To perform a PET scan, doctors must first inject patients with a radiotracer, a drug engineered to accumulate in the area of the body being studied and to emit low-level radiation detectable by a scanner.

The compounds are considered very safe. But because they degrade quickly, many laboratories produce them themselves, under protocols agreed upon with the F.D.A.

The agency regulates the allowable radiation levels and the purity of the drugs. If a drug contains too many impurities  unknown chemicals that may or may not be related to the tracer itself  then its effects in the body are unpredictable.

“There could be a patient safety issue, for one,” said Dr. Barry Siegel, chairman of the radioactive drug research committee at Washington University in St. Louis. “And there could be a scientific validity issue. If you’re exposing people to radiation and getting garbage data, then that becomes an ethical problem.”

That is particularly true when it comes to psychiatric research. Radiotracers that target receptors in the brain, as used in many of the Columbia studies, are more prone than other PET drugs to be biologically active  to affect mood or behavior, especially in those who already suffer from severe depression or other mental problems. “You have to have additional quality-assurance procedures if you’re using agents that bind receptors in brain,” said Dr. Dennis P. Swanson, chairman of the radiation safety commission at the University of Pittsburgh.

The F.D.A.’s latest investigation, which took place from Jan. 5 to Jan. 21, listed six categories of violations. It found that since 2007, “at least 10 batches” of drugs had been “released and injected into human subjects” with impurities that exceeded the level the lab had agreed to set. At least four injections “had impurity masses that more than doubled the maximum limit implemented.”

The report highlighted an equation that the lab routinely used, resulting in injections that exceeded the limit for acceptable impurities. The lab did not adequately check “the identity, strength and purity of each active ingredient prior to release” for injection into patients, the report said.