Electrophysiological considerations relevant to the limiting of pulsed electric and magnetic fields

The objective of the study was to theoretically examine the stimulation threshold of large myelinated axons and to use the results to formulate criteria for exposure limits of pulsed magnetic fields. The induced electric fields were calculated with a homogeneous tissue equivalent prolate spheroid. The stimulation level of the field was computed with the SENN model by using a folded axon with 2 mm separation for the Ranvier nodes. In the case of rectangular induced electric field pulses, the asymptotic stimulation level for electric field strength was 10 Vm(-1) with pulse durations greater than 100 micro(s) and for integrated electric field strength 1 x 10(-3) V m(-1)s with pulse durations less than 100 micro(s). The latter threshold level was exceeded in the surface of the prolate spheroid when the magnetic flux density changed by more than 7.4 mT within 100 micro(s). For sinusoidal bursts, the threshold amplitude of the magnetic field decreased asymptotically to a minimum value 2.5 mT when the carrier frequency and burst duration exceeded 5 kHz and 100 micro(s), respectively. If the same safety criteria are applied for pulsed and continuous exposure, the peak limits for induced current densities and magnetic field can exceed the amplitude values of the limits for continuous exposure only by a factor varying from 3 to 10.