Virtual colonoscopy is an examination of the large bowel using computerized X-ray techniques, such as computed tomography (CT scan), or magnetic resonance imaging (MRI). The purpose of the examination is to look for polyps or tumors in the large intestine. It provides images of the colon but does not allow for biopsy or direct look.

Description of the procedure
Like standard colonoscopy, virtual colonoscopy requires a bowel preparation to cleanse the colon. This will usually require clear liquid diet and laxatives the day prior to the procedure. On the day of the examination, a tube is placed into the rectum and the colon is then filled with air or water at which time an abdominal-pelvic CT scan or an MRI is performed. Because air or water is pumped into the colon, some cramping may result.

Accuracy of virtual colonoscopy
Recently published studies suggest that virtual colonoscopy is almost as accurate as standard colonoscopy in finding large polyps, but probably not as accurate in finding small polyps. It is not useful for examining details of the lining of the colon.

Need for follow-up standard colonoscopy
If polyps are detected, or if there was inadequate bowel cleansing, conventional colonoscopy will still be needed to remove the polyps found, and to examine areas that were inadequately cleansed. In a recent large study, over half the people who had virtual colonoscopy needed to have a follow-up standard colonoscopy.

Reasons for the examination
Virtual colonoscopy should be considered for those individuals who have an obstruction in the colon that prevents standard colonoscopy. It is not appropriate for individuals with family history of colon cancer, prior polyps, anemia, or other risk factors for having polyps. It is also not appropriate for individuals with diarrhea, rectal bleeding, or other conditions that may require direct examination of the lining of colon. It may be considered for screening in individuals with no digestive symptoms and low risk for colon polyps who would prefer to avoid the sedation given during standard colonoscopy in return for the risk of needing a second procedure.

Conclusions
At this time the American Cancer Society, the U.S. Multi-Society Task Force on Colorectal Cancer, and the U.S. Preventive Services Task Force have not recommended virtual colonoscopy for the purpose of screening for colorectal cancer. Patients who undergo virtual colonoscopy may expose themselves to false negative and false positive results leading to missed diagnoses and/or the need for additional testing.