Calculation of electric fields and currents induced in a millimeter-resolution human model at 60 Hz using the FDTD method

The finite-difference time-domain (FDTD) method has previously been used to calculate induced currents in anatomically based models of the human body at frequencies ranging from 20 to 915 MHz and resolutions down to about 1.25 cm. Calculations at lower frequencies and higher resolutions have been precluded by the huge number of time steps that would be needed in these simulations. This paper describes a method used to overcome this problem and efficiently calculate induced currents in an MRI-based, 6-mm-resolution model of the human under a high-voltage transmission line. This model is significantly higher resolution than the 1.31-cm-resolution model previously used; therefore, it can be used to pinpoint locations of peak current densities in the body. Proposed safety guidelines would allow external electric fields of 10 kV/m and 25 kV/m for exposure to 60 Hz fields of the general public and workers, respectively. For this external electric field exposure of 10 kV/m, local induced current densities as high as 20 mA/m² are found in the head and trunk with even higher values (above 150 mA/m²) in the legs. These currents are considerably higher than the 4 or even 10 mA/m² that have been suggested in the various safety guidelines, thus indicating an inconsistency in the proposed guidelines. In addition, several ratios of E/H typical of power line exposures were examined, and it was found that the vertical electric field couples strongly to the body, whereas the horizontal magnetic field does not. Bioelectromagnetics 19:293–299, 1998. © 1998 Wiley-Liss, Inc.     

A frequency-dependent finite-difference time-domain formulation for induced current calculations in human beings.

The finite-difference time-domain (FDTD) method has been used to calculate SARs and induced currents involving whole-body or partial-body exposures of models to spatially uniform or nonuniform (far-field or near-field), to sinusoidally varying EM fields, or to transient fields such as those associated with electromagnetic pulses. However, a weakness of the FDTD algorithm is that the dispersion of the tissue's dielectric properties is ignored and frequency-independent properties are assumed. Although this is permissible for continuous-wave or narrow-band irradiation, the results may be highly erroneous for short pulses, in which ultra-wide bandwidths are involved. In some recent publications, procedures are described for one- and two-dimensional problems for media in which the complex permittivity epsilon * (omega) may be described by a single-order Debye relaxation equation or a modified version thereof. These procedures based on a convolution integral describing D(t) in terms of E(t) cannot be extended to human tissues for which multiterm Debye relaxation equations must generally be used. We describe here a new differential-equation approach that can be used for general dispersive media. We illustrate the use of this approach by one- and three-dimensional examples of media for which epsilon * (omega) is given by a multiterm Debye equation, and for an approximate two-thirds muscle-equivalent model of the human body. Based on a single run involving a Gaussian pulse, the frequency-dependent FDTD [(FD)2TD] method allows calculations of SARs and induced currents at various frequencies by taking the Fourier components of the induced E fields. The (FD)2TD method can also be used to calculate coupling of the short (ultra-wideband) pulses to the human body.     

Detection of the third and fourth heart sounds using Hilbert-Huang transform

Background

Magnetic nanoparticles are gaining great roles in biomedical applications as targeted drug delivery agents or targeted imaging contrast agents. In the magnetic nanoparticle applications, quantification of the nanoparticle density deposited in a specified region is of great importance for evaluating the delivery of the drugs or the contrast agents to the targeted tissues. We introduce a method for estimating the nanoparticle density from the displacement of tissues caused by the external magnetic field.

Methods

We can exert magnetic force to the magnetic nanoparticles residing in a living subject by applying magnetic gradient field to them. The nanoparticles under the external magnetic field then exert force to the nearby tissues causing displacement of the tissues. The displacement field induced by the nanoparticles under the external magnetic field is governed by the Navier's equation. We use an approximation method to get the inverse solution of the Navier's equation which represents the magnetic nanoparticle density map when the magnetic nanoparticles are mechanically coupled with the surrounding tissues. To produce the external magnetic field inside a living subject, we propose a coil configuration, the Helmholtz and Maxwell coil pair, that is capable of generating uniform magnetic gradient field. We have estimated the coil currents that can induce measurable displacement in soft tissues through finite element method (FEM) analysis.

