Virtual colonoscopy (VC) is an imaging or x-ray test that looks for cancer, polyps, or other disease in the large intestine (colon).
Colonoscopy - virtual; CT colonography; Computed tomographic colonography; Colography - virtual
How the Test is Performed
Virtual colonoscopy is different from a regular colonoscopy. Regular colonoscopy uses a long, lighted tool called a colonoscope that is inserted into the rectum and large intestine.
Virtual colonoscopy 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, since 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 then view the images on a video monitor.
How to Prepare for the Test
Your bowels need to be completely empty 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 bowels. This is called bowel preparation. Steps may include using enemas, not eating solid foods for 2 or 3 days before the test, and 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
You will likely be told to stop taking aspirin, ibuprofen, naproxen, or other blood-thinning medicines for several days before the test. Keep taking your other medicines unless your doctor tells you otherwise.
You will need to stop taking iron pills or liquids a few days before the test, unless your health care provider tells you it is okay 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 wear a hospital gown for the procedure.
How the Test will Feel
The x-rays are painless. Pumping air into the colon can create cramping or gas pains.
Why the Test is Performed
Virtual colonoscopy may 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 virtual colonoscopy. The reason is that virtual colonoscopy does not allow the doctor to remove tissue samples or polyps.
Other times, a virtual colonoscopy 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
- Colorectal cancer
- Inflammatory bowel disease
- Lower gastrointestinal (GI) bleeding
Regular colonoscopy may be done (on a different day) after a virtual colonoscopy if:
- No cause for bleeding or other symptoms were found. Virtual colonoscopy can miss some smaller problems in the colon.
- Problems that need a biopsy were seen on a virtual colonoscopy
Risks of virtual colonoscopy include:
- Exposure to radiation from the CT scan
- Nausea, vomiting, bloating, or rectal irritation from medicines used to prepare for the test.
Differences between virtual and conventional colonoscopy include:
- Virtual colonoscopy can view the colon from many different angles. This is not as easy with regular colonoscopy.
- Virtual colonoscopy 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.
- Virtual colonoscopy 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 virtual colonoscopy.
- Virtual colonoscopy is often not able to detect polyps smaller than 10 mm. Regular colonoscopy can detect polyps of all sizes.
Itzkowitz SH, Potack J. Colonic polyps and polyposis syndromes. In: Feldman M, Friedman LS, Brandt LJ, eds. Sleisenger and Fordtran's Gastrointestinal and Liver Disease Pathophysiology/Diagnosis/Management. 9th ed. Philadelphia, Pa: Elsevier Saunders; 2010:chap 122.
National Comprehensive Cancer Network. NCCN Clinical Practice Guidelines in Oncology (NCCN Guidelines): Colorectal cancer screening. Version 2.2013. Available at: www.nccn.org/professionals/physician_gls/pdf/colorectal_screening.pdf. Accessed October 2013.
Philip AK, Lubner MG, Harms B. Computed tomographic colonography. Surg Clin N Am. 2011;91:127–139.
Reviewed By: George F. Longstreth, MD, Department of Gastroenterology, Kaiser Permanente Medical Care Program, San Diego, California. Also reviewed by David Zieve, MD, MHA, Bethanne Black, and the A.D.A.M. Editorial team.