MRI, or « Magnetic Resonance Imagery », is a relatively new technique that rests on the electromagnetic properties of atomic nuclei.
The patient is placed at the center of a very powerful magnet which forces the nuclei of Hydrogen atoms in the pateint's body to align themselves with the magnetic field. Then, a radio wave, perpendicular to the main field, is sent to excite the Hydrogen atoms, which then begin to resonate.
At this stage, the Hydrogen atoms absorb the energy.
Then the radio source is turned off. The Hydrogen atoms return to their original positions, releasing the stored energy in the form of a new radio wave.
It is this wave, released by the atoms, which is detected and then analyzed by a computer in order to convert it to an image.