ABSTRACT
Nuclear magnetic resonance imaging (MRI) and spectroscopy (MRS) are powerful noninvasive medical examination procedures that are gaining rapid clinical acceptance. The imaging procedure entails the concurrent exposure of subjects to a high static magnetic field, an Rf electromagnetic field and time-varying pulsed magnetic gradient fields. Biological effects of NMR exposure conditions (i.e., the combination of static magnetic fields, extremely low frequency (ELF) gradient magnetic fields and Rf radiation have been investigated by several authors in order to prove that no unacceptable hazardous effects are associated with medical applications of NMR in vivo spectroscopy (MRS) and NMR imagine (MRI). There is, however, not yet any special study of the effect of gradient switching, although the exposure to the short pulse lengths used in clinical NMR imaging (i.e., less than 2 T/ sec) is close to the threshold of 3 T/sec producing retinal stimulation. The biological effects of all combined fields used in NMR imaging have been studied by several investigators. 5,32,35,39
