In terms of safety, the problem is that, as current flows through the device, it becomes a huge and extremely powerful electromagnet. Ferromagnetic materials—which, at room temperature, include iron, cobalt, nickel and some metal alloys—are attracted to it with a great deal of force. That means metallic objects that come close to a switched on MRI can become dangerous projectiles.

Before a patient is brought in for a scan, they’re asked whether they have any medical or cosmetic implants containing metal in their body. These can include pacemakers, stents, piercings and screws in bones. Metal residue from gunshot wounds must also be reported. The staff then check whether the objects could cause problems. In the end, most metal objects inside the body pose no danger to patients. But if they’re overlooked, things can get ugly.

Projectile fragments and metal shavings that have penetrated the tissue as a result of gunshot wounds or accidents may travel a few millimeters during the scan. Doctors consider very carefully whether an MRI is too risky in the presence of such foreign bodies and then switch to other imaging procedures if necessary. Small metal particles also sometimes move back and forth around their own axis and in confined spaces. This can cause them to heat up dangerously.