Medical nuclear magnetic resonance imaging: I. Physical principles.

For over 30 years, nuclear magnetic resonance (NMR) spectroscopy has been used for chemical analysis and for studying molecular behavior, but NMR imaging is a recent addition to the methods available to radiologists for investigating the interior of the body. It uses radiofrequency radiation in the presence of a magnetic field to produce anatomical cross sections. The images may be simple maps of the concentration of the hydrogen nucleus, or they may depend on tissue relaxation times, which describe how rapidly hydrogen nuclei exchange energy with their surroundings. In this article, the basic concepts and physical principles of conventional NMR spectroscopy are introduced. In subsequent articles, the various approaches to producing NMR images will be outlined, and the types of information obtainable from NMR scanners will be discussed.