Since long before the Fukushima nuclear-plant disaster, doctors and others have been alarmed by Americans' increased exposure to radiation. But Japan's nuclear disaster has reawakened fear of the invisible enemy that's also used to discover diseases and save human lives. Even radiologists say Americans are getting too much of a good thing, but not from fallout, airport scanners or cell phones. Doctors are ordering seven times more radiation scans than they were 30 years ago, while diagnoses of life-threatening conditions have hardly risen at all. Are so many scans really needed for medicine or to avoid lawsuits, pay back investments in expensive machines and satisfy the demands of patients?