Results

From the displacement data obtained from FEM analysis of a soft-tissue-mimicking phantom, we have calculated nanoparticle density maps. We obtained the magnetic nanoparticle density maps by approximating the Navier's equation to the Laplacian of the displacement field. The calculated density maps match well to the original density maps, but with some halo artifacts around the high density area. To induce measurable displacement in the living tissues with the proposed coil configuration, we need to apply the coil currents as big as 10⁴A.

Conclusions

We can obtain magnetic nanoparticle maps from the magnetically induced displacement data by approximating the Navier's equation under the assumption of uniform-gradient of the external magnetic field. However, developing a coil driving system with the capacity of up to 10⁴A should be a great technical challenge.     

Numerical evaluation of the fields induced by body motion in or near high-field MRI scanners

In modern magnetic resonance imaging , both patients and health care workers are exposed to strong, non-uniform static magnetic fields inside and outside of the scanner, in which body movement may be able to induce electric currents in tissues which could be potentially harmful. This paper presents theoretical investigations into the spatial distribution of induced E-fields in a tissue-equivalent human model when moving at various positions around the magnet. The numerical calculations are based on an efficient, quasi-static, finite-difference scheme. Three-dimensional field profiles from an actively shielded 4 T magnet system are used and the body model projected through the field profile with normalized velocity. The simulation shows that it is possible to induce E-fields/currents near the level of physiological significance under some circumstances and provides insight into the spatial characteristics of the induced fields. The methodology presented herein can be extrapolated to very high field strengths for the evaluation of the effects of motion at a variety of field strengths and velocities.     

Insights from the clustering of microarray data associated with the heart disease

Heart failure (HF) is the major of cause of mortality and morbidity in the developed world. Gene expression profiles of animal model of heart failure have been used in number of studies to understand human cardiac disease. In this study, statistical methods of analysing microarray data on cardiac tissues from dogs with pacing induced HF were used to identify differentially expressed genes between normal and two abnormal tissues. The unsupervised techniques principal component analysis (PCA) and cluster analysis were explored to distinguish between three different groups of 12 arrays and to separate the genes which are up regulated in different conditions among 23912 genes in heart failure canines'' microarray data. It was found that out of 23912 genes, 1802 genes were differentially expressed in the three groups at 5% level of significance and 496 genes were differentially expressed at 1% level of significance using one way analysis of variance (ANOVA). The genes clustered using PCA and clustering analysis were explored in the paper to understand HF and a small number of differentially expressed genes related to HF were identified.     

Comparison of coupling of humans to electric and magnetic fields with frequencies between 100 Hz and 100 kHz

Recent laboratory and epidemiological results have stimulated interest in the hypothesis that human beings may exhibit biological responses to magnetic and/or electric field transients with frequencies in the range between 100 Hz and 100 kHz. Much can be learned about the response of a system to a transient stimulation by understanding its response to sinusoidal disturbances over the entire frequency range of interest. Thus, the main effort of this paper was to compare the strengths of the electric fields induced in homogeneous ellipsoidal models by uniform 100 Hz through 100 kHz electric and magnetic fields. Over this frequency range, external electric fields of about 25–2000 V/m (depending primarily on the orientation of the body relative to the field) are required to induce electric fields inside models of adults and children that are similar in strength to those induced by an external 1 μT magnetic field. Additional analysis indicates that electric fields induced by uniform external electric and magnetic fields and by the nonuniform electric and magnetic fields produced by idealized point sources will not differ by more than a factor of two until the sources are brought close to the body. Published data on electric and magnetic field transients in residential environments indicate that, for most field orientations, the magnetic component will induce stronger electric fields inside adults and children than the electric component. This conclusion is also true for the currents induced in humans by typical levels of 60 Hz electric and magnetic fields in U.S. residences. Bioelectromagnetics 18:67–76, 1997. © 1997 Wiley-Liss, Inc.     

