What is virtual colonoscopy?
Virtual colonoscopy (VC), or CT colonography, is a method of screening the colon for precancerous polyps. Using a CAT (computed tomography, or CT) scanner and computer methods of reconstructing the images, the colon can be evaluated without a colonoscope and without sedation.
Virtual colonoscopy takes the information produced by a CT scanner and processes it to produce an image of the colon's inner surface. When the colon is properly cleansed and distended (expanded) with air or carbon dioxide, and when the CT information is processed, health care providers can look at the inner lining or surface to detect polyps.
Why do we need another test to detect precancerous colon polyps?
It is estimated that several million people in the United States alone have not been properly screened for colorectal polyps. The best method for screening is a regular colonoscopy. However, there are simply not enough endoscopists to screen such a large number of patients. Other methods, such as sigmoidoscopy, evaluate only a part of the colon. Tests to detect small amounts of blood in the stool can miss polyps and even cancers. The barium enema is not nearly as sensitive as a regular colonoscopy.
If a noninvasive test could be developed that could select those patients with a polyp, then fewer and more focused colonoscopies could be performed. The combination of newer, faster CT scanners with computer software that would produce images equivalent to an endoscope seems to be a logical step in the development of such a test.
How does virtual colonoscopy work?
CT scanners produce their images as the patient lies on a "bed" that is pulled through a short tunnel. Within the tunnel, there is a rotating x-ray tube on one side. On the other side is a set of detectors that receive the transmitted x-rays after they have passed through the body.
A continuous volume of information is obtained from this exam, and can be processed in virtual reality computers. Because the inside of the colon has been distended, providers can look inside the colon and have a view almost exactly the same as the colonoscopists.
How is a virtual colonoscopy done?
- As with standard colonoscopy, the patient must undergo a preparation that includes a liquid diet and a colon cleansing.
- On the morning of the procedure, the patient arrives in the radiology department and changes into an examination gown.
- A small tube is placed in the rectum and gas is slowly pumped into the colon (insufflation) manually (by hand) or with a medical device called an automated insufflator, which maintains a constant pressure. With manual insufflation, room air is used. With the automated insufflator, CO2 (carbon dioxide) is used to inflate the colon. (Many patients find CO2 to be more comfortable.) No sedation is given for the CT colonography.
- Once the appropriate amount of distension is achieved, the patient is scanned with the CT machine. The scan time takes approximately 15 seconds, during which time the patient holds his or her breath. The patient is scanned both on the stomach and back. This allows fluid and stool to fall away from the dependent portion of the colon.
- After the exam, the patient goes to the bathroom to expel the gas and then gets dressed. The patient may resume normal activity after the procedure.
Results and Follow-Up
What happens if a polyp is detected during a virtual colonoscopy?
If a polyp is detected, the patient will be contacted by his or her physician to discuss alternatives. If the polyp is small (less than 5-7 mm), the physician may recommend close followup. If the polyp is greater than 1 cm, then a standard colonoscopy will likely be requested.
More about virtual colonoscopy
While the accuracy of CT colonography is equal to that of regular colonoscopy in polyps greater than 7 to 8 mm, CT colonography is not as good as regular colonoscopy for smaller polyps. Fortunately, these smaller polyps usually are not cancerous.
While CT colonography cannot differentiate a non-precancerous polyp from a precancerous polyp, neither can regular colonoscopy. Only a biopsy can provide this information. Biopsy can only be performed with a regular colonoscopy.