Here's an odd by-product of body art. In an MRI machine, a tattoo can start generating heat to the point where it burns your skin.

In 2011, a football player went in for an MRI, presumably to see what horrible, horrible things his chosen profession was doing to his body. He came out of the MRI machine worse than he went in. The authors of a paper about the incident explained that he had "an immediate and sustained cutaneous reaction," which means "he got a burn." The burn was at the site of a tattoo on his lower body.

An MRI puts a person in a strong magnetic field. This causes the protons in their body to synchronize, lining up in the same direction. The machine then sends bursts of radio waves through the body. This knocks the protons out of sync, but eventually they line back up. This re-alignment involves the protons sending out radio signals, which the machine interprets, figuring out where each proton is, and delivering a map of the inside of the body to doctors. Because different kinds of tissues respond at different rates, an MRI can even distinguish one kind of tissue from another.

The strong magnetic field in the MRI will grab hold of any metal. Patients and doctors who work with the machine are instructed to remove any jewelry or other metal objects before they go in the room with the machine, but they can't remove tattoos. While most manufacturers deny using iron oxides in their dyes, the industry isn't tightly regulated, and iron oxides sometimes turn up in red, orange, brown, or black ink. Inside an MRI machine, electric currents start up in iron-heavy inks, and the resulting heat burns the skin.

This can be simply a nuisance. It can also be very nasty, especially in cases of women who have eye-liner or lipstick tattoos. If you're getting a tattoo, inquire about what's in the ink. If you're getting an MRI with a tattoo, let the doctors know before you go in.

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Image: Chris Garver of Miami Ink.

[Sources: Tattoo-Induced Skin Burn During Magnetic Resonance Imaging, Some Tattoo Inks Can Burn Your During an MRI, How an MRI Works]