New York City ? A respected brain-imaging center run by Columbia University has halted some research after federal officials repeatedly complained that patients were getting drugs that failed purity tests.

The Food and Drug Administration found in a series of inspections that the center had failed to correct manufacturing problems in a lab that makes experimental drugs injected into psychiatric patients to help capture images of brain activity.

In one warning letter, an FDA office in New York described problems dating back to at least 2004. It cited a litany of violations, including a failure to reject batches of medication that didn’t pass required tests.