Magnetic resonance imaging (MRI) is a non-invasive, non-ionizing medical procedure which provides images of soft tissue and anatomic structures. The SAR values are typically calculated using computational electromagnetic software body models often obtained from MRI scans. The finite difference time domain (FDTD) technique is most often used, because of its simplicity and compatibility with numerical models of the human body. In response to this demand, a model of a pregnant female was developed and the FDTD numerical modeling to evaluate the SAR values was performed.

So far, the numerical modeling of the nanostructures in metal films has considered perfect conductors, or used effective surface impedance boundary conditions. Here, we incorporate the Drude response of metals within a finite-difference time-domain (FDTD) method, which allows for modeling the electromagnetic field propagation within the metal. Numerical modeling was performed for a specific 2D structure consisting of a slit surrounded by a periodic array of grooves in a thin gold film. By using the FDTD method with a realistic metal response, we can account for tunneling through the metal film, which was not shown with past models. The modeled structures were fabricated using focused-ion beam milling.

Contact currents flow when a conducting object such as an animal touches conductive surfaces at different potentials. This completes a path for current flow through the body. These currents provide an additional coupling mechanism between the human body and low-frequency external fields to that due to direct induction effects. Recent research indicates that childhood exposure to residential contact currents may play a role in explaining any possible association between residential magnetic fields and childhood leukemia. To verify this hypothesis, laboratory experiments with rodents are planned. Thus, it is important to understand the relationship between fields induced in rodents and humans. Results from numerical computations are reported here. They are based on high-resolution anatomically based inhomogeneous models of adult and child male humans and male and female rats and mice, for a variety of 60-Hz contact current scenarios. It is hoped that this work will aid in the design of experiments involving rodents and in the interpretation of results as applied to humans. It is found that for geometrically similar models, the induced electric-field scales in an anticipated inverse-square manner with the geometric scaling factor. For dissimilar models, scaling can provide a crude estimate for translating induced field results between species. However, numerical modeling provides the most suitable analysis tool for more accurate estimates.

Heterogeneous model of the human body and scalar potential finite difference method are used to compute electric filed in tissue. This field is compared to the previously obtained thresholds for stimulation of peripheral nerves.

Numerical computations are used to evaluate electric field dosimetry for high-resolution anatomically based inhomogeneous models of a human male child, and male and female rats and mice, under exposure to 60 Hz uniform magnetic field sources of three perpendicular orientations. The goal is to compare the child data to previously computed adult dosimetry and to evaluate the accuracy of linear scaling of organ dosimetry between species. It is expected that this work will aid in the design and interpretation of experiments involving rodents.It is found that child-to-adult and mouse-to-rat organ dosimetry shows the expected linear dependence on the geometric scale factor between models. The comparison between mice and the human child shows that postural and individual organ differences do have significant effects, and that care is required in scaling-based extrapolation of rodent experiment results to humans. However, for unrestrained animals, linear scaling appears to be a reasonable and conservative approach. Most of the rodent organ fields, for at least one field orientation, are greater than those expected from linear scaling.

Human exposure to external 50/60-Hz electric and magnetic fields induces electric fields within the body. These induced fields can cause interference with implanted pacemakers. In the case of exposure to magnetic fields, the pacemaker leads are subject to induced electromotive forces, with current return paths being provided by the conducting body tissues. Modern computing resources used in conjunction with millimeter-scale human body conductivity models make numerical modeling a viable technique for examining any such interference. In this paper, an existing well-verified scalar-potential finite-difference frequency-domain code is modified to handle thin conducting wires embedded in the body. The effects of each wire can be included numerically by a simple modification to the existing code. Results are computed for two pacemaker lead insertion paths, terminating at either atrial or ventricular electrodes in the heart. Computations are performed for three orthogonal 60-Hz magnetic field orientations. Comparison with simplified estimates from Faraday's law applied directly to extracorporeal loops representing unipolar leads underscores problems associated with this simplified approach. Numerically estimated electromagnetic interference (EMI) levels under the worst case scenarios are about 40 /spl mu/T for atrial electrodes, and 140 /spl mu/T for ventricular electrodes. These methods could also be applied to studying EMI with other implanted devices such as cardiac defibrillators.

