CT (computed tomography) colonography is a new type of x-ray exam that applies the well-known CT technique to examining the interior of the colon, or large intestine. CT itself is a noninvasive type of radiography that uses a rotating beam of x-rays and detectors placed at varying angles to obtain cross-sectional images, or "slices," of a particular part of the body. Special computerized analysis converts the images into detailed three-dimensional pictures of the inside of the colon. In this way it is possible to accurately demonstrate a wide range of abnormal changes including tumor masses. Another name for CT colonography is "virtual colonoscopy," meaning that it enables the radiologist to "see" inside the colon without having to insert a viewing instrument—the colonoscope—into the bowel.

Who Should Have CT Colonography, and Why?

Colon cancer is a serious and common condition. With about 150,000 Americans being diagnosed with colon cancer each year, it is the third most frequent cancer in this country and the second most common cause of cancer deaths. Most often colon cancers start when malignant cells form within polyps (benign growths) attached to the inner surface of the large bowel. If these polyps are detected early enough, taking them out will prevent colon cancer from developing. Up until recently, the standard means of screening for polyps has been colonoscopy, which involves inserting a long, flexible tube into the colon through which the physician can see any polyps that are present and remove them. Many people are not willing to have colonoscopy. In fact, a current estimate of the proportion of persons who are screened by colonoscopy or other methods—including barium enema x-ray exam or fecal occult blood test (FOBT)—is only about 40 percent of the target population. This means that a majority of persons eligible for screening do not have it. For colonoscopy—the most certain of these methods—the figure is much lower.

There is wide agreement that everyone older than 50 years should be screened by some method, as well as those who for any reason are at increased risk of developing colon cancer. This applies to individuals who have had polyps in the past, have a family history of colon cancer, or test positive for blood in the stool. CT colonography may prove to be especially helpful:

for elderly patients who are frail or infirm.
when colonoscopy is attempted but incomplete because the full length of the colon is not visualized. Roughly 10 percent to 20 percent of patients fall into this category. In this circumstance the colon will already have been cleansed, and CT colonography may be done on the same day.
if a tumor is found on colonoscopy and is large enough to block passage of the scope, CT colonography will permit a thorough check of the entire colon.

A number of studies carried out at university-affiliated hospitals throughout the country suggest that CT colonography is a very sensitive means of detecting polyps in the colon. A radiologist highly practiced in this method, Dr. Michael Zalis from Massachusetts General Hospital in Boston, acknowledges that it is premature to think of replacing colonoscopy with CT colonography as the primary means of screening for colon cancer. At the same time, he believes that there is sufficient evidence to use CT colonography as an alternative to colonoscopy for patients who otherwise would not be screened in any way. Most medical authorities have adopted a policy of screening average-risk persons every three to five years. An alternative would be for everyone to have colon screening at least once, perhaps when reaching 60 years of age.

What It's Like to Have CT Colonography

Following bowel cleansing the evening before, you will be asked to empty your bowels just before the exam. Then a small flexible tube is placed in the rectum. Your privacy will be respected. After the tube is in place, either you or a technologist will inflate the colon with air using a hand-held squeeze bulb. Some patients temporarily feel slight abdominal cramping, or "gas pains," but significant pain is infrequent. Usually the colon is adequately distended at the point that the patient begins to feel uncomfortable. A muscle-relaxing drug may be injected intravenously to relax the bowel and lessen discomfort, but this usually is not necessary. The CT scan, first done with the patient lying face-up and then turned to the prone position, takes only about 10 minutes, and the entire procedure can be completed within 20 minutes. Imaging in both positions will help show the radiologist the difference between a polyp and fecal material that was not cleared out. Unlike colonoscopy, you will not receive a sedative and so will be able to drive home or return to work.

Bowel preparation (cleansing) is vital to obtain accurate results with CT colonography. You will be asked to limit your intake to clear liquids (broth, tea, juice) and jello the day before the exam. You will be instructed to take either a set of pills or a cathartic liquid to help empty your bowels the evening before the exam. The diarrhea that follows will leave a minimum of fluid in the colon and permit clear pictures of its inner lining so that even very small polyps can be detected. Virtually all patients tolerate the bowel preparation well. If you have heart, liver, or kidney disease, you should check with your physician to be sure that the bowel preparation you use will be safe. You may resume your regular diet immediately after the procedure.

No serious complications have been reported from CT colonography. The risk that instilling air will injure the colon is quite small. Although CT colonography does involve the use of x-rays, the radiation dose is very low—lower, in fact, than needed for a barium enema radiographic exam.

Interpreting the Results

Most people do not have polyps, and for them CT colonography may avoid the need for colonoscopy. If one or more polyps are detected by CT colonography, your physician will be informed and probably will recommend therapeutic colonoscopy to remove the abnormal tissue. Most polyps grow slowly. Only if they are larger than about one-half inch in diameter is there a substantial chance that cancer cells will already be present. If CT colonography is negative for polyps, your radiologist may recommend a follow-up exam after three years have passed. As more experience is gained with CT colonography, the interval between follow-up exams is expected to get longer.

In addition to two-dimensional images recorded at different angles, which present different views of the entire colon, three-dimensional (3-D) images of the colon's interior are obtained that closely resemble what is seen with colonoscopy. It is the 3-D view that prompted the term "virtual colonoscopy," as it provides what amounts to a movie of the entire colon from an "insider's" perspective. Recently researchers have devised a computer-aided detection (CAD) system for evaluating colonic polyps. Because this is a highly sensitive process that detects most polyps, and because most positive readings prove to be accurate, computer-aided detection has the potential to make CT colonography a practical means of screening large groups of people. Another advantage of computer-aided detection is that it requires less time to evaluate the findings.

How Well Does CT Colonography Perform?

CT colonography reportedly detects a great majority of colonic polyps measuring a half-inch or more in size—lesions which have significant potential for becoming cancerous. However, the exam is less sensitive in detecting smaller polyps, which are less likely to progress to cancer. Some researchers believe that a policy of removing only polyps of a certain size, as opposed to removing all those detected, might be just as effective in saving lives while minimizing risk and cost. A study conducted at several centers and sponsored by the American College of Radiology (ACR) found that CT colonography detected 80 percent of polyps measuring a half-inch or more. The study results indicate that even if CT colonography detects only half the polyps present, but is more acceptable to patients causing a higher proportion of them to be screened, the procedure will reduce deaths from colon cancer more than if a perfect test were available but was used in only 10 percent of the population.

Advantages of CT Colonography

It is a minimally invasive procedure, and avoids the low but real risk of perforating the colon that may occur with conventional colonoscopy.
It is well tolerated by patients, who require no sedation.
CT colonography is substantially less costly than colonoscopy.

Disadvantages of CT Colonography

Patients are exposed to radiation, though in an acceptably low dose.
Compared to colonoscopy, CT colonography provides less detail of the inner lining of the colon, and it is in black and white. The lack of color makes it harder to evaluate lesions.
Small polyps are located more reliably by colonoscopy (although conventional colonoscopy still misses up to 20 percent of polyps and up to five percent of colon cancers).
Like the barium enema radiographic exam, CT colonography is strictly a diagnostic procedure. Any polyps that are found have to be removed by conventional colonoscopy.
Most insurance companies as yet do not cover CT colonography as a screening test for colonic polyps. They may, however, cover the cost of the exam if you have symptoms related to the colon.

Trade-offs between cost, risk, and accuracy place CT colonography midway between the two standard techniques of evaluating the colon: conventional colonoscopy and the barium enema radiographic test.