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Virtual colonoscopy is generally safe. Risks include:

• A tear (perforation) can occur in the colon or rectum wall due to the colon and rectum being inflated with air or carbon dioxide.
• Radiation exposure may be a concern. However, the level of radiation used during the type of CT scan in virtual colonoscopy is lower than the amount used in a diagnostic CT scan.

Before a virtual colonoscopy, you'll need to empty your colon. Any residue in your colon may obscure the images taken during the exam.

To empty your colon, follow your doctor's instructions carefully. You may be asked to:

• Follow a special diet the day before the exam. Typically, you won't be able to eat that day. Drinks may be limited to clear liquids. You may not be able to eat or drink anything after midnight the night before the exam.
• Take a laxative the night before the exam. You may also need to use a suppository to clear any remaining residue in your colon.
• Take medication to identify stool in your colon. The medication will help the doctor differentiate between colon abnormalities and stool residue.
• Adjust your usual medications. Remind your doctor of your medications at least a week before the exam. You may need to temporarily stop taking certain medications days or hours before the exam.

During the exam

You'll wear a gown, but likely no other clothes. Sedation usually isn't necessary, but you may be given medication to relax your colon.

You'll begin the exam lying on your side on the exam table, usually with your knees drawn toward your chest. The doctor will place a small tube (catheter) inside your rectum to fill your colon with air or carbon dioxide. The air or gas helps create clear images and may cause a feeling of pressure in your abdomen.

For the next part of the exam, you'll lie on your back. The exam table will be moved into the CT machine, and your body will be scanned. Then you'll turn over to lie on your abdomen or your side, and your body will be scanned again.

You may be asked to turn and hold various other positions, as well as hold your breath at times. If necessary, a contrast agent may be given through a vein in your arm to help create clearer images.

A virtual colonoscopy typically takes about 10 to 15 minutes.

In a multicenter study of people who had virtual colonoscopy to screen for colon cancer, nearly 93 percent of participants labeled their overall experience "excellent" or "good," and 93 percent indicated they would choose the procedure for their next screening.

After the exam

Most of the air or gas will be removed from your colon through the catheter in your rectum. You may feel bloated or pass gas for a few hours after the exam as you clear the remaining air or gas from your colon. Walking may help relieve any discomfort, and you can return to your usual diet and activities right away.

Your doctor will review the results of the colonoscopy and then share them with you. Your test results may be:

• Negative, if the doctor doesn't find any abnormalities in the colon. If you're at average risk of colon cancer — you have no colon cancer risk factors other than age — your doctor may recommend repeating the exam in five years.
• Positive, if the images reveal any polyps or other abnormal tissue in the colon. Depending on the size and number of polyps, your doctor may recommend a traditional colonoscopy to obtain samples of the abnormal tissue or remove the polyps. In some cases, the traditional colonoscopy or polyp removal can be done the same day as the virtual colonoscopy