An electric field induced in the retina and brain at threshold magnetic flux density causing magnetophosphenes

Abstract

For magnetic field exposures at extremely low frequencies, the electrostimulatory response with the lowest threshold is the magnetophosphene, a response that corresponds to an adult exposed to a 20 Hz magnetic field of nominally 8.14 mT. In the IEEE standard C95.6 (2002), the corresponding in situ field in the retinal locus of an adult-sized ellipsoidal was calculated to be 53 mV m(-1). However, the associated dose in the retina and brain at a high level of resolution in anatomically correct human models is incompletely characterized. Furthermore, the dose maxima in tissue computed with voxel human models are prone to staircasing errors, particularly for the low-frequency dosimetry. In the analyses presented in this paper, analytical and quasi-static finite-difference time-domain (FDTD) solutions were first compared for a three-layer sphere exposed to a uniform 50 Hz magnetic field. Staircasing errors in the FDTD results were observed at the tissue interface, and were greatest at the skin-air boundary. The 99th percentile value was within 3% of the analytic maximum, depending on model resolution, and thus may be considered a close approximation of the analytic maximum. For the adult anatomical model, TARO, exposed to a uniform magnetic field, the differences in the 99th percentile value of in situ electric fields for 2 mm and 1 mm voxel models were at most several per cent. For various human models exposed at the magnetophosphene threshold at three orthogonal field orientations, the in situ electric field in the brain was between 10% and 70% greater than the analytical IEEE threshold of 53 mV m(-1), and in the retina was lower by roughly 50% for two horizontal orientations (anterior-posterior and lateral), and greater by about 15% for a vertically oriented field. Considering a reduction factor or safety factors of several folds applied to electrostimulatory thresholds, the 99th percentile dose to a tissue calculated with voxel human models may be used as an estimate of the tissue's maximum dose.

Analysis of human brain exposure to low‐frequency magnetic fields: A numerical assessment of spatially averaged electric fields and exposure limits

“…A similar finding is reported by Hirata et al [2011], in which the E 99 values in the brains of anatomically based models at 20 Hz were found to be 10-70% higher than the IEEE [2002] exposure limits. A similar finding is reported by Hirata et al [2011], in which the E 99 values in the brains of anatomically based models at 20 Hz were found to be 10-70% higher than the IEEE [2002] exposure limits.…”

“…1 kHz. A similar finding is reported by Hirata et al [2011], in which the E 99 values in the brains of anatomically based models at 20 Hz were found to be 10-70% higher than the IEEE [2002] exposure limits.…”

“…The E 99 values are the least affected quantity with respect to grid size variation and stay at 95-100% of the analytical peak value for the investigated grid sizes. As the E 99 value is both stable and close to the analytical peak value in this particular exposure scenario (a homogeneous sphere in a uniform B-field), it was often employed as a substitute of the E max value to obtain a more stable approximation of the peak induced E-field in an anatomical model [Dimbylow, 2005[Dimbylow, , 2006Hirata et al, 2009Hirata et al, , 2010Hirata et al, , 2011.…”

“…Numerous studies have been published for LF numerical dosimetry [Dawson et al, 1997[Dawson et al, , 1998[Dawson et al, , 2001Dimbylow, 1998Dimbylow, , 2005Dimbylow, , 2006Caputa et al, 2002;Dimbylow and Findlay, 2009;Hirata et al, 2009Hirata et al, , 2010Hirata et al, , 2011Bakker et al, 2011;Laakso and Hirata, 2012]. While the effects of spatial averaging algorithms have been investigated by Hirata et al [2010], the grid size of the voxelized anatomical models used in that study is 2 mm.…”

Anecdotal report of magnetophosphene perception in 50 mT 20, 50 and 60 Hz magnetic fields

“…Advancing knowledge on MP is thus critical to protect populations from any possible effects on brain function. Indeed, if MP is an established phenomenon, the dose-response relationship and perception a martine.souques@edf.fr threshold remain uncertain at power frequencies, as estimates are based on the extrapolation from experimental data acquired at lower frequencies (D'Arsonval, 1896;Lövsund et al, 1980;Hirata et al, 2011). …”