Mayo Clinic Health Library

Virtual colonoscopy

Updated: 06-18-2011


Virtual colonoscopy is an exam used to detect changes or abnormalities in the large intestine (colon) and rectum.

During virtual colonoscopy, an imaging technique known as computerized tomography (CT) is used to produce hundreds of cross-sectional images of the abdominal organs. The images are combined and digitally manipulated to provide a detailed view of the inside of the colon and rectum. Unlike traditional colonoscopy, virtual colonoscopy doesn't require sedation or the insertion of a scope into the colon.

Virtual colonoscopy is one option used to screen for colon cancer. Discuss your colon cancer screening options with your doctor to determine whether virtual colonoscopy is the right option for you.


Why it's done

Virtual colonoscopy is used to screen for colon cancer.

Because virtual colonoscopy involves the entire abdominal and pelvic area, problems unrelated to colon cancer — such as an aortic aneurysm in the abdominal area or a tumor in the kidney, liver, pancreas or spleen — could potentially be found as well.



Virtual colonoscopy carries few risks:

  • A tear (perforation) can occur in the colon or rectum wall due to the colon and rectum being inflated with air or carbon dioxide.
  • An adverse reaction to the contrast agent sometimes used can occur.
  • Radiation exposure during a CT scan may be a concern. The level of radiation used during a screening CT scan, such as a virtual colonoscopy, is lower than the amount of radiation used in a diagnostic CT scan. Radiation used during a virtual colonoscopy is unlikely to be dangerous, but the effect of cumulative radiation exposure after repeated exams is unknown.

How you prepare

Before a virtual colonoscopy, you'll need to clean out (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 the day before the exam. Drinks may be limited to clear liquids — plain water, tea and coffee without milk or cream, broth, and clear carbonated beverages. 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. Your doctor may suggest taking a laxative, either in pill or liquid form. You may also need to use a suppository to clear any remaining residue in your colon.
  • Take medication to identify stool in your colon. Before the exam, you may need to take a prescription stool-tagging medication. 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.

What you can expect

During the exam
During a virtual colonoscopy, you'll wear a gown but likely no other clothes. Sedation isn't usually necessary, but you may be given an injection of medication to reduce the likelihood of stomach cramps during the exam.

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 — which helps create clear images — 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 (intravenously) to help create clearer images.

A virtual colonoscopy typically takes about 10 minutes.

After the exam
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. Consult your doctor if your signs and symptoms don't improve on their own.



Your doctor will review the results of the colonoscopy and then share the results with you.

  • Negative result. A virtual colonoscopy is considered 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 result. A virtual colonoscopy is considered 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 can be done the same day as the virtual colonoscopy — but you'll need someone to take you home afterward because a traditional colonoscopy is typically done under sedation.

If your doctor is concerned about the quality of the view obtained with virtual colonoscopy, a repeat virtual colonoscopy or another screening test may be recommended.