Distension of the Colon
Once the colon has been prepared, the examination is ready to be performed. It should be noted that different institutions use different CT techniques. What is outlined below is the NYU technique, which we have found is easy to perform and is very successful in detecting colorectal polyps with diameters of 10 mm or more. The procedure is performed by a trained technologist or nurse.
Immediately before the examination, the patient is asked to evacuate any residual fluid from the rectum. Therefore, easy access to a close bathroom is essential. Either room air or carbon dioxide can be used to inflate the colon.
The use of room air is easy, clean, and inexpensive. Proponents of carbon dioxide argue that because it is easily absorbed from the colon it causes less cramping after the procedure than room air. In our experience, carbon dioxide is associated with less delayed discomfort. While cramping may be a problem in some patients after inflation of the colon with room air, most find the examination to be quick and minimally uncomfortable.
We use a small rubber catheter to inflate the colon with a hand-held bulb syringe. This catheter is much smaller than a barium enema tip and a balloon is not used. Patients do not feel this catheter being inserted. Patients are encouraged to keep the gas in. We ask them to let the technologist know when they are just beginning to feel uncomfortable from the distension. Generally this signals that the colon is well distended.
Approximately 40 puffs with a hand-held bulb syringe are enough to distend the colon. However, we do not use a set strict number of puffs since the length of an individual colon is variable. Also, if the valve between the small and large intestines is not working properly, more gas will be required for optimal distension.
We do not use a bowel relaxant (i.e., glucagon) for virtual colonoscopy (35). This minimizes cost and patient anxiety since no intravenous needles are used. At NYU, we have been very successful in distending the colon sufficiently without a bowel relaxant. After the colon is inflated, the catheter is left in the rectum. With the patient lying on his or her back, a single scout CT image is taken to verify adequate bowel distension. If the bowel is adequately distended, the CT examination is performed. If the bowel is not distended enough, additional air is pumped into the rectum and then the necessary images are obtained.
Finally, regarding patient preparation, there is some controversy about the use of intravenous contrast administration. One study found improved detection of polyps after the administration of intravenous (IV) contrast material (43). Occasionally polyps may be obscured by residual fluid.
After administering IV contrast the view of a polyp will be enhanced and it may become visible despite the fluid. However, the downside of the routine administration of contrast is cost, need for IV access, and risk of allergy from the iodinated contrast material. However, when it is known that a lesion in the colon exists it may be valuable to use IV contrast, since it may show a clearer outline of the abnormality. It may also allow the radiologist to more accurately stage the tumor (to determine how advanced it is).