Conventional vs. Virtual Colonoscopy

Colonoscopy is a procedure that allows your doctor to check your rectum and large intestine (colon) for inflammation, abnormal growths such as polyps, and ulcers. Most often, colonoscopies are used to screen for early signs of cancer.

Charles S. Marn, MD, Medical College of Wisconsin Associate Professor of Radiology and Gastroenterology, who provides care at the Radiology Department, says polyps are of particular concern to doctors. Although only a small percentage of them become cancerous, all must be removed.

The polyp is like a calling card, Dr. Marn says. It gives notice that sometime in the future there's a possibility of developing cancer. "The polyp-cancer sequence is a relatively slow, long one," he explains. "You have a wide window to catch that cancer. If the patient has a polyp removed today and it's one that would have turned into cancer, we've protected that patient from getting cancer years from now."

There are two options for the procedure: conventional colonoscopy and virtual colonoscopy. Both provide a thorough examination and are effective in screening for colon cancer, but there are differences.

Conventional colonoscopy remains the accepted technique, notes Dr. Marn, but improvements in technology are allowing the virtual method to close the gap. "We are coming to an understanding that virtual colonoscopy can often do the same things that optical colonoscopy can do in terms of detection," he says.

Conventional Colonoscopy
In a conventional colonoscopy (sometimes called optical colonoscopy), the doctor inserts a colonoscope - a long, flexible, lighted tube - into the patient's rectum and slowly guides it up through the colon. Pain medication and a mild sedative help the patient stay relaxed and comfortable during the 30- to 60-minute procedure. A tiny camera in the scope transmits an image of the lining of the colon, so the doctor can examine it on a video monitor. If an abnormality is detected, the doctor can remove it or take tissue samples using tiny instruments passed through the scope.

The colon must be completely empty for this procedure to be effective. To prepare, patients undergoing the procedure will typically be asked to follow a liquid diet for one to three days beforehand and drink up to a gallon of a special laxative fluid the day before.

"This is the main problem with optical colonoscopy," Dr. Marn says, "It requires an uncomfortable prep." The laxatives will cause very frequent bowel movements, necessary to completely clear the system of any fecal matter. In addition, even though it's performed on an outpatient basis, the procedure requires light sedation. "For most people in the working world that's a full day off of work."

Dr. Marn says the procedure is very safe, but there is a small risk of perforation or other complications.

Virtual Colonoscopy
Virtual colonoscopy (VC) uses X-rays and computers to produce two- and three-dimensional images of the colon and display them on a screen. VC is performed with computed tomography (CT), sometimes called a CAT scan. Research is ongoing about performing the same procedure with Magnetic Resonance Imaging (MRI) scanners.

Unlike conventional colonoscopy, virtual colonoscopy can be performed even though some liquid or fecal matter remains in the colon, Dr. Marn says. The prep usually involves a mild laxative as well as a "tagging" agent, either barium or iodine, added to the meals immediately prior to the exam. The tagging agent causes fecal material to show up on the computer screen.

The examination typically takes only about 10 minutes and does not require sedation.

During the procedure, the doctor will ask you to lie on your back on a table. A thin tube is inserted into the rectum, and air is pumped through the tube to inflate the colon for better viewing. The table moves through a scanner to produce a series of two-dimensional images, which are combined to create a three-dimensional picture that can be viewed on a video screen. The scanning procedure is repeated with the patient lying face-down.

The milder prep procedure and lack of sedation make this procedure attractive to many people, Dr. Marn says. However, if an abnormality is found, the patient must return for a conventional colonoscopy.

The risks from CT scanning are nearly zero, Dr. Marn says. While the procedure uses radiation, its benefits far outweigh the slight radiation exposure.

Advantages and Disadvantages
If an abnormality is detected during a conventional colonoscopy, the doctor can remove it or take tissue samples using tiny instruments passed through the colonoscope. During a virtual colonoscopy this is not possible; if abnormalities are found the patient will still have to have a conventional colonoscopy

Virtual colonoscopy does not show as much detail as a conventional colonoscopy, so some very small polyps might not show up on the images.

Though virtual colonoscopy has been in use for about a decade, it is still considered experimental and therefore is typically not covered by insurance, Dr. Marn says. Insurance will only cover VC for patients who, for medical reasons, should not have conventional colonoscopy. However, newly published research documenting the safety and efficacy of VC may move insurers to broaden the payment criteria and use of the technique.

The advantages of VC are that it is more comfortable than conventional colonoscopy because there is no colonoscope inserted into the rectum and through the intestine; no sedation is needed and patients can return to their activities after the procedure; and VC takes less time than a conventional colonoscopy.

Screening Recommendations
People with no personal or family history of colon or rectal cancer should start being screened at age 50, Dr. Marn says. If no polyps or other abnormalities are found, the patient is advised to return in 10 years. Anyone with a personal or family history of colon cancer should be screened by age 40, he says. Those patients will be asked to return in three to five years, especially polyps are found.

Unfortunately it's difficult to get people in for screening, especially if they feel they are not at risk for cancer. But the vast majority of people with colon cancer have no pre-existing risk factors, Dr. Marn says.

"So when we advocate to the population for screening," Dr. Marn says, "we have to take healthy people - people who don't know or perceive the risk - and tell them that we want them to invest some personal time in getting this done for the sake of proving that they don't have colon cancer." Also, people are less likely to seek out a procedure is perceived as intrusive or unpleasant.

Cancer screening saves lives, Dr. Marn stresses. "The simple fact of the matter is, if you fully applied colon cancer screening, you would eliminate almost all colon cancer in the population."

Dr. Marn says that the medical community has an obligation to increase screening. Having CBS anchor Katie Couric and others advocate for cancer screening helps, but more is needed. "We have patients walking into this institution with advanced colon cancer every week," he says. "There's no reason why that should be happening."

Dr. Marn strongly urges patients to take an active role in their health care. Part of that process is becoming informed of the alternatives available for cancer screening. "Offering patients options is a smart thing," he says. "The only bad option is to do nothing."

This article includes information from the National Institute of Diabetes and Digestive and Kidney Diseases.

MCW Health News presents up-to-date information on patient care and medical research by the physicians of the Medical College of Wisconsin.