Soheil Hashemi

Three-dimensional analysis, modeling, and simulation of the effect of static magnetic fields on neurons

The effect of static magnetic fields on neuron function has been studied. None of the possible explanations are decisive or fully consistent with evidence in the literature. Therefore, the purpose of this paper is to examine the different possibilities, for the first time, through a three-dimensional modeling strategy in an effort to find out which possibility or combination is effective on cell function. A full-wave analysis was employed to simulate various effects of magnetic fields. The possibilities included force exerted on mobile ions, magnetophoretic force exerted on ions with permeability different from intracellular or extracellular fluids, magnetophoretic force exerted on sensor proteins in ion channels, and magnetophoretic pressure exerted on the membrane and spatial rotation of anisotropic diamagnetic particles. According to the simulations, the last two possibilities are more likely to be effective; therefore, their corresponding equations in this article were formulated to verify the results of the literature experiments. Bioelectromagnetics. 38:128-136, 2017.

Room shielding with frequency-selective surfaces for electromagnetic health application

Use of frequency-selective surfaces (FSSs) is proposed to shield rooms against electromagnetic fields in order to achieve secure indoor communications and reduce human exposure to external fields. The secure room is designed using two-layer FSSs with an FR4 substrate to cover 10–12 GHz frequency band. Different elements in each layer and shift in the position of elements are the reasons for more than 3 GHz bandwidths in X band. The performance of the structure is also stable versus the misaligned position of layers. An equivalent circuit model is proposed for the structure and results show −20 dB isolation between inside and outside of the room in the desired frequency band. Bio tissue is located inside the cubic structure with FSS walls and the results of the specific absorption rate are demonstrated and compared in two rooms with FSS cover on concrete walls and a room with concrete walls. The 17 × 17 cm 2 two-layer FSS is fabricated for the measurement and the results are presented. The designed FSS can be used in the construction of wave-isolated room for any application.

MODELLING DISPERSIVE BEHAVIOR OF EXCITABLE CELLS

Most of the materials have nearly constant electromagnetic characteristics at low frequencies. Nonetheless, biological tissues are not the same; they are highly dispersive, even at low frequencies. Cable theory is the most famous method for analyzing nerves though a common mistake when studying the model is to consider a constant parameter versus frequency. This issue is discussed in the present article, and the analysis of how to model the dispersion in the cable model is proposed and explained. The proposed dispersive model can predict the behavior of excitable cells versus stimulations with single frequency or wide-band signals. In this article, the nondestructive external stimulation by a coil is modeled and computed by finite difference method to survey the dispersion impact. Also, 5% to 80% difference is shown between the results of dispersive and nondispersive models in the 5 Hz to 4 kHz investigation. The disagreement expresses the dispersion notability. The proposed dispersive method assists in accurate device design and signal form optimization. Noise analysis is also achieved by this model, unlike the conventional models, which is essential in the analysis of single neurons or central nervous system, EEG and MEG records.

Three-dimensional FDTD modeling of neurons to solve EEG and MEG forward problem

Neuronal activities including calcium sodium current, ligands current, and synaptic transmembrane current create electromagnetic fields. Here, an analytic method is suggested to obtain the electromagnetic fields and potential signals resulting from the function of nerve cells inside the brain. Modeling simulates the behavior of cells three‐dimensionally. The proposed method employs the electric scalar potential and magnetic vector potential to solve the time‐domain three‐dimensional equations using the partial differential method. All ion flows are considered as electrical current densities. In this method, the brain and desired cells are meshed to solve the problem using the numerical method. As an example, the electric fields, magnetic fields, and signals generated by cingulum nerve fibers are illustrated and compared in Cz, Fz, and T3 electrode positions. A direct analysis method based on the same mechanism and biophysics of the nervous system is proposed. Employing this direct method leads not only to a better understanding of neuronal activity but also to a more accurate vision regarding the accuracy/inaccuracy of experimental and inverse methods. The analysis of these data provides insights into the brain function processes.

Modelling response of excitable cells induced by electromagnetic fields

Prediction and targeting of neural stimulation need an accurate analysis to model the activity of the nervous system in the presence of electric field. In this paper, fundamental of neural activity modelling in the vicinity of electric field is explained. The analysis is derived using continuity equation and cable theory. Inaccuracy of the formulation in the literature is also discussed and modified analysis is introduced in detail. Non-destructive external stimulation by a coil stimulator is modelled and simulated by finite difference method. Stimulation performance vs. frequency and geometry of stimulator is discussed. Accurate and authentic neuronal modelling in the presence of induced electric field is deduced only by satisfying Maxwell equations. Unlike our proposed method, most of the researches have not considered this issue. Therefore, at very low frequency, stimulation results show more than 70% difference in calculating the threshold of excitability between the proposed method and the one in the literature.