What is virtual colonoscopy?
Virtual colonoscopy (VC) or CT colonography is a method of screening the colon for precancerous polyps. Using a CAT 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 this information to produce an image of the colon’s inner surface. When the colon is properly cleansed and expanded (distended) with room 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.
How does it 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. The information from these exams 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 completely the same as the colonoscopists.
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. 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 is it 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 where he or she then changes into an examination gown.
- A small tube is placed in the rectum and gas is slowly pumped into the colon manually or using 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. CO2 is thought by many to be more comfortable.
- Once the appropriate amount of distension is achieved, the patient is scanned with the Computer Tomography (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.
- There are two methods that can be used to deal with a polyp identified on a CT colonography. If the patient desires to have a same day diagnostic colonoscopy if a polyp is identified (avoiding another prep), he or she waits for approximately 45 minutes to 1 hour in order to determine whether a polyp has been identified. This time is needed to carefully examine the colon using computer images. If a polyp is identified, the patient then goes to the colonoscopy suite where a formal colonoscopy is performed to remove the polyp. If no polyp is found, the patient goes home. Alternatively, the patient may prefer to go home and wait to see if a polyp is identified on the CT colonography.
- No sedation is given for the CT colonography. However, the patient must still have a responsible person available to drive home should a colonoscopy be needed (sedation will be given for this).
- The patient may resume normal activity after the procedure.
More about CT colonography
While the accuracy of CT colonography is equivalent to regular colonoscopy in polyps greater than 7 to 8mm, CT colonography is not as good as regular colonoscopy for smaller polyps. Fortunately, these smaller polyps usually are not cancerous. 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 via a regular colonoscopy. If an abnormality is detected on a CT colonography, the patient will need a regular colonoscopy to determine the nature of that polyp.