Varying Magnetic (Gradient) Fields

Varying magnetic fields are necessary in order to obtain images from MRI scanners. Changing magnetic field induce electrical currents in conductors ( this is how an electrical generator works). In patients with metal in their body, the potential exists for electrical currents being induced in the metal with subsequent heating. This may occur with metal foreign bodies or some surgical implants. It does not universally occur and some patients with hip prostheses, for example, may be scanned without harm.
Very rapidly changing magnetic fields as may be achieved with echo planar imaging can cause nerve stimulation. This stimulation can effect motor nerves with resulting muscle contraction as well as the retina with resulting flickering lights called "magnetophosphenes".