Q. Do M.R.I. scans cause any harm?

A. Magnetic resonance imaging, or M.R.I., is considered one of the safest technologies for looking deep inside the body, because it doesn’t carry the radiation risk of X-rays or PET scans.

“Over all, M.R.I. is a very safe test,” said Dr. Max Wintermark, chief of neuroradiology at Stanford University.

Most concerns about M.R.I.s involve people with metal, such as shrapnel, embedded in their bodies, or someone with an implanted medical device, like a cochlear implant or an older pacemaker. The imaging system’s strong magnetic field can slightly shift or heat up embedded metal and disrupt the activities of medical devices. It can also draw metal objects into the magnetic field, and there are still occasional accidents when standard safety procedures are not followed and M.R.I. magnets have sucked in hospital beds, screwdrivers, oxygen tanks and other metal objects.

“That’s why we take extreme precautions to know if a patient has a device, so we can take appropriate measures to make it safe for them, too,” Dr. Wintermark said. In some cases, people with implants or embedded metal cannot safely get an M.R.I. and must use a different scanning technology instead.