When the nuclei of atoms like hydrogen (1H), carbon (13C), phosphorus (31P), and sodium (23Na) are subjected to a static magnetic field, they can be induced to emit electromagnetic radiation. This phenomenon, called nuclear magnetic resonance (NMR), was first demonstrated experimentally by the research groups of Bloch and Purcell in 1946. Because the spectrum of the radiation emitted depends on the type of nucleus and its chemical environment, NMR spectroscopy was rapidly exploited by chemists and has become a major tool for the investigation of molecular structure and the analysis of complex mixtures. More recently the NMR response of living systems subjected to known gradients of a static magnetic field has been exploited in medicine to construct images of the concentration and environment of hydrogen (protons) in the water and fat of soft tissues in the body.