Virtual colonoscopy (VC) is an imaging or x-ray test that looks for cancer, polyps, or other disease in the large intestine (colon). The medical name of this test is CT colonography.
How the Test is Performed
VC is different from regular colonoscopy. Regular colonoscopy uses a long, lighted tool called a colonoscope that is inserted into the rectum and large intestine.
VC is done in the radiology department of a hospital or medical center. No sedatives are needed and no colonoscope is used.
The exam is done as follows:
- You lie on your left side on a narrow table that is connected to an MRI or CT machine.
- Your knees are drawn up toward your chest.
- A small, flexible tube is inserted into the rectum. Air is pumped through the tube to make the colon bigger and easier to see.
- You then lie on your back.
- The table slides into a large tunnel in the CT or MRI machine. X-rays of your colon are taken.
- X-rays are also taken while you lie on your stomach.
- You must stay very still during this procedure, because movement can blur the x-rays. You may be asked to hold your breath briefly while each x-ray is taken.
A computer combines all the images to form three-dimensional pictures of the colon. The doctor can view the images on a video monitor.
How to Prepare for the Test
Your bowels need to be completely empty and clean for the exam. A problem in your large intestine that needs to be treated may be missed if your intestines are not cleaned out.
Your health care provider will give you the steps for cleansing your bowel. This is called bowel preparation. Steps may include:
- Using enemas
- Not eating solid foods for 1 to 3 days before the test
- Taking laxatives
You need to drink plenty of clear liquids for 1 to 3 days before the test. Examples of clear liquids are:
- Clear coffee or tea
- Fat-free bouillon or broth
- Sports drinks
- Strained fruit juices
Keep taking your medicines unless your doctor tells you otherwise.
You will need to ask your provider if you need to stop taking iron pills or liquids a few days before the test, unless your provider tells you it is OK to continue. Iron can make your stool dark black. This makes it harder for the doctor to view inside your bowel.
CT and MRI scanners are very sensitive to metals. Do not wear jewelry the day of your exam. You will be asked to change out of your street clothes and wear a hospital gown for the procedure.
How the Test will Feel
The x-rays are painless. Pumping air into the colon may cause cramping or gas pains.
After the exam:
- You may feel bloated and have mild abdominal cramping and pass a lot of gas.
- You should be able to return to your regular activities.
Why the Test is Performed
VC may be done for the following reasons:
- Follow-up on colon cancer or polyps
- Abdominal pain, changes in bowel movements, or weight loss
- Anemia due to low iron
- Blood in the stool or black, tarry stools
- Screen for cancer of the colon or rectum (should be done every 5 years)
Your doctor may want to do a regular colonoscopy instead of a VC. The reason is that VC does not allow the doctor to remove tissue samples or polyps.
Other times, a VC is done if your doctor was not able to move the flexible tube all the way through the colon during a regular colonoscopy.
Normal findings are images of a healthy intestinal tract.
What Abnormal Results Mean
Abnormal test results may mean any of the following:
- Colorectal cancer
- Abnormal pouches on the lining of the intestines, called diverticulosis
- Colitis (a swollen and inflamed intestine) due to Crohn's disease, ulcerative colitis, infection, or lack of blood flow
- Lower gastrointestinal (GI) bleeding
Regular colonoscopy may be done (on a different day) after a VC if:
- No cause for bleeding or other symptoms were found. VC can miss some smaller problems in the colon.
- Problems that need a biopsy were seen on a VC.
Risks of VC include:
- Exposure to radiation from the CT scan
- Nausea, vomiting, bloating, or rectal irritation from medicines used to prepare for the test
- Perforation of the intestine when the tube to pump air is inserted (extremely unlikely).
Differences between virtual and conventional colonoscopy include:
- VC can view the colon from many different angles. This is not as easy with regular colonoscopy.
- VC does not require sedation. You can usually go back to your normal activities right away after the test. Regular colonoscopy uses sedation and often the loss of a work day.
- VC using CT scanners expose you to radiation.
- Regular colonoscopy has a small risk of bowel perforation (creating a small tear). There is almost no such risk from VC.
- VC is often not able to detect polyps smaller than 10 mm. Regular colonoscopy can detect polyps of all sizes.
Colonoscopy - virtual; CT colonography; Computed tomographic colonography; Colography - virtual
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Review Date 1/16/2020
Updated by: Michael M. Phillips, MD, Clinical Professor of Medicine, The George Washington University School of Medicine, Washington, DC. Internal review and update on 06/03/2021 by David Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.