These two conclusions, in documents to be released Tuesday, are the result of a yearlong investigation by the F.D.A. into hundreds of radiation overdoses in 2008 and 2009 that occurred during a specific diagnostic test for a stroke, called a CT brain perfusion scan.
The agency had earlier concluded that equipment failure played no role in the overdoses, which left dozens of patients in California and Alabama with a variety of ailments, including hair loss, confusion and memory problems.
Dr. Alberto Gutierrez, an F.D.A. official who oversees diagnostic devices, said Monday that the full extent of the overdoses might never be known because of imprecise guidelines governing how and when radiation overdoses should be reported.
“We actually don’t have a good description of what is a reportable event in radiation,” Dr. Gutierrez said. “There is some need for the agency to clarify what is reportable and for user facilities to have a system by which they can track and report incidents to the agency and to the manufacturer.”
Dr. Gutierrez specifically cited Huntsville Hospital in Alabama for its “unsatisfactory” response to brain scan overdoses there. Given the weakness of reporting rules, the F.D.A. said it could not definitively say how many radiation overdoses occurred at the hospital.
A Huntsville Hospital spokesman, Burr Ingram, said that his facility took any recommendation from the F.D.A. seriously and that it was already working to bring its reporting in line with the agency’s guidelines.
The New York Times had previously reported that Huntsville had some of the biggest brain scan overdoses and that the company that supplied the scanners, GE Healthcare, had concluded that the hospital used high levels of radiation to get clearer images.
The F.D.A. sent a letter Monday to the Medical Imaging Technology Alliance, spelling out its recommendations, including one that asked manufacturers to do a better job organizing dose-related information in their user manuals. The agency said that it hoped to discuss its recommendations with manufacturers at the end of this month, and that it expected the companies to act on at least some of them.
The executive director of the manufacturers’ group, Dave Fisher, said his organization had been working with the F.D.A. on these issues over the last year. “In the end, the optimal solution will require action by all stakeholders including, manufacturers, users and regulators,” Mr. Fisher said.
Higher-than-expected doses were reported in at least a half dozen hospitals, with most occurring at Cedars-Sinai Medical Center in Los Angeles, where the overdoses were first discovered. The hospital told The Times that it had found 269 patients who received more radiation than expected. The F.D.A. said that it had to rely in part on numbers provided by manufacturers and that its number was somewhat lower.
