The American Cancer Society recommends a colorectal screening every 3 to 5 years for people over the age of 50. Screening involves the identification of polyps (small benign neoplasms) on the bowel wall; surgical adenomatous 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 the detection of most polyps, but the colonoscope is not able to reach all portions of the colon in 5 to 15 percent of patients, and the procedure carries a risk of colon perforation. Barium enemas lack diagnostic accuracy for smaller polyps and colonoscopy requires time-consuming sedation. The unwieldy nature of these procedures has led some to look to powerful new imaging techniques to simplify the procedure.

A new procedure, computed tomographic (CT) colonography, or virtual colonoscopy, is gaining approval as an alternative to the traditional techniques. Like traditional colonoscopy, virtual colonoscopy begins with full bowel and colon cleansing and air distension. Rather than inserting a colonoscope into the anus, thin-section spiral CT scans of the abdomen (both supine and prone) are taken and processed into a variety of two- and three-dimensional reconstructions. The three-dimensional endoluminal reconstructions can be viewed from any angle, and include the entire colon, including “blind spots” caused by colonic folds and the areas beyond occlusive colorectal cancers.

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 5 or 6 millimeters, the reported accuracy is the same as 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 polypectomy. However, with good cooperation between radiologists and endoscopists, patients who require polypectomy can be referred for same-day colonoscopy and polypectomy and avoid repeating the bowel preparation.

Virtual colonoscopy is ideal 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.

Virtual colonoscopy currently is offered at the Columbia campus of NewYork-Presbyterian Hospital, and is expected to be available at the Weill Cornell campus in the fall. For more information on virtual colonoscopy, call Dr. Louis Germaine in the Department
of Radiology at the Columbia campus (305-2602).