Before sliding into an MRI machine, patients are supposed to remove all metal from their bodies: underwire, jewelry, piercings, everything. The machines work using extremely strong magnets, and there are all kinds of horror stories of people leaving metal in their skin or on their body, to disastrous effects.

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But it’s not just piercings and jewelry people have to worry about. According to Jennifer J. Brown at Everyday Health, the ink in some tattoos can become a problem in the MRI machine:

During MRI, skin or eye irritation — even first degree burns — can result when dyes in tattoos, even from tattooed eyeliner, heat up. Covering them isn't likely to help, and if skin irritation or burning happens, the MRI must be stopped at once to avoid a burn.

While it’s not common, there are a handful of case reports and studies that look at burns—some serious—that have occurred inside an MRI machine. The likely culprit is the metallic, iron oxides found in some tattoo pigments.

“These compounds can theoretically create an electric current that increases the local skin temperature, enough to cause a cutaneous burn,” explain the authors of one paper that reports on a professional football player who was burned in this way. The authors argue that this might be particularly important for sports doctors to pay attention to, “given the frequent occurrence of cosmetic tattoos in athletes requiring magnetic resonance imaging to diagnose a musculoskeletal injury.”

Aside from being painful, tattoos with magnetic ink can also distort the MRI image. “MRI involves the use of a strong magnetic field to obtain images of structures within the body,” writes nurse Myrna Armstrong, “and the metallic compounds in tattoo pigments, especially iron oxide, distort that field."

But the burns can happen to people without iron oxides in their tattoo ink, too, it seems. This case study reports a woman with permanent makeup tattooed onto her eyelids experiencing first degree burns during an MRI. (Ouch.) The authors of this study actually reached the manufacturers of the woman’s tattoo ink. They write:

The manufacturer's certificate of analysis of the chemical characteristics of the tattoo components identified various heavy metals (e.g., lead, copper, zinc, chrome, arsenic, cadmium, barium, and mercury) but no ferric oxides. The manufacturer denies previous MRI-associated incidents.

Because tattoo inks aren’t regulated by the FDA, it’s hard to say whether the manufacturer in this case is disclosing the full list of ingredients or not. That said, the cases of burning during MRIs are few and far between, and a tattoo should not deter a person from getting into the machine, if a doctor orders the scan. According to the FDA:

There have been reports of people with tattoos or permanent makeup who experienced swelling or burning in the affected areas when they underwent magnetic resonance imaging (MRI). This seems to occur only rarely and apparently without lasting effects.There have also been reports of tattoo pigments interfering with the quality of the MRI image. This seems to occur mainly when a person with permanent eyeliner undergoes MRI of the eyes. However, the risks of avoiding an MRI when your doctor has recommended one are likely to be much greater than the risks of complications from an interaction between the MRI and tattoo or permanent makeup. Instead of avoiding an MRI, individuals who have tattoos or permanent makeup should inform the radiologist or technician.

In other words, it’s probably riskier to avoid the MRI than to simply tell your doctor about your tattoos before going into the machine. The chances of having an adverse reaction are low, and if your doctor thinks you need an MRI, you should probably get one.