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Virtual Colonoscopy
The American Cancer Society recommends colorectal screening every three to five years for people
over age 50. Screening involves the identification of polyps (small, noncancerous growths) on the
bowel wall; the removal of suspicious polyps can prevent them from developing into malignant
growths. The most common screening procedures are traditional colonoscopy and barium enema.
Traditional colonoscopy is accurate in detecting most polyps, but the instrument used--a
colonoscope--is not able to reach all sections of the colon in about 5% to 15% of patients. In
addition, the procedure requires sedation and carries a very small risk of colon perforation. The
next most common screening test, --the barium enema-does not have these drawbacks, but it lacks
diagnostic accuracy for smaller polyps. The unwieldy nature of these screening procedures has led
researchers to look to powerful new imaging techniques to simplify screening for colorectal cancer.
A new procedure, computerized tomographic (CT) colonography, or virtual colonoscopy, is gaining
approval at medical centers across the US as an alternative to the traditional techniques. Like
traditional colonoscopy, virtual colonoscopy begins with pre-exam full bowel and colon cleansing and
air distension. Rather than inserting a colonoscope into the rectum, thin-section spiral CT scans of
the abdomen are taken and processed via virtual reality software into a variety of two- and
three-dimensional reconstructions. The three-dimensional reconstructions include the entire colon,
including "blind spots" caused by tissue folds in the colon as well as areas blocked from view by
strictures. Furthermore, the colon can be viewed from any angle, which can add to the amount of
information gleaned from the test.
Since 1994, the accuracy of this procedure has improved rapidly, largely due to advances in
ultra-fast spiral CT scanners and better image processing software. The reported polyp detection
rates of virtual colonoscopy surpass those of double-contrast barium enema and have begun to
approach those of traditional colonoscopy; for polyps greater than 1 centimeter, the reported
accuracy is similar to that of traditional colonoscopy.
The technique is fast (15 minutes of patient time), examines the entire colon, and requires no
sedation. It is quick for radiologists as well; with proper training, a full virtual colonoscopy
exam can be reviewed in as little as 10 minutes. Unlike conventional colonoscopy, the procedure
cannot be used for the removal of polyps found during the exam. However, with good cooperation
between radiologists and endoscopists, patients who require polyp removal can be referred for
same-day virtual colonoscopy and polyp removal through conventional colonoscopy and so avoid
repeating the bowel preparation.
Virtual colonoscopy can be used for regular screening of the average risk patient (those over 50
with no previous history of cancer or polyps), but has particular utility in patients who have had
unsuccessful or incomplete traditional colonoscopies, as well as the frail and elderly.
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