Currents induced in anatomic models of the human for uniform and nonuniform power frequency magnetic fields

We have used the quasi-static impedance method to calculate the currents induced in the nominal 2 x 2 x 3 and 6 mm resolution anatomically based models of the human body for exposure to magnetic fields at 60 Hz. Uniform magnetic fields of various orientations and magnitudes 1 or 0.417 mT suggested in the ACGIH and ICNIRP safety guidelines are used to calculate induced electric fields or current densities for the various glands and organs of the body including the pineal gland. The maximum 1 cm(2) area-averaged induced current densities for the central nervous system tissues, such as the brain and the spinal cord, were within the reference level of 10 mA/m(2) as suggested in the ICNIRP guidelines for magnetic fields (0.417 mT at 60 Hz). Tissue conductivities were found to play an important role and higher assumed tissue conductivities gave higher induced current densities. We have also determined the induced current density distributions for nonuniform magnetic fields associated with two commonly used electrical appliances, namely a hair dryer and a hair clipper. Because of considerably higher magnetic fields for the latter device, higher induced electric fields and current densities were calculated.     

Magnetic induction at 60 Hz in the human heart: a comparison between the in situ and isolated scenarios

Numerical modelling is used to estimate the electric fields and currents induced in the human heart and associated major blood vessels by 60 Hz external magnetic fields. The modelling is accomplished using a scalar-potential finite-difference code applied to a 3.6-mm resolution voxel-based model of the whole human body. The main goal of the present work is a comparison between the induced field levels in the heart located in situ and in isolation. This information is of value in assessing any health risks due to such fields, given that some existing protection standards consider the heart as an isolated conducting body. It is shown that the field levels differ significantly between these two scenarios. Consequently, data from more realistic and detailed numerical studies are required for the development of reliable standards.     

Self-Recognition of One's Own Fall Recruits the Genuine Bodily Crisis-Related Brain Activity

While bipedalism is a fundamental evolutionary adaptation thought to be essential for the development of the human brain, the erect body is always an inch or two away from falling. Although the neural mechanism for automatically detecting one''s own body instability is an important consideration, there have thus far been few functional neuroimaging studies because of the restrictions placed on participants'' movements. Here, we used functional magnetic resonance imaging to investigate the neural substrate underlying whole body instability, based on the self-recognition paradigm that uses video stimuli consisting of one''s own and others'' whole bodies depicted in stable and unstable states. Analyses revealed significant activity in the regions which would be activated during genuine unstable bodily states: The right parieto-insular vestibular cortex, inferior frontal junction, posterior insula and parabrachial nucleus. We argue that these right-lateralized cortical and brainstem regions mediate vestibular information processing for detection of vestibular anomalies, defensive motor responding in which the necessary motor responses are automatically prepared/simulated to protect one''s own body, and sympathetic activity as a form of alarm response during whole body instability.     

Role of exercise-induced reactive oxygen species in the modulation of heat shock protein response

Abstract

The multiple roles that have been associated with heat shock proteins (HSPs), inside and outside cells are remarkable. HSPs have been found to play a fundamental role in multiple stress conditions and to offer protection from subsequent insults. Exercise, because of the physiological stresses associated with it, is one of the main stimuli associated with a robust increase of different HSPs in several tissues. Given the combination of physiological stresses induced by exercise, and the ‘cross-talk’ that occurs between signaling pathways in different tissues, it is likely that exercise induces the HSP expression through a combination of ‘stressors’, among which reactive oxygen species (ROS) could play a major role. Indeed, although an imbalance between ROS production and antioxidant levels results in oxidative stress, causing damage to lipids, proteins, and nucleic acids with a possible activation of the programed cell death pathway, at moderate concentrations ROS play an important role as regulatory mediators in signaling processes. Many of the ROS-mediated responses actually protect the cells against oxidative stress and re-establish redox homeostasis. The aim of this review is to provide a critical update on the role of exercise-induced ROS in the modulation of the HSP's response, focusing on experimental results from animal and human studies where the link between redox homeostasis and HSPs’ expression in different tissues has been addressed.     

