A child gets rushed into an emergency room and doctors order a CT scan ? one of the most reliable diagnostic tools to check for maladies from appendicitis to traumatic injuries. But computed tomography imaging, the technology that took away the guesswork, has caused another medical puzzle. Doctors increasingly order the tests but don't really know how many of the procedures expose a child to enough radiation to cause cancer. Dr. David Gaw, a Nashville orthopedic surgeon, worries that doctors are needlessly exposing children to radiation by ordering CT scans too quickly and too often.