But because they emit much more radiation than an X-ray, and because their use has grown swiftly in recent years, physicians need to think before they order CT scans, say the report's authors, who are from Columbia University Medical Center. ''Part of the issue is that physicians often view CT studies in the same light as other radiologic procedures, even though radiation doses are typically much higher,'' they said. This will require a shift in thinking. Doctors will have to ask themselves two questions when considering ordering a CT scan. First, is it medically necessary? Second, would a CT scan show something that couldn't be detected by a less-radioactive technology like ultrasound? As for the companies that make CT scanners, they probably shouldn't worry too much about the business implications -- as they tend to make X-ray, ultrasound and other imaging systems too. UPDATE: The Medical Imaging and Technology Alliance released a statement citing that the benefits of CT scanning far outweigh the cancer-related risks. It also highlighted a report on reduced radiation doses.