Investigation of the spinal cord as a natural receptor antenna for incident electromagnetic waves and possible impact on the central nervous system

The effects of electromagnetic field (EMF) exposure on biological systems have been studied for many years, both as a source of medical therapy and also for potential health risks. In particular, the mechanisms of EMF absorption in the human or animal body is of medical/engineering interest, and modern modelling techniques, such as the Finite Difference Time Domain (FDTD), can be utilized to simulate the voltages and currents induced in different parts of the body. The simulation of one particular component, the spinal cord, is the focus of this article, and this study is motivated by the fact that the spinal cord can be modelled as a linear conducting structure, capable of generating a significant amount of voltage from incident EMF.

In this article, we show, through a FDTD simulation analysis of an incoming electromagnetic field (EMF), that the spinal cord acts as a natural antenna, with frequency dependent induced electric voltage and current distribution. The multi-frequency (100–2400 MHz) simulation results show that peak voltage and current response is observed in the FM radio range around 100 MHz, with significant strength to potentially cause changes in the CNS. This work can contribute to the understanding of the mechanism behind EMF energy leakage into the CNS, and the possible contribution of the latter energy leakage towards the weakening of the blood brain barrier (BBB), whose degradation is associated with the progress of many diseases, including Acquired Immuno-Deficiency Syndrome (AIDS).     

The effect of alpha-tocopherol in water-soluble form on human erythrocyte membranes while in contact with oxygen

The effect of water soluble tocopherol forms (dipotassium salt d,l-alpha-tocopheryl-phosphate) on lipid peroxidation induced by oxygen in membranes of human erythrocytes has been studied. An erythrocyte was considered as a structure reflecting the state of plasmatic membranes of organs and tissues. It has been established that this substance doses optimum for membrane protection against lipid peroxidation are not in excess of 1.5-5.0 mg per 1 kg of human body mass; the dose by an order higher may exert a toxic effect.     

The properties of human body phantoms used in calculations of electromagnetic fields exposure by wireless communication handsets or hand-operated industrial devices

According to international guidelines, the assessment of biophysical effects of exposure to electromagnetic fields (EMF) generated by hand-operated sources needs the evaluation of induced electric field (E _in) or specific energy absorption rate (SAR) caused by EMF inside a worker's body and is usually done by the numerical simulations with different protocols applied to these two exposure cases. The crucial element of these simulations is the numerical phantom of the human body. Procedures of E _in and SAR evaluation due to compliance analysis with exposure limits have been defined in Institute of Electrical and Electronics Engineers standards and International Commission on Non-Ionizing Radiation Protection guidelines, but a detailed specification of human body phantoms has not been described. An analysis of the properties of over 30 human body numerical phantoms was performed which has been used in recently published investigations related to the assessment of EMF exposure by various sources. The differences in applicability of these phantoms in the evaluation of E _in and SAR while operating industrial devices and SAR while using mobile communication handsets are discussed. The whole human body numerical phantom dimensions, posture, spatial resolution and electric contact with the ground constitute the key parameters in modeling the exposure related to industrial devices, while modeling the exposure from mobile communication handsets, which needs only to represent the exposed part of the human body nearest to the handset, mainly depends on spatial resolution of the phantom. The specification and standardization of these parameters of numerical human body phantoms are key requirements to achieve comparable and reliable results from numerical simulations carried out for compliance analysis against exposure limits or within the exposure assessment in EMF-related epidemiological studies.     

