MR physics: Introduction
Nuclear magnetic resonance (NMR), a property of atoms first observed by Bloch  and Purcell  in 1946, has proven to be an informative technique in many fields of study, particularly in chemistry and physics. The magnetic resonance signal is very rich in measurable characteristics - including initial strength, frequency of oscillation, and rate of recovery and decay - that reflect the nature of a population of atoms, the structure of their environment, and the way in which the atoms interact with this environment. Furthermore, one can manipulate the external magnetic environment in space and time to modify the NMR signal without significantly affecting material structure.
Relatively recently, magnetic resonance was extended to the in vivo study of human anatomy. This was made possible by new, practical methods for exciting signal from limited volumes , and for generating spatial maps of this signal . Relying primarily on the differential decay and recovery characteristics of the proton NMR signal (generally termed relaxation behavior), this technology can generate images with high contrast among various soft tissues and organs. As a result, magnetic resonance imaging (MRI) has become the modality of choice in many diagnostic studies of the head, spine, and joints. With ongoing developments to improve the image quality, acquisition speed and quantitative accuracy of related measures of local signal characteristics, the range of clinical applications for MRI continues to expand rapidly.