NMR Imaging in Biomedicine

Abstract

The first successful nuclear magnetic resonance (NMR) experiments in bulk matter were carried out by Bloch and his collaborators at Stanford and by Purcell and his collaborators at Harvard, and were published in 1946. In the 38 yr since those first papers, NMR techniques have been widely applied in many areas of physics, chemistry, biochemistry, and biology. These applications are based almost entirely on the fact that the quantized energy levels available in a magnetic field to a nucleus with spin angular momentum and a magnetic moment are separated by an energy that is proportional to the strength of the magnetic field at the nucleus. This field is always slightly different from an applied extermal magnetic field because of perturbations by the nearby electrons and magnetic nuclei and by local and long-range magnetic susceptibility effects. These small differences result in shifts in the resonance frequency. Also, the fluctuations in the magnetic field at the nucleus caused by its motions and those of nearby nuclei and impaired electrons determine the transition rates between the nuclear Zeeman energy levels and the rates at which phase coherence of the detectable component of the nuclear magnetization are lost. Such changes in the relaxation times are useful because of their sensitivities to molecular motions and local environments.