Selective absorption of very high-frequency radio waves by certain atomic nuclei subjected to a strong stationary magnetic field .
Nuclei that have at least one unpaired proton or neutron act like tiny magnets. When a strong magnetic field acts on such nuclei, it sets them into precession . When the natural frequency of the precessing nuclear magnets corresponds to the frequency of a weak external radio wave striking the material, energy is absorbed by the nuclei at a frequency called the resonant frequency. NMR is used to study the molecular structure of various solids and liquids. Magnetic resonance imaging , or MRI, is a version of NMR used in medicine to view soft tissues of the human body in a hazard-free, noninvasive way.