MRI is the abbreviation for Magnetic Resonance Imaging.
To better understand the risk MRIs pose to tattooed and pierced individuals it is important to know how Magnetic Resonance Imaging works:
The main component of the MRI system, or scanner, is a powerful magnet. It generates a magnetic field which causes the hydrogen atoms in your body to resonate, or vibrate in sync. This excitation due to the energy they receive causes them to emit energy of a slightly longer wavelength, which is detected by the scanner.
In short, an MRI uses a powerful magnet, low-intensity radio waves and computer technology to create detailed images of the soft tissues, muscles, nerves, and bones in your body. Like a normal magnet, an MRI magnet will attract anything that has iron in it. However, because a magnet is so powerful its attraction to metallic items will be extremely strong, increasing the risk of injury if tissue should get in the way of the iron molecules and the magnet.
Though you may immediately think that piercings would be the primary concern, tattoos actually pose more of a risk to patients receiving MRI scans. Most professional piercing jewelry is made of implant grade surgical stainless steel which is non-ferrous and is considered MRI safe (titanium, wood, and acrylic are also all considered MRI safe); that said it is strongly recommended that you replace any metallic jewelry with acrylic, wood, or Teflon for such procedures to minimize the risk.
Iron oxide is a metallic substance found in some pigments of tattoo ink, because this ink is embedded under the skin it poses a greater risk because the iron particles will be strongly attracted to the magnet in the MRI machine. These days professional tattoo inks contain little to no iron oxide particles, however cosmetic tattoo ink (permanent eyeliner, lip liner, etc) often contains measurable amounts which have been shown to cause reactions to MRI scans. If this ferrous ink is embedded under your skin and you go for an MRI scan the magnet will attract the minuscule molecules of iron in the ink and they will try to exit your skin. This often causes tingling, discomfort, swelling, itching, and can lead to nerve damage if the iron oxide pigment particles are able to exit the skin shearing a nerve upon exit. As daunting as this sounds, reactions are quite rare and none have been officially documented stating permanent damage arising from a tattoo and an MRI encounter.
Should you ever be in need of an MRI scan make sure to either swap out your metallic jewelry for acrylic/wood/Teflon jewelry or remove it altogether, and if you are tattooed notify the MRI technicians of the location(s) of your tattoo(s) and immediately notify them if you feel any tingling, numbness, burning, or other discomfort in the area of your tattoo(s) during the scan.