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Thomas B. Shope, Ph.D., a radiation physicist in the FDA's Center for Devices and Radiological Health, says the agency's concern is that "some of the kinds of things screening CT may find are not necessarily of any health significance." In addition, because no screening test is 100 percent accurate, the FDA is concerned that many people will get false-positive results, leading them to seek additional, possibly risky tests or surgical procedures. While there is a small danger of this when symptoms exist, its occurrence is far more probable when they do not. Further, the use of any X-ray imaging procedure is always accompanied by a concern about the possible effects of radiation exposure.

"The effective dose from a CT procedure can be hundreds of times larger than the effective dose from a conventional radiographic procedure," says Shope.

During a CT scan, an X-ray tube housed inside a doughnut-shaped machine rotates around and transmits radiation through a person's body at various angles. Detectors inside the machine measure the radiation transmitted through the body and these data are converted into electrical signals. A computer gathers these signals and produces three-dimensional images that are displayed on a monitor. A technologist or radiologist can change the contrast or brightness of the displayed image, or use other image processing or display techniques to emphasize areas or tissues of interest.

Using CT systems for diagnosis in medicine has been accepted as a valuable medical practice based on a wealth of experience. And physicians may use it for any condition or disease, as long as they deem the use legitimate within the doctor-patient relationship. Therefore, although the device was approved as a diagnostic tool for specific purposes, the FDA has limited authority to control how it is actually used. The agency continues to approve CT systems and their enhancements on the basis that they are to be used in the diagnoses of symptomatic people.

The American College of Radiology "does not believe there is sufficient scientific evidence to justify recommending total body computed tomographic (CT) screening for patients with no symptoms or a family history suggesting disease." The organization says there is no evidence that the procedure is either cost-effective, or effective in prolonging life.

Robert Smith, director of cancer screening at the American Cancer Society, says his organization also "discourages full-body scanning" for the same reasons.