A physical phenomenon in which atomic nuclei in a substance exposed to a magnetic field absorb electromagnetic radiation and re-radiate it at a resonance frequency determined by the strength of the magnetic field and the magnetic properties of the isotope of the absorbing atoms. If material is exposed to a magnetic field that is not uniform everywhere, the resonance frequencies of the nuclei will depend on their location within the field. This allows the molecular structure of the material to be displayed as an image.