Under the action of the magnetic field with magnetic atoms can produce different energy levels, if the addition of an energy and energy is exactly equal to the energy difference between the two neighboring energy levels, the energy absorbed by the atom produced by the jump, that is, to produce resonance. It is the use of nuclear magnetic resonance, based on the energy released in different structural environments within the material, different attenuation. By applying a gradient magnetic field to detect the electromagnetic waves emitted, you can know the location and type of the atomic nuclei of this object, based on which you can draw an image of the internal structure of the object to form a nuclear magnetic resonance image.