And that appears to be what happened this summer.

Higher doses

A typical completed stress procedure, with rest and stress scans, with Rb-82 exposes patients to about 3 to 4 mSv, Di Carli said. However, based on models from Los Alamos National Laboratory, where the recalled generators were sent, the FDA estimates the patients received 90 mSv.

Although higher than normal, the FDA said the amount of radiation the patients received was comparable to doses from cumulative exposures of other heart scans. In a follow-up letter to customers Bracco said a patient would need over 1,000 times the dose to have a problem.

"Anything below 100 mSv is still considered low-level radiation," Di Carli said. He also noted that the 90 mSv number mentioned by the FDA is an "ultra-conservative" estimate.

"I don’t know where that comes from. As far as we can tell, there was no specific body measurement," he said.

Ultimate causes still unknown

A September Herald-Tribune article cited officials who suggested that a supply shortage may have been the ultimate cause of the problem. Bracco alerted customers earlier this year that because some cyclotrons -- particle accelerators that make the raw material used in the CardioGen generators -- were down for maintenance, it would have limited supplies from March through July. The newspaper said Bracco extended its 28-day expiration limit for the product to 45 days, before revising it back to 42 days. However, the Florida clinic scanned one of the patients on at least the 44th day, the paper said.

But Di Carli said, although this has been a common suggestion, that his lab tested their generator, which was also bumped up to the six-week cycle, and there was nothing out of the ordinary.

"I think it’s very dangerous to take sides, because the facts are not out there," he said. "It’s premature to say what happened."

Still, in a July announcement about the recall, the FDA faulted CardioGen manufacturing procedures for not being "sufficient to ensure reliable performance of the generator." However, in its most recent letter, Bracco said it has been in "constant communication" with the watchdog agency, and that it has been "revalidating its manufacturing process and reviewing labeling and user training to ensure proper generator use."

The price of a return

The lack of CardioGen may have had an impact on both patients and cardiac PET scanner prices, as the main alternative for PET perfusion studies, 13N-ammonia, has such a short half-life (about 10 minutes) it's generally only usable for the few institutions that have a cyclotron on-site. Di Carli estimates only a handful of the 100 facilities across the country that had been using CardioGen have one.

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