We humans are made of flesh and bone, of cartilage, sinews, muscles and tendons.
But a surprising number of us carry around embedded metal as well. The potential list of metal bits is long: stents or shunts, surgical screws or plates, artificial joints or implanted
electrodes, dental posts, body piercings or even so called permanent makeup, eyeliner or eyebrows that are tattooed on.
What does all that metal mean if a doctor suggests you should get a diagnostic scan using magnetic resonance imaging, more commonly referred to as an MRI? As the name suggests, the machines use powerful magnets to produce detailed images of soft tissue, organs and joints. Metal is a problem in the rooms where MRIs are housed.
In a highly publicized and tragic example of why, a six-year-old boy from New York State died in 2001 from injuries incurred when an oxygen tank left in the testing room flew across the room and struck him in the head when the magnet was turned on. In another cautionary incident, a metal worker who had a metal sliver in his eye – the result of an old work accident – lost his vision when the magnet caused the shard to move, severing his optic nerve.
Those stories are rare. But there are more common reports of minor incidents, such as people complaining of burns when their tattoos heated up while they were in an MRI. Why? The inks used to draw the body art contain metals, which are embedded in the skin.
"There are thousands of incidents a year of minor injuries," says Dr. Jerry Froelich of the University of Minnesota, speaking of MRI-related events in the United States reported to the Food and Drug Administration.
"The major ones like an oxygen tank that kills somebody makes the news. But there are thousands of other ones that are reported or not reported that are minor instances that should not have occurred."
So, who should stay out of an MRI?
Figuring out the answer to that question is a complicated business and a moving target, given the fast changing field of medical devices. People who administer MRI scans and the professional organizations that oversee the field of radiology give it serious and ongoing study.
"I do spend a lot of time reviewing paperwork to determine whether or not our research subjects are safe or not safe. And almost a day doesn't go by that I don't get a request from a researcher: 'I have this patient with such and such and such. Are they safe?'" says Froelich, who is chair of the American College of Radiology's MRI safety committee.
In research, scientists like Froelich are using increasingly powerful magnets as they strive for greater detail in the images the machines make. They can currently capture neuron bundles in the brain and are trying to get to the point where they can see individual neurons.
The goal, he explains, is to see deeper and deeper into the brain in hopes of identifying the cause of traumatic brain injury, psychiatric diseases or other problems. "… We know that we're getting closer and closer to seeing what's happening on a cellular level within the body."
The stronger the magnet, the more important the issue of embedded metal becomes.
When the devices were first developed in the 1970s, the chief fear was shrapnel. Over time the concerns have expanded, as more metal is being used in surgeries, in implanted medical devices and in dentistry.
There are pacemakers and implanted defibrillators. Neurostimulators – electrodes implanted in the brain – and cochlear implants to give hearing to people who would otherwise be deaf. There are penile implants. Artificial joints. Prosthetic eyes. Intrauterine device or IUDs for birth control. Diabetic pumps.
Nicotine patches or dermal patches that deliver pain medication through the skin are sometimes backed with metal foil. Makeup and hairspray may contain tiny bits of metal and some MRI clinics will instruct patients not to use these products on the day of their scan.
People who have some implanted devices cannot undergo MRI scans, though decisions are made on a case-by-case basis. For instance while some new pacemakers are designated MRI-compatible, many can't be put into an MRI.
Likewise ear implants, breast-tissue expanders and certain types of older model surgical clips are considered contraindications for MRIs, says Dr. Anish Kirpalani, co-director of the MRI program at St. Michael's Hospital in Toronto.
Some metal – the posts used to hold dental implants, for instance – aren't a safety issue. But depending on how much metal there is and where it is in conjunction to the part of the body being scanned, the metal could distort the image being captured.
Kirpalani says his clinic has a regular handheld magnet and will use it at times to test whether something metal in or on a patient is likely to pose a problem. He and Froelich describe detailed intake procedures aimed at finding out whether patients are safe to scan.
The goal is to get as much detail as possible. For example, people who have implanted devices are asked for specifics, down to model and manufacturer. People who have had operations of any sort are queried to see whether they might unknowingly have metal clips or pins implanted. When available, medical records are checked.
With detailed information, many people can be safely scanned. "But if they don't have medical records and we don't know what's in them, then commonly they cannot have an MRI," Froelich says.
In fact, if your doctor orders an MRI for you, you should come armed with as much information as possible about metal in and on your body.
"You need to be open," Kirpalani says. "That's one thing I would stress, that people disclose these kinds of things to us."