A virtual colonoscopy is used to identify signs of colorectal cancer. It also is used to locate polyps and growths of tissue in the colon and rectum, which may be precancerous. The procedure, also called computed tomographic (CT) colonography, uses a CT scan to construct images of these areas, which make up your large intestine. After the scan, a computer combines the images of the colon and rectum to produce both two- and three-dimensional interior views. In some cases, magnetic resonance imaging (MRI) is used instead of CT scanning.
The difference between virtual colonoscopy and conventional colonoscopy is how the images of your colon and rectum are retrieved. Conventional colonoscopy uses a colonoscope, which is a long, lighted tube. This flexible tube, which is placed inside your colon and rectum, uses a small camera to send images of the areas to a video monitor. Virtual colonoscopy uses a CT or MRI to take pictures of the inside of the colon from outside of your body.
Virtual colonoscopy is used for early detection of colon cancer in men and women. It allows a physician to look at your colon and rectum to find abnormal areas, which may indicate polyps or tumors. With early detection, some polyps can be found and removed before they can turn into cancer. If cancer is already present, it is more likely to be curable if it is found early.
US Preventive Services Task Force Recommendations
The United States Preventive Services Task Force (USPSTF) recommends screening for colorectal cancer using fecal occult blood testing, sigmoidoscopy, or colonoscopy in adults, beginning at age 50 years and continuing until age 75. This is an A recommendation—in other words, there is a high certainty that net benefit is substantial. The USPSTF recommends against routine screening for colorectal cancer in adults age 76 to 85 years. The USPSTF do not include either CT colonography (CTC or so-called virtual colonoscopy) or DNA stool tests as screening tests, saying that there was not enough current evidence to judge the harms and benefits of the new technology.
American Cancer Society Recommendations
The American Cancer Society recommends that men and women who do not have a family history of colon cancer or other high-risk factors start regular colorectal screenings at age 50. Screening every five years is advised for patients who have had a virtual colonoscopy. If you are at a high risk for colon cancer, you physician will set your screening schedule. High-risk factors include a family history of polyps or cancer, diabetes, obesity, Crohn’s disease, and ulcerative colitis.
To allow your physician to get the clearest images, your colon and rectum must be emptied before a virtual colonoscopy. This is called bowel prep.
- You will be told to follow a clear liquid diet for a day or two before the exam.
- You usually won’t be able to eat or drink anything the night before the exam.
- You will be given a strong laxative, and possibly a suppository, to help clear your bowels.
- You may be given a contrast agent in pill form to begin taking at home. The contrast agent will highlight your large intestine to help your physician tell the difference between a stool and a small polyp.
Tell your physician about any medications you are taking at least one week before the exam in case any changes need to be made to your medication schedule.
A virtual colonoscopy is usually done in a hospital, but it is an outpatient procedure. This means that you will not be checked into the hospital and can leave again on the day of the test. It takes 10 to 15 minutes to complete and you will not be sedated. The testing process is as follows:
- Before the exam, you will change into a hospital gown.
- If you are having a CT virtual colonoscopy, you may be asked to drink a liquid contrast agent. (In some cases, you may take the contrast in pill form at home.)
- You will be asked to lie on a table.
- A thin, flexible tube will be placed into your rectum.
- For CT procedures, room air or carbon dioxide is pumped slowly into the tube. This inflates the rectum and allows for closer examination.
- For MRI exams, a contrast agent is used to enlarge the area. This is administered rectally.
- The table you are lying on will slide into the CT or MRI machine.
- Your physician may ask you to hold your breath for a few moments in order to get a steady image.
- You will have at least two scans, each lasting about 15 seconds.
- You will be scanned while lying on your back and then on your stomach.
- After the exam, you will be able to release the gas in the bathroom.
- You can return to work or normal activities after the exam.
There are few risks associated with virtual colonoscopy. The air in your colon may make you feel bloated, but the feeling should dissipate once you pass the air from your body. Rarely, some patients have a negative reaction to the contrast agent. Though uncommon, mild reactions can include:
Moderate to severe responses to the contrast agent, though rare, resemble an allergic reaction and may include:
- hives or itching
- difficulty breathing or swallowing
- distress or confusion
- rapid heartbeat
- bluish skin color
There is a very small risk that inflating your colon could cause injury. The American Cancer Society considers this risk to be much lower with virtual colonoscopy than conventional colonoscopy. Also, like other types of CT scan, this test subjects you to a small, but safe, amount of radiation.
- Virtual colonoscopy does not use a colonoscope, so it may be more comfortable for some patients.
- Patients do not need a sedative during virtual colonoscopy, while one is usually given for conventional colonoscopy.
- Patients can resume normal activities after a virtual colonoscopy. However, patients usually require recovery time from the sedatives used in a conventional colonoscopy.
- Virtual colonoscopy lasts about 10 to 15 minutes versus about 30 to 60 minutes for conventional colonoscopy.
- Virtual colonoscopy allows a physician to examine a colon that may be constricted by swelling or abnormal growth.
- Virtual colonoscopy has less risk of colon tearing, or perforation, than conventional colonoscopy.
- In virtual colonoscopy, a tube is placed into your rectum to expand it with gas. Gas is not used during a conventional colonoscopy.
- A physician can’t remove a tissue sample or polyp during a virtual colonoscopy. If tissue removal (biopsy) is necessary, you will need to have a conventional colonoscopy.
- Virtual colonoscopy is not as sensitive as conventional colonoscopy for finding polyps less than 10 millimeters in size.
- Some health insurance plans may not cover the cost of virtual colonoscopy.
After your virtual colonoscopy, a radiologist will review the CT or MRI images. If your physician is not satisfied with the images, you may need to repeat the scan or have another type of screening.
The results of a virtual colonoscopy are negative if there are no abnormalities in your colon. The results of the test are positive if any abnormalities or polyps are found. If your results are positive, your physician may recommend a conventional colonoscopy, during which abnormal tissue samples can be obtained and polyps can be removed.
Other abdominal problems not associated with colorectal cancer may also be found during virtual colonoscopy. Tumors in the abdomen or the areas surrounding the kidney, liver, pancreas, or spleen may be detected since these organs lie near your large intestine.