Two types of self‐resonant antennas, meander and sinusoidal are experimentally characterized. Input impedance, bandwidth, and radiation pattern data for such antennas are given. It is demonstrated that antennas that are 34% shorter than quarter‐wave monopoles and have 3.8 dBi of peak gain can be designed with acceptable bandwidth. © 2002 Wiley Periodicals, Inc. Microwave Opt Technol Lett 35: 143–145, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.10541

Dosimetry is evaluated for live-line workers exposed to 50 Hz non-uniform magnetic fields from typical high-voltage transmission lines in the United Kingdom. The configurations involve twin-, triple- and quadruple-conductor transmission line bundles. Scenarios include three worker postures for the twin and triple bundles, and four postures for the quadruple bundle. The postures are selected to simulate worst case scenarios representative of work practices and result in highest values of dosimetric measures in critical organs. Only single-phase bundles are considered, as adjacent bundles of differing phase result only in a small reduction of the dosimetric measures. Reported data include various measures of the electric field and current density induced in tissues, as well as of the current density averaged over 1 cm2 areas normal to the current flow. A value of this latter quantity of 10 mA m(-2) is suggested as a threshold for neural tissue in the UK and international regulations. Critical tissues considered in this study include the retina, spinal cord, brain and cerebrospinal fluid. Some discussion is devoted to problems associated with the concept of current-density averaging, and two algorithms are considered. For a nominal load of 1 kA per subconductor, averaged current densities exceed the guideline bounds, only for a small number of postures and bundle configurations, in the brain, retina and cerebrospinal fluid. Non-averaged current densities in the cerebrospinal fluid exceed the suggested bound for all scenarios modelled, as well as in the retina for three postures involving a quadruple bundle.

Contact currents occur when a person touches conductive surfaces at different potentials, thereby completing a path for current flow through the body. Such currents provide an additional coupling mechanism between the human body and external low-frequency fields. The resulting fields induced in the body can cause interference with implanted cardiac pacemakers. Modern computing resources used in conjunction with millimeter-scale human body conductivity models make numerical modeling a viable technique for examining any such interference. An existing well-verified scalar-potential finite-difference frequency-domain code has recently been modified to allow for combined current and voltage electrode sources, as well as to allow for implanted wires. Here, this code is used to evaluate the potential for cardiac pacemaker interference by contact currents in a variety of configurations. These include current injection into either hand, and extraction via: 1) the opposite hand; 2) the soles of both feet; or 3) the opposite hand and both feet. Pacemaker generator placement in both the left and right pectoral areas is considered in conjunction with atrial and ventricular electrodes. In addition, the effects of realistically implanted unipolar pacemaker leads with typical lumped resistance values of either 20 k/spl Omega/ and 100 k/spl Omega/ are investigated. It is found that the 60-Hz contact current interference thresholds for typical sensitivity settings of unipolar cardiac pacemaker range from 24 to 45 /spl mu/A.. Voltage and electric field dosimetry are also used to provide crude threshold estimates for bipolar pacemaker interference. The estimated contact current thresholds range from 63 to 340 /spl mu/A for bipolar pacemakers.

This paper presents a comparison of anatomically realistic human models and numerical codes in the dosimetry of power frequency magnetic fields. The groups at the University of Victoria and the National Radiological Protection Board have calculated the induced electric fields in both their 'UVic and 'NORMAN' models using independently developed codes. A detailed evaluation has been performed for a uniform magnetic field at 60 Hz. Comparisons of all dosimetric metrics computed in each particular model agree within 2% or less. Since in situ measurements cannot be performed in humans, and achievable accuracy of measurements in models and animals is not likely to be better than 10-15%, the comparisons presented should provide confidence limits on computational dosimetry. An evaluation of the effect of model size, shape and resolution has also been performed and further illuminated the reasons for differences in induced electric fields for various human body models.

Induced electric field and current density in a child's body exposed to a 60-Hz electric field are calculated and compared with those for an adult's body. Because of the different proportions of the child body relative to those of the adult body, differences in the induced electric field and current density values in various organs are observed. These results are interpreted in terms of international guideline limits, and hypotheses regarding plausible interactions.

