What is a virtual colonoscopy?
A virtual colonoscopy uses computed tomography (CT) or magnetic resonance imaging (MRI) to make a three-dimensional image of the interior lining of your large intestine (colon) and rectum. A virtual colonoscopy can be used to screen for precancerous and cancerous growths in the colon or rectum (colorectal cancer), such as polyps or tumors.
Virtual colonoscopy (also called computed tomographic colonography, or CTC) is still being studied to determine whether the results are as accurate as a regular colonoscopy. Virtual colonoscopy is not yet widely available.
Virtual colonoscopy is one of many tests that may be used to screen for colon cancer. Which screening test you choose depends on your risk, your preference, and your doctor. Talk to your doctor about what puts you at risk and what test is best for you.
For more information on screening tests for colon cancer, see:
Why is it done?
A virtual colonoscopy may be done to:
- Screen for colorectal cancer or polyps.
- Monitor the growth of polyps.
- Screen for recurrence of colorectal cancer in people who have had surgery for this disease.
- Evaluate abnormal results from other colon tests.
- Replace a regular colonoscopy if the thin lighted scope cannot be inserted in the colon for some reason, such as when a tumor blocks the passage.
Virtual colonoscopy is not commonly done for other conditions at this time but may prove to be useful to:
- Detect other problems or diseases of the anus, rectum, or large intestine (colon).
- Check the source of internal bleeding.
- Check the cause of chronic diarrhea.
- Monitor the treatment of inflammatory bowel disease.
How do I prepare for it?
Virtual colonoscopy requires the same cleansing colon prep as a regular colonoscopy. Your doctor will tell you what to do before the test.
If you are having virtual colonoscopy using CT, you will be given some liquid to drink after your bowel prep. This drink is a contrast medium that makes the large intestine easier to see in the pictures.
How is it done?
Virtual colonoscopy is usually done in the radiology department at a hospital or medical center. The test is most often done by a doctor who specializes in performing and interpreting diagnostic imaging tests (radiologist). The doctor may also have an assistant.
You will need to take off most of your clothes. You will be given a gown to wear during the test.
First, you will be asked to lie on your back on the table of the CT or MRI machine. A thin tube is inserted into the rectum through the anus. For CT, air or carbon dioxide gas will be pumped through the tube. (There may be some discomfort as the gas expands your colon.) For MRI, a contrast liquid will be given through the tube into the colon. This helps the doctor see all parts of your colon.
The table will slide into the CT or MRI machine so pictures of your colon can be taken. You may be asked to hold your breath when pictures are taken. This can help prevent blurry pictures. Then the procedure will be repeated while you lie face down.
The test usually takes about 10 minutes.
How does a virtual colonoscopy differ from a regular colonoscopy?
Virtual colonoscopy has advantages and disadvantages compared to a regular colonoscopy.
- Virtual colonoscopy is less invasive, safer, and takes less time than a regular colonoscopy.
- A thin tube to insert air into the colon is placed in the rectum rather than a long flexible tube that is moved up your colon.
- No medicine to relax or sedate you is needed, so you will be able to drive yourself home and resume normal activities.
- Virtual colonoscopy can show the inside of a colon that is narrow due to inflammation or an abnormal growth.
- Virtual colonoscopy may not show polyps smaller than 10 mm (0.4 in.).
- Virtual colonoscopy is not covered by all health insurance plans. Check with your insurance plan before having the test.
- If a polyp is found, regular colonoscopy will be needed to confirm the diagnosis and remove the polyp so it can be looked at under a microscope.
- This is a newer test and isn't as widely available as regular colonoscopy.
What do I need to consider about a virtual colonoscopy?
If you have an average risk for colorectal cancer, virtual colonoscopy may be an appropriate screening test for you. But if you are at an increased risk, you may require a regular colonoscopy because tissue biopsies or polyp removal can be done at the same time.
Virtual colonoscopy requires the same cleansing colon prep as a regular colonoscopy. If an abnormality is found on virtual colonoscopy, you may need a regular colonoscopy to remove and test the abnormal tissue.
Virtual colonoscopy is not available everywhere because it requires special equipment and special training to interpret the test. The initial evaluations on virtual colonoscopy were done by highly trained radiologists, so the results of your virtual colonoscopy may vary widely depending on the amount of experience your doctor's office or clinic has with the procedure.
Virtual colonoscopy is being studied to determine whether the results are as accurate (sensitivity) as a regular colonoscopy. More evidence is needed to show that virtual colonoscopy is a benefit in people with average risk for colon cancer and whether this screening method can prevent cancer. Colon preps that are more comfortable are also being studied.
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Other Works Consulted
- Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.
- Gupta S, et al. (2008). Variation of agreement in polyp size measurement between computed tomographic colonography and pathology assessment: Clinical implications. Clinical Gastroenterology and Hepatology, 6(2): 220–227.
- Kim DH, et al. (2007). CT colonography versus colonoscopy for the detection of advanced neoplasia. New England Journal of Medicine, 357(14): 1403–1412.
- Levin B, et al. (2008). Screening and surveillance for the early detection of colorectal cancer and adenomatous polyps, 2008: A joint guideline from the American Cancer Society, the U.S. Multi-Society Task Force on Colorectal Cancer, and the American College of Radiology. CA: A Cancer Journal for Clinicians, 58(3): 130–160.
- Pagana KD, Pagana TJ (2010). Mosby's Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby Elsevier.