Most seasoned travelers know that their watches and belt buckles can set off airport metal detectors.

A recent study also shows that patients who have certain medical procedures might set off security sensors designed to find “dirty” bombs or other radioactive weapons.

Patients may emit small amounts of radiation after being treated with radioactive iodine, for example, or after being injected with compounds used in PET scans, bone scans and cardiac stress tests, says Lionel Zuckier, a radiology professor at New Jersey Medical School.

Patients have been treated with “radiopharmaceuticals” for years, and 16 million nuclear medicine procedures are performed each year, according to the Society of Nuclear Medicine. Since 2001, however, doctors say they’re hearing about patients setting off portable radiation detectors used in subways, tunnels and other places.

Patients injected with a material called FDG before having a PET scan stop emitting a detectable level of radiation within 24 hours. But patients undergoing iodine therapy for thyroid conditions emit radiation for 95 days.

Many doctors say they now provide patients with detailed explanations of treatments, along with phone numbers, just in case patients are stopped by security. Chaitanya Divgi, a nuclear medicine specialist at New York’s Memorial Sloan-Kettering Cancer Center, says security officers have called about his patients 15 to 20 times since 2001.