Contact currents occur when a person touches conductive surfaces at different potentials and completes a path for current flow through the body. Such currents provide an additional coupling mechanism to that, due to the direct field effect between the human body and low-frequency external fields. The scalar potential finite difference method, with minor modifications, is applied to assess current density and electric field within excitable tissue and bone marrow due to contact current. An anatomically correct adult model is used, as well as a proportionally downsized child model. Three pathways of contact current are modeled: hand to opposite hand and both feet, hand to hand only, and hand to both feet. Because of its larger size relative to the child, the adult model has lower electric field and current-density values in tissues/unit of contact current. For a contact current of 1 mA [the occupational reference level set by the International Commission on Non-ionizing Protection (ICNIRP)], the current density in brain does not exceed the basic restriction of 10 mA/m(2). The restriction is exceeded slightly in the spine, and by a factor of more than 2 in the heart. For a contact current of 0.5 mA (ICNIRP general public reference level), the basic restriction of 2 mA/m(2) is exceeded several-fold in the spine and heart. Several microamperes of contact current produces tens of mV/m within the child's lower arm bone marrow.

Design-development and performance evaluation of antennas on handheld telephones can be done experimentally or numerically. Numerical modeling approach or a combination of experiment and numerical modeling, offer many advantages compared with purely experimental approach, which has been often used till now. However, modeling poses many challenges and in order to be considered reliable has to be verified. In this work, modeling of an antenna consisting of two helices is verified by a comparison with measurements made in the laboratory of the handset manufacturer, and by use of two different computer codes. The numerical method used in this investigation is the finite difference time domain (FDTD). A comparison of the measured (in manufacturer's laboratory) electric and magnetic field in four planes at distances 1-4 cm from the antenna with the computed values shows agreement within 15% (which corresponds to the uncertainty in measurements). This agreement is for two operational positions of the antenna in free space. Performance of the handset antenna in the vicinity of the user's head is evaluated. Investigations include the input impedance, far-field radiation pattern and power deposited in the human tissue. The influences of the ear shape and various positions of the handset with respect to the user's head have been evaluated

In a collaborative effort, electromagnetic interference (EMI) is evaluated from a global system For mobile communication telephone with one model of a hearing aid used in the ear canal. Since the electromagnetic fields cannot be measured in the ear canal, a reliable method of their modeling with the finite-difference time-domain method is established. Very good agreement has been achieved between the measured and computed electric and magnetic fields in free space in very close proximity to the telephone, Subsequently, electric and magnetic fields in the ear canal are computed for two models of the ear, and three positions of the telephone. The computed fields are compared with the acoustic measurements for a small number of humans subjected to the EMI test.

This paper presents evaluation of EMI with a hearing aid using electromagnetic modeling and measurements, and acoustic measurements. For modeling the FDTD technique is used, and the fields around a commercial telephone are computed in free space and near a heterogeneous model of the human head. Very good agreement of computed near fields of the telephone in free space is obtained, provided that its antenna is correctly represented. Computed field levels in the ear canal correlate well with the measured acoustic interference levels.

The possibility of interference by low-frequency external electric fields with cardiac pacemakers is a matter of practical concern. For pragmatic reasons, experimental investigations into such interference have used contact electrode current sources. However, the applicability to the external electric field problem remains unclear. The recent development of anatomically based electromagnetic models of the human body, together with progress in computational electromagnetics, enable the use of numerical modeling to quantify the relationship between external field and contact electrode excitation. This paper presents a comparison between the computed fields induced in a 3.6-mm-resolution conductivity model of the human body by an external electric field and by several electrode source configurations involving the feet and either the head or shoulders. The application to cardiac pacemaker interference is also indicated.

In this research, the authors use a high-resolution (cubic voxels with 3.6-mm edges) model of the human body, modified to include a pacemaker, and previously developed and verified computational methods to evaluate potential differences for representative placements of cardiac pacemaker electrodes under several source configurations. Various EMI scenarios are considered.

Cellular telephones such as the global system for mobile communication (GSM) and personal communication services (PCS) are well known to cause electromagnetic interference (EMI) with hearing aids. The magnitude of the acoustic interference depends on several parameters of the telephone and aid. For a given hearing aid and telephone, the EMI depends on the electric and magnetic fields in the location of the hearing aid. The magnitude and direction of electromagnetic fields from a cellular telephone are very different in the ear canal compared to those in free space. In this contribution we present the results of numerical modeling of one telephone and one hearing aid. This hearing aid was extensively investigated by the US Food and Drug Administration laboratory. They evaluated the ratios of acoustic signals at 217 Hz in the ear canal and free space from the same telephone. The telephone was for SGM, and operated at about 900 MHz with TDMA. Electric and magnetic fields in free space in very close proximity to the telephone were measured in its manufacturer's research laboratory.