An MRI scanner is essentially a big magnet that produces a powerful magnetic field of around 3 tesla (or 3 million microtesla) — millions of times larger than the fields we’re normally exposed to. But because it’s a static magnetic field, MRI scanners don’t exert any noticeable effect on the body. That would change, however, if the patient inside the scanner were to rapidly move his or her head back and forth. “Moving quickly induces a time-varying field, so by doing that you are inducing currents in different structures of your brain,” says Legros. Those currents may lead to nausea, loss of balance, a metallic taste in your mouth, or in some cases, magnetophosphenes.