Investigation of the spinal cord as a natural receptor antenna for incident electromagnetic waves and possible impact on the central nervous system

The effects of electromagnetic field (EMF) exposure on biological systems have been studied for many years, both as a source of medical therapy and also for potential health risks. In particular, the mechanisms of EMF absorption in the human or animal body is of medical/engineering interest, and modern modelling techniques, such as the Finite Difference Time Domain (FDTD), can be utilized to simulate the voltages and currents induced in different parts of the body. The simulation of one particular component, the spinal cord, is the focus of this article, and this study is motivated by the fact that the spinal cord can be modelled as a linear conducting structure, capable of generating a significant amount of voltage from incident EMF. In this article, we show, through a FDTD simulation analysis of an incoming electromagnetic field (EMF), that the spinal cord acts as a natural antenna, with frequency dependent induced electric voltage and current distribution. The multi-frequency (100-2400?MHz) simulation results show that peak voltage and current response is observed in the FM radio range around 100?MHz, with significant strength to potentially cause changes in the CNS. This work can contribute to the understanding of the mechanism behind EMF energy leakage into the CNS, and the possible contribution of the latter energy leakage towards the weakening of the blood brain barrier (BBB), whose degradation is associated with the progress of many diseases, including Acquired Immuno-Deficiency Syndrome (AIDS).     

Magnetic nanoparticle-based hyperthermia for cancer treatment

Nanotechnology involves the study of nature at a very small scale, searching new properties and applications. The development of this area of knowledge affects greatly both biotechnology and medicine disciplines. The use of materials at the nanoscale, in particular magnetic nanoparticles, is currently a prominent topic in healthcare and life science. Due to their size-tunable physical and chemical properties, magnetic nanoparticles have demonstrated a wide range of applications ranging from medical diagnosis to treatment. Combining a high saturation magnetization with a properly functionalized surface, magnetic nanoparticles are provided with enhanced functionality that allows them to selectively attach to target cells or tissues and play their therapeutic role in them. In particular, iron oxide nanoparticles are being actively investigated to achieve highly efficient carcinogenic cell destruction through magnetic hyperthermia treatments. Hyperthermia in different approaches has been used combined with radiotherapy during the last decades, however, serious harmful secondary effects have been found in healthy tissues to be associated with these treatments. In this framework, nanotechnology provides a novel and original solution with magnetic hyperthermia, which is based on the use of magnetic nanoparticles to remotely induce local heat when a radiofrequency magnetic field is applied, provoking a temperature increase in those tissues and organs where the tumoral cells are present. Therefore, one important factor that determines the efficiency of this technique is the ability of magnetic nanoparticles to be driven and accumulated in the desired area inside the body. With this aim, magnetic nanoparticles must be strategically surface functionalized to selectively target the injured cells and tissues.     

Obtaining source current density related to irregularly structured electromagnetic target field inside human body using hybrid inverse/FDTD method

Inverse method is inherently suitable for calculating the distribution of source current density related with an irregularly structured electromagnetic target field. However, the present form of inverse method cannot calculate complex field–tissue interactions. A novel hybrid inverse/finite-difference time domain (FDTD) method that can calculate the complex field–tissue interactions for the inverse design of source current density related with an irregularly structured electromagnetic target field is proposed. A Huygens’ equivalent surface is established as a bridge to combine the inverse and FDTD method. Distribution of the radiofrequency (RF) magnetic field on the Huygens’ equivalent surface is obtained using the FDTD method by considering the complex field–tissue interactions within the human body model. The obtained magnetic field distributed on the Huygens’ equivalent surface is regarded as the next target. The current density on the designated source surface is derived using the inverse method. The homogeneity of target magnetic field and specific energy absorption rate are calculated to verify the proposed method.     

