanced heart, lung, liver, or kidney diseases, virtual colonoscopy may be a safer and less invasive alternative to traditional colonoscopy.

New England Journal of Medicine, October 2007