"They not only are not detecting a sizable number of polyps, they are calling polyps that aren't there," Johnson said.

Then there's the question of exposure to radiation, which he described as "not inconsequential."

Radiologists and advocates of virtual colonoscopy acknowledge the concerns but argue that the test has proven its value.

For example, though virtual colonoscopy might not be as accurate, it is effective in finding the polyps that most often lead to colon cancer, Yee said.

"Virtual colonoscopy has been shown through multiple studies to be as sensitive as normal colonoscopy to detect clinically significant lesions of 10 millimeters or larger," she said.

Yee added that radiation concerns are overblown. "It's been shown that virtual colonoscopy is a low-dose examination," she said.

Researchers also are working to make virtual colonoscopy even better, Yee said. New computerized, post-processing techniques are being developed to improve the test's accuracy and lower the radiation dose, and doctors are working to develop a form of the test for which the patient would not need to take laxatives in advance.

At the moment, though, the American College of Gastroenterology still would prefer that people have a conventional colonoscopy done every 10 years because, as Johnson said, it's a better test that allows doctors to immediately remove any polyps that are found.

However, if the drawbacks of a normal colonoscopy are enough to dissuade a person from undergoing colon cancer screening, then they should consider virtual colonoscopy as an alternative test that they should have every five years, according to ACG guidelines.

The hope is that virtual colonoscopy "will bring in individuals who aren't ever considering being screened, who need
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