On Tuesday, the U.S. Food and Drug Administration issued a warning to doctors regarding a popular radioactive drug used in heart scans. The agency has asked that doctors stop using the drug CardioGen-82 during heart scan procedures because of the potential risk of increased radiation exposure.

CardioGen-82 is manufactured by Bracco Diagnostics and is often used in positron emission tomography (PET) scans on the heart in order to diagnose heart disease. Following the FDA’s warning, Bracco issued a voluntary recall of its radioactive drug.

The FDA opened its investigation into CardioGen-82 after being notified that two patients who underwent PET scans with the drug were found to have detectable levels of radiation several months later. The drug’s maker is working closely with the FDA and other regulatory agencies in order to further investigate the problem.

CardioGen-82 has been used in the United States since 1989, and the FDA has stated that the amount of radiation exposure associated with use of the drug is normally lower than radiation exposure from other types of heart scans.