The body is imaged with radio, not with X-rays

MRI maps hydrogen nuclei in a magnetic field, using radio waves. The scan itself is not ionizing radiation. The word ‘nuclear’ was dropped from the name because it frightened people.

A large magnet lines up a tiny fraction of the hydrogen nuclei in the body. A pulse of radio, at the frequency those nuclei prefer, tips them. As they relax they emit radio of their own, and the frequency is tuned by a gradient in the magnet so that each place in the body sings at a slightly different pitch. The image is a map of where the singing was loud, which is where the hydrogen was, which is where the water and the fat were. Paul Lauterbur published the imaging method in 1973. Peter Mansfield made it fast enough to be medicine.

There is no radioactive source in the scanner and no X-ray tube. The magnet, however, is strong enough to pull a steel cylinder across the room, and the scanner is loud, and some contrast agents are not nothing. ‘No radiation’ is true in the narrow sense patients ask about, and it is a bad sentence if it is heard as ‘no hazards at all’.