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If you've ever had a colonoscopy — that much dreaded procedure in which a physician inserts a lighted tube into your rectum and snakes it up your large intestine, looking for abnormal growths that could lead to colon cancer — it's easy to see the appeal of the so-called virtual colonoscopy. The procedure is far less invasive: a small device blows air into the rectum to inflate the bowel while a C.T. scanner takes X rays. Unfortunately, the results from early virtual colonoscopies did not measure up. In some studies, they missed half the polyps that needed to be caught.

So there was a great deal of interest last week in a New England Journal of Medicine report suggesting that a new, 3D virtual colonoscopy (aided by a special staining potion that patients drink in advance) may be as good as or better than a traditional colonoscopy. If these results are replicated — and if Medicare and other insurers will cover the $400 to $2,000 cost of the screening test — it could dramatically change the way colon cancer is detected, encourage more screenings and probably save lives. Currently, only about half the folks who should be screened actually undergo any of the available tests for colon cancer.

The virtual procedure is not a walk in the park. You have to fast for a day and cleanse your bowel before any colonoscopy. And if the doctor finds a polyp that must be removed, you still have to undergo a conventional colonoscopy — right away, ideally, so you won't have to prep twice. That's why virtual colonoscopies probably work best for those at lowest risk of colon cancer — with no symptoms and no family history of the disease, says Dr. Perry Pickhardt, the radiologist who led the study while at the National Naval Medical Center in Bethesda, Md.

There's another important point to consider. Doctors agree that polyps measuring 10 mm or more in diameter need to be removed, although during a conventional colonoscopy, even smaller, very low-risk growths are taken out. In a virtual colonoscopy, current practice is just to watch them until the next procedure to see if they grow. "Some patients and some doctors aren't going to be comfortable leaving anything behind," predicts Dr. Thomas LaMont of Boston's Beth Israel Deaconess Medical Center. So while the virtual colonoscopy is providing ever better results, the conventional colonoscopy is still the gold standard for preventing colon cancer.

• Christine Gorman