- Written by Los Angeles Colonoscopy
What is virtual colonoscopy?
Virtual colonoscopy or CT colonography is an examination of the colon, without actually performing a direction visualization of the colon. Unlike conventional technique such as colonoscopy in which an instrument is inserted inside the colon, images of the large intestine are taken using computerized tomography (CT). A computer then puts the images together to create a three-dimensional images of the inside of the large intestine, or virtual images.
What is computerized tomography (CT)?
CT is an examination that takes hundreds of cross-sectional x rays in a few seconds. Like putting together a loaf of bread from its many slices, a computer puts cross-sectional x-ray pictures together to form whole images of internal organs.
How to Prepare for Virtual Colonoscopy
The instructions for the exam is usually provided by the center that performs virtual colonoscopy. The process is called bowel prep. The bowel prep for virtual colonoscopy is almost identical to the bowel prep for conventional colonoscopy. Generally, all solids must be emptied from the gastrointestinal (GI) tract by following a clear liquid diet for 1 to 3 days before the procedure. Acceptable liquids include
- fat-free bouillon or broth
- strained fruit juice
- plain coffee
- plain tea
- sports drinks, such as Gatorade
A laxative will be required the night before virtual colonoscopy. A laxative is medicine that loosens stool and increases bowel movements. Laxatives are usually swallowed in pill form or as a powder dissolved in water.
After the bowel prep, patients undergoing CT drink a liquid called contrast media that causes the large intestine to appear very bright during CT. Contrast media helps the doctor identify abnormal tissues.
How is virtual colonoscopy performed?
Virtual colonoscopy is performed wherever the CT scanner or MRI unit is located—usually in the radiology department of a hospital or medical center. The procedure takes about 10 minutes and does not require sedation.
- Patients will lie face up on a table.
- A thin tube will be inserted through the anus and into the rectum. For CT, carbon dioxide gas will be pumped through the tube to expand the large intestine for better viewing. For MRI, contrast media will be given rectally to expand the large intestine.
- The table will move through the CT scanner or MRI unit to produce a series of cross-sectional images of the colon.
- At various points during the procedure, the doctor may ask patients to hold their breath to steady the images.
- The procedure will be repeated while patients lie face down.
After the procedure, cross-sectional images taken by CT or MRI are processed to create three-dimensional, computer-generated images of the large intestine. A radiologist evaluates the results to identify any abnormalities. If abnormalities are found, conventional colonoscopy may be performed the same day or at a later time.
How is virtual colonoscopy different from conventional colonoscopy?
The main difference between virtual and conventional colonoscopy is how the doctor sees inside the colon. Conventional colonoscopy uses a long, lighted, flexible tube called a colonoscope to view the inside of the colon, whereas virtual colonoscopy uses CT or MRI.
What are the advantages of virtual colonoscopy?
Virtual colonoscopy has several advantages over other procedures:
- Virtual colonoscopy does not require the insertion of a colonoscope into the entire length of the colon. Instead, a thin tube is inserted through the anus and into the rectum to expand the large intestine with air.
- No sedation is needed. A patient can return to usual activities or go home after the procedure without the aid of another person.
- Virtual colonoscopy provides clearer, more detailed images than a conventional x ray using a barium enema—sometimes called a lower GI series.
- Virtual colonoscopy takes less time than either conventional colonoscopy or a lower GI series.
- Virtual colonoscopy can see inside a colon that is narrowed due to inflammation or the presence of an abnormal growth.
What are the disadvantages of virtual colonoscopy?
Virtual colonoscopy has several disadvantages:
- As with conventional colonoscopy, virtual colonoscopy requires bowel prep and the insertion of a tube into the rectum for expanding the large intestine with gas or liquid.
- Virtual colonoscopy does not allow the doctor to remove tissue sample or polyp (polypectomy).
- Virtual colonoscopy does not detect pre-cancerous polyps smaller than 10 millimeters.
- Many health insurance including Medicare plans do not pay for virtual colonoscopy cancer screening.
- Virtual colonoscopy is a newer technology and is not available in many locations.
- Radiologist may not be as experienced with the reading of virtual colonoscopy, since it is relatively new and was not included in formal training of currently practicing radiologist.
There has been increased excitement recently over the development of virtual colonoscopy that utilizes CT scan (a form of X-ray examination) to circumvent colonoscopy in the detection of colonic neoplasm. However, there is an increased concern regarding the high dose of ionizing radiation exposure required to perform virtual colonoscopy. This test may, in fact, be associated with an excessive life-time risk of developing cancer at 1/270 according to one study. Additionally, its relative low sensitivity for lesion smaller than 1cm, as well as the need to undergo colonoscopy anyway if a lesion is found on CT scan has tempered enthusiasm for its use.
Please note that we do not perform virtual colonoscopy, here at the Los Angeles Endoscopy Center.