Patients must follow a strict bowel-emptying procedure the day before, just as they would for a standard colonoscopy. The test is performed in the radiology department of a hospital or medical center. Unlike conventional colonoscopy, no sedatives are needed and no scope is used.

You will lie down on your left side on a narrow table that is connected to an MRI or CT machine. Your knees will be up toward your chest. A small, flexible tube will be inserted into your rectum. Air is pumped through the tube to make the colon bigger and easier to see. After this is done, you’ll be asked to lie on your back.

Next, the table slides into a large tunnel through the machine, where x-rays create images of your colon. You must stay very still during this procedure, since movement can blur the pictures. You may be asked to hold your breath briefly while each picture is taken.

A computer combines all the images to form a three-dimensional picture of the colon, which are viewed on a video monitor.

Pictures are also taken while you lie on your stomach.

The exam takes about 20 minutes.

Preparations can vary, depending on your health care provider. Everyone undergoing any type of colonoscopy must completely empty their bowels before the exam. This may be done using an enema or laxatives combined with a liquid diet the day before the test. The bowel must be completely emptied until no solid matter remains.

Unfortunately, diarrhea is common during preparation. Drink plenty of clear liquids, such as apple juice and chicken broth, to avoid dehydration.

Make sure you tell your health care provider about any medications you are taking. You’ll be told which you can take, and which you should temporarily stop. CT and MRI scanners are very sensitive to metals. Don’t wear jewelry the day of your exam. You’ll be asked to wear a hospital gown for the procedure.

The x-rays are painless. Pumping air into the colon can create temporary cramping or gas pains.

Virtual colonoscopy may be performed to:

• Diagnose colorectal cancer
• Look for polyps
• Follow up after an incomplete or failed colonoscopy
• Follow up after a previous finding of colon cancer or polyps
• Examine those with a family history of colon cancer

Normal findings are images of a healthy intestinal tract.

• Polyps
• Colorectal cancer
• Diverticulosis
• Lower gastrointestinal (GI) bleeding
• Tumor
• Inflammatory bowel disease

• Radiation exposure
• Nausea, vomiting, bloating, or rectal irritation caused prep medications

Differences between virtual and conventional colonoscopy include:

• Virtual colonoscopy uses no sedation, and patients are usually able to resume normal activities immediately after the test. Conventional colonoscopy involves sedation, and usually the loss of a work day.
• If a polyp is found with virtual colonoscopy, the patient must have a conventional colonoscopy to remove the abnormality. Conventional colonoscopy allows for the immediate removal of polyps .
• Both procedures can spot polyps that have reached a large, worrisome size. However, virtual colonoscopy is not as detailed as a conventional colonoscopy, and may miss polyps smaller than 10 millimeters in diameter or flat lesions.
• Virtual colonoscopy can view the colon from many different angles. This is not easy with conventional colonoscopy.
• Virtual colonoscopy is not currently recommended as a screening tool for the early detection of colon cancer. Routine screening for colon cancer involves conventional colonoscopy, as well as other tests.

Virtual Colonoscopy, The Medical Letter, February 14, 2005; 1202:15

Van Dam, J. et al. CT Colonography. Gastroenterology. 2004; 127: 970-984 

Lieberman, D. Colonoscopy: Good as Gold? Ann Intern Med. 2004; 141: 401-403 

Pickhardt, P. et al. Location of Adenomas Missed by Optical Colonoscopy. Ann Intern Med. 2004; 141:352-359