A multi-scale computational approach based on TMS experiments for the assessment of electro-stimulation thresholds of the brain at intermediate frequencies.

In recent years, human exposure to electromagnetic fields (EMF) at intermediate frequencies (300 Hz-10 MHz) has risen, mainly due to the growth of technologies using these fields. The current safety guidelines/standards defined by international bodies (e.g. ICNIRP and IEEE) established basic restrictions for limiting EMF exposure. These limits at intermediate frequencies are derived from threshold values of the internal electric field that may produce transient effects, such as the stimulation of the nervous system. However, there are some discrepancies between the basic restrictions of those guidelines/standards. The aim of this study is to investigate the excitation thresholds of the nervous system exposed to intermediate-frequency electromagnetic fields, with the purpose of extrapolating the threshold-frequency curves which are compared with existing basic restrictions prescribed by the international guidelines/standards. Our investigation was based on transcranial magnetic stimulation (TMS) experiments, physiological measurements, and individualized MRI-based computer simulations for the determination of brain stimulation thresholds. The combined approach with established biological axon models enabled the extrapolation of the measured thresholds for sinusoidally varying electric fields. The findings reveal that the exposure limits are significantly conservative for the brain, especially at frequencies in the range of 300 Hz-5 kHz.

Modelling of induced electric fields based on incompletely known magnetic fields.

Determining the induced electric fields in the human body is a fundamental problem in bioelectromagnetics that is important for both evaluation of safety of electromagnetic fields and medical applications. However, existing techniques for numerical modelling of induced electric fields require detailed information about the sources of the magnetic field, which may be unknown or difficult to model in realistic scenarios. Here, we show how induced electric fields can accurately be determined in the case where the magnetic fields are known only approximately, e.g. based on field measurements. The robustness of our approach is shown in numerical simulations for both idealized and realistic scenarios featuring a personalized MRI-based head model. The approach allows for modelling of the induced electric fields in biological bodies directly based on real-world magnetic field measurements.

Dielectric polarization transients in biological tissue moving in a static magnetic field.

Movement of a body in a static magnetic field gives rise to the Lorentz force that induces in the medium both electric currents and dielectric polarization. It is usually assumed that the conductivity of biological tissues is sufficiently high in order to neglect dielectric phenomenon arising from non-equilibrium of polarization charges. However, the permittivity of biological tissues is extremely high and the relaxation time of free charges is relatively low. In this study, we examined the effect of dielectric polarization on the electric field (EF) induced by human movements in a strong magnetic field (MF). Analytic equations for brain and bone equivalent spheres translating and rotating in a uniform MF were derived from Maxwell equations. Several examples were computed by using Fast Fourier Transform to examine transient dielectric effects in a time domain. The results showed that dielectric polarization transients do arise, but in the case of homogeneous medium, they are vanishingly small. In contrast, the local dielectric transients are not vanishingly small in heterogeneous medium. However, due to limited acceleration and deceleration of normal human movements, the transients are relatively small, at maximum a few dozen percent of the EF induced by the change of the magnetic flux. Taking into account the high uncertainty in numerical simulation, the dielectric transients can be neglected in the case of biological materials but not in the case of many non-biological materials of low conductivity. Bioelectromagnetics. 37:409-422, 2016. © 2016 Wiley Periodicals, Inc.

Why intra-epidermal electrical stimulation achieves stimulation of small fibres selectively: a simulation study.

The in situ electric field in the peripheral nerve of the skin is investigated to discuss the selective stimulation of nerve fibres. Coaxial planar electrodes with and without intra-epidermal needle tip were considered as electrodes of a stimulator. From electromagnetic analysis, the tip depth of the intra-epidermal electrode should be larger than the thickness of the stratum corneum, the electrical conductivity of which is much lower than the remaining tissue. The effect of different radii of the outer ring electrode on the in situ electric field is marginal. The minimum threshold in situ electric field (rheobase) for free nerve endings is estimated to be 6.3 kV m(-1). The possible volume for electrostimulation, which can be obtained from the in situ electric field distribution, becomes deeper and narrower with increasing needle depth, suggesting that possible stimulation sites may be controlled by changing the needle depth. The injection current amplitude should be adjusted when changing the needle depth because the peak field strength also changes. This study shows that intra-epidermal electrical stimulation can achieve stimulation of small fibres selectively, because Aß-, Aδ-, and C-fibre terminals are located at different depths in the skin.

Computational estimation of decline in sweating in the elderly from measured body temperatures and sweating for passive heat exposure.

Several studies reported the difference in heat tolerance between younger and older adults, which may be attributable to the decline in the sweating rate. One of the studies suggested a hypothesis that the dominant factor causing the decline in sweating was the decline in thermal sensitivity due to a weaker signal from the periphery to the regulatory centres. However, no quantitative investigation of the skin temperature threshold for activating the sweating has been conducted in previous studies. In this study, we developed a computational code to simulate the time evolution of the temperature variation and sweating in realistic human models under heat exposure, in part by comparing the computational results with measured data from younger and older adults. Based on our computational results, the difference in the threshold temperatures for activating the thermophysiological response, especially for sweating, is examined between older and younger adults. The threshold for activating sweating in older individuals was found to be about 1.5 °C higher than that in younger individuals. However, our computation did not suggest that it was possible to evaluate the central alteration with ageing by comparing the computation with the measurements for passive heat exposure, since the sweating rate is marginally affected by core temperature elevation at least for the scenarios considered here. The computational technique developed herein is useful for understanding the thermophysiological response of older individuals from measured data.