New trends in antigen uptake in the gut mucosa

The mucosa represents a large surface of the human body that is in contact with the external environment. Mucosal tissues are colonized by an extremely dense and diverse micro flora of commensal bacteria, which compete with the growth of pathogenic strains. The mucosal sites continuously sample foreign material via specific cells, such as M cells and dendritic cells. Many new ways of antigen uptake have been described recently, especially by M cells and dendritic cells in the intestine. Depending on various factors, antigen presentation in the mucosa can lead to tolerance or initiation of an immune response. Mucosal vaccine strategies will certainly require eliciting specific antigen uptake because this initial step has a crucial role in controlling the outcome of immune responses.     

Miniature-probe measurements of electric fields induced by 60 Hz magnetic fields in rats

Extremely-low-frequency (ELF) magnetic fields interact with an animal by inducing internal electric fields, which represent the internal dose from an external exposure. In this study, an electric field probe of approximately 2 mm resolution was used to measure fields induced in rat carcasses by a 60 Hz magnetic field at 1 mT. With the rat lying on its side, the probe was inserted through a small hole in the body wall, and scanned at 5 mm increments from the side with frontal and axial exposure (field horizontal) and from the front with lateral exposure (field vertical). The induced electric field declined from a maximum at the entrance to the abdomen and crossed zero to negative (180° phase shift) values within the body as expected. In general, the magnitudes of the measurements inside the abdomen were less than expected from whole-body calculations that used homogeneous-ellipsoidal models of a rat in the three orientations. The low measurements did not appear to be explained by perpendicular field components, by conductivity differences between the tissue and the probe path, or by air in the lungs. The low measurements probably result from inhomogeneities in actual rats that include conductivity differences between tissues and biological membranes. For example, an alternative model considered the abdominal cavity to be electrically isolated from the body by the diaphragm and the peritoneum and calculations from this model were in better agreement with the measurements inside the abdomen (than were the whole-body calculations). Therefore, inhomogeneities in conductivity and biomembranes such as the peritoneum should be considered in order to fully understand ELF-induced field dosimetry. © 1996 Wiley-Liss, Inc.     

Magnetic particle motions within living cells. Physical theory and techniques.

Body tissues are not ferromagnetic, but ferromagnetic particles can be present as contaminants or as probes in the lungs and in other organs. The magnetic domains of these particles can be aligned by momentary application of an external magnetic field; the magnitude and time course of the resultant remanent field depend on the quantity of magnetic material and the degree of particle motion. The interpretation of magnetometric data requires an understanding of particle magnetization, agglomeration, random motion, and both rotation and translation in response to magnetic fields. We present physical principles relevant to magnetometry and suggest models for intracellular particle motion driven by thermal, elastic, or cellular forces. The design principles of instrumentation for magnetizing intracellular particles and for detecting weak remanent magnetic fields are described. Such magnetic measurements can be used for noninvasive studies of particle clearance from the body or of particle motion within body tissues and cells. Assumptions inherent to this experimental approach and possible sources of artifact are considered and evaluated.     

Synchronization of neuron population subject to steady DC electric field induced by magnetic stimulation

Electric fields, which are ubiquitous in the context of neurons, are induced either by external electromagnetic fields or by endogenous electric activities. Clinical evidences point out that magnetic stimulation can induce an electric field that modulates rhythmic activity of special brain tissue, which are associated with most brain functions, including normal and pathological physiological mechanisms. Recently, the studies about the relationship between clinical treatment for psychiatric disorders and magnetic stimulation have been investigated extensively. However, further development of these techniques is limited due to the lack of understanding of the underlying mechanisms supporting the interaction between the electric field induced by magnetic stimulus and brain tissue. In this paper, the effects of steady DC electric field induced by magnetic stimulation on the coherence of an interneuronal network are investigated. Different behaviors have been observed in the network with different topologies (i.e., random and small-world network, modular network). It is found that the coherence displays a peak or a plateau when the induced electric field varies between the parameter range we defined. The coherence of the neuronal systems depends extensively on the network structure and parameters. All these parameters play a key role in determining the range for the induced electric field to synchronize network activities. The presented results could have important implications for the scientific theoretical studies regarding the effects of magnetic stimulation on human brain.