Introduction

Results from the successful experiments demonstrating the phenomenon of nuclear magnetic resonance (NMR) or nuclear induction in solids and liquids were published almost simultaneously by Bloch et al. working at Stanford University and Purcell et al. working at Harvard University. The excitation of the nuclear spin is induced by a pulse of electromagnetic radiation transmitted at the resonance frequency. In a standard NMR imaging experiment, a receiver coil surrounds the area of interest in the patient. NMR imaging offers some special advantages over other diagnostic imaging methods used in medicine. Superimposed upon the constant magnetic field of the earth is also an additional dynamic field resulting from fluctuating electric currents in the ionosphere. The amplitude of this dynamic or oscillating field is frequency dependent, generally decreasing in amplitude as the frequency increases. In NMR imaging, the time of exposure to dynamic magnetic fields is very short, but of high magnitude.