Beginning in February 2008, each time a patient at Cedars-Sinai Medical Center in Los Angeles received a CT brain perfusion scan, the dose displayed would have been eight times higher than normal. No standard medical imaging procedure would use so much radiation, which one expert said is on par with the levels used to blast tumors. The U.S. Food and Drug Administration and Cedars-Sinai has revealed that 206 stroke patients who received scans at the hospital were overdosed with radiation. Now doctors and safety experts around the country face the question: How did the problem go undetected for 18 months?