More than 400 people across the country were subjected to radiation overdoses during botched CT brain perfusion scans, according to a New York Times investigation. What’s more, the Times found that some people received much higher overdoses than federal regulators first reported.

According to alerts issued by the Food & Drug Administration (FDA) in October and December 2009, machines involved in the botched CT scans, made by General Electric and Toshiba, had been wrongly calibrated and subjected patients who underwent brain perfusion scans following a stroke to excessive doses of radiation.

A CT perfusion scan, which can identify a stroke through a series of blood flow images, already requires a higher radiation dose than other scans. According to the Times, the FDA has determined that a machine malfunction was not behind the radiation overdoses seen in California and elsewhere. But the FDA has been unable to determine exactly what went wrong.

Patients who received the radiation overdoses have reported hair loss and skin reddening. Some of those who received the worst overdoses have also suffered from headaches, memory loss and confusion. Many are now at risk for brain damage and cancer.

The CT scan machines had been set at the higher level since February 2008, but went undetected for 18 months. It wasn’t until patients began complaining of hair loss and skin reddening that the error was found.

According to the FDA alerts, at least 269 patients at Cedar-Sinai Medical Center in Los Angeles were injured in this way. Another 37 received radiation overdoses at Providence Saint Joseph Medical Center in Burbank, while 65 more at Alabama’s Huntsville Hospital underwent botched CT scans. At the time, the FDA was also investigating 10 incidents at Glendale Adventist Medical Center in California.

Now the Times is reporting that a total of eight hospitals, six of which are located in California, have been implicated. Other hospitals where overdoses are known to have occurred include Los Angeles County and University of Southern California Medical Center, where one patient received seven and a half times the amount generally used; Bakersfield Memorial Hospital, where 16 people received up to five and a half times too much; South Lake Hospital in central Florida, where an unknown number of patients received 40 percent more than usual; and an unidentified hospital in San Francisco.

The New York Times is also reporting that the Alabama patients were subjected to the worst overdoses, receiving up to 13 times the amount of radiation generally used in the test. An inquiry by GE Healthcare, the maker of the scanners used there, found that technicians purposely used high levels of radiation to get clearer images. Experts told the Times that such a practice is “unjustified and potentially dangerous.”

What’s more, the FDA had no idea about how bad the overdoses at Huntsville Hospital were, until the Times itself made the agency aware. Now, the FDA is considering expanding its investigation there.

Officials at Cedars-Sinai told the Times that a feature on GE scanners used there could have caused the overdoses. That feature is supposed to automatically adjust the radiation dose according to a patient’s size and body part. At Cedars-Sinai and Glendale Adventist, technicians used the automatic feature for their brain perfusion scans. But according to the Times, “when used with certain machine settings that govern image clarity, the automatic feature did not reduce the dose — it raised it.”

The hospitals claim that GE trainers never fully explained the automatic feature. According to the Times, Cedars-Sinai said GE never mentioned the “counterintuitive” nature of the automatic feature during multiple training sessions there.

Experts interviewed by the Times also faulted the scanners, saying a better designed machine might have prevented the overdoses by alerting operators or shutting down when a dose got too high.