Numerical evaluation of radio frequency power deposition in human models during MRI.

Concerns have been expressed about safety of MRI examination of two groups of people, namely pregnant mothers and cardiac pacemaker bearers. The main uncertainty relates to excessive heating by pulsed radio frequency (RF) fields. To address these issues, numerical evaluations of the power deposition are performed for a 27-week old fetus in a simplified model of the mother, and a realistic model of a human torso with an in situ pacemaker including its leads. The evaluations are supplemented with organ dosimetry for a realistic model of the human torso. An ideal non-resonant and two resonant birdcage coils operating at 64 MHz (corresponding to 1.5 T MRI) are evaluated. All simulations are performed with the finite difference time domain (FDTD) method.

Evaluation of interactions of electric fields due to electrostatic discharge with human tissue.

Electrostatic discharges (ESDs) produce in the human tissue very strong electric fields of short duration. Possible biophysical interactions are evaluated by comparing the fields in subcutaneous fat/skin to the thresholds for peripheral nerve stimulation, and by computations of membrane potential and electric fields in cytoplasm of a typical cell in bone marrow. It is found that a 4-A peak ESD event is capable of stimulation of nerves located in subcutaneous fat of the lower arm of the hand eliciting a spark, with tens of kV/m and pulse duration of approximately 80 ns. For the same ESD event, the transmembrane potential (TMP) reaches 32 mV with a pulse duration of approximately 200 ns (half-width duration). The electric field in the cytoplasm of a bone marrow cell changes from about 8.8 kV/m to--2 kV/m in about 200 ns.

Electric fields in the human body due to electrostatic discharges.

Electrostatic discharges (ESDs) occur when two objects at different electric potentials come close enough to arc (spark) across the gap between them. Such discharges may be either single-event or repetitive (e.g., 60 Hz). Some studies have indicated that ESDs may be a causative factor for health effects in electric utility workers. Moreover, a hypothesis has recently been forwarded imperceptible contact currents in the human body may be responsible for health effects, most notably childhood leukemia. Numerical modeling indicates that the electric fields in human tissue resulting from typical contact currents are much greater than those induced from typical exposures to electric and magnetic fields at power line frequencies. Numerical modeling is used here to compute representative spark-discharge dosimetry in a realistic human adult model. The frequency-domain scalar potential finite difference method is applied in conjunction with the Fourier transform to assess electric fields in selected regions and tissues of interest in the body. Electric fields in such tissues as subcutaneous fat (where peripheral nerves may be excited), muscle and bone marrow are of the order of kilovolts per meter in the lower arm. The pulses, however, are of short duration (approximately 100 ns).

Human body exposure to power lines: relation of induced quantities to external magnetic fields.

Induced electric field and corresponding current density values in various organs of the human body can be computed numerically using a heterogeneous, anatomically representative voxel model. Such computations are available for uniform magnetic fields of various directions with respect to the body. The highest exposure levels occur for non-uniform fields, most often in occupational settings. Various organ induced dosimetric measures of the induced quantities can also be computed, although the associated computational complexity and effort are greater than for uniform fields. A simplified method of estimation of the induced measures is described and validated. The method is based on evaluation of the external (exposure) magnetic flux density in locations corresponding to those occupied by various organs and dosimetry for the uniform fields. Computations of the external fields are relatively simple even for very complex geometries of current-carrying conductors. Computational methods are available for external fields. The external magnetic fields can also be measured. Detailed organ dosimetry is already published. In this contribution, the proposed simplified dosimetry is verified using accurate, numerically computed dosimetry for four non-uniform field exposure scenarios. For most dosimetric measures and organs, the proposed method gives conservative estimates. Only in rare cases when a large organ is in a weak exposure field compared to the whole-body average exposure, the induced dosimetric measures may be underestimated by up to 10%. Another exception is the maximum induced electric field in spatially distributed tissues such as bone marrow, muscle, or skin when a part of the limb is in a very strong magnetic field close to the conductor. However, both of these situations are easily recognizable from the mutual configuration of the human body and the current-carrying conductors. Thus, additional corrections can be applied to the estimates.

Modelling fields induced in humans by 50/60 Hz magnetic fields: reliability of the results and effects of model variations.

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.

Dosimetry in models of child and adult for low-frequency electric field.

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.

Electric fields in the human body resulting from 60-Hz contact currents.

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/m2. 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/m2 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.

Evaluation of biological effects, dosimetric models, and exposure assessment related to ELF electric- and magnetic-field guidelines.

Several organizations worldwide have issued guidelines to limit occupational and public exposure to electric and magnetic fields and contact currents in the extremely low frequency range (<3 kilohertz). In this paper, we evaluate relevant developments in biological and health research, computational methods for estimating dosimetric quantities, and exposure assessment, all with an emphasis on the power frequency (60 hertz in North America, 50 hertz in Europe). The aim of each guideline is to prevent acute neural effects of induced electric fields. An evaluation of epidemiological and laboratory studies of neurobiological effects identified peripheral nerve stimulation as the response most suitable for establishing a magnetic-field guideline. Key endpoints that merit further study include reversal of evoked potentials; cardiovascular function, as measured by heart rate and heart rate variability; and sleep patterns. High-resolution computations of induced electric fields and current densities in anatomically correct human models are now achieved with finite-difference methods. The validity and limitations of these models have been demonstrated by computations in regular geometric shapes, using both analytic and numeric computations. Calculated values for average dosimetric quantities are typically within a few percent for the two approaches. However, maximum induced quantities are considerably overestimated by numerical methods, particularly at air interfaces. Overestimates are less pronounced for the upper 99th percentile level of a dosimetric quantity, making this measure a more useful indicator of maximum dose. Neural stimulation thresholds are dependent on the electric field around the excitable cell rather than on the current density, making the former preferable for expression of basic restrictions based on nervous system function. Furthermore, modeling data indicate that the induced electric field is much less strongly influenced by tissue conductivity than is the induced current density. In the electric utility industry, most magnetic-field exposures at or near guideline levels occur in highly nonuniform fields. Two methods are described for simplified estimation of induced quantities in such fields, with each method using as input modeling results for uniform field exposure. These methods have practical value for assessing occupational exposures relative to guideline levels.

Pacemaker interference and low-frequency electric induction in humans by external fields and electrodes.

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.

Inter-laboratory comparison of numerical dosimetry for human exposure to 60 Hz electric and magnetic fields.

In recent years, with the availability of high resolution models of the human body, numerical computations of induced electric fields and currents have been made in more than one laboratory for various exposure conditions. Despite the verification of computational methods, questions are often asked about the reliability of these data. In this paper, computational results from two laboratories that presented data in compatible formats are compared, supplemented with additional data from the third laboratory. Two exposures to uniform fields at 60 Hz are evaluated. The human body models used in the computations are different and so are the computation al methods and codes. There are some differences in the conductivity values used for some of the tissues, as well. The results of the comparison confirm that these data are reliable, as the overall agreement is reasonably good and the differences can be rationally explained. This comparison also underscores the importance of accurate data on the dielectric properties of tissues.

Are wireless phones safe? A review of the issue.

Most wireless phones and their corresponding base stations operate at a very low power output and in the radiofrequency range of 800 to 2000 Megahertz. Current international guidelines protect against thermal biological effects in terms of the local or whole-body specific absorption rate (SAR). Potential non-thermal bio-effects resulting from the use of wireless phones are not established and laboratory (i.e., in vitro, in vivo) studies have shown conflicting results. Epidemiological studies of potential human health effects are few but are expected to emerge in the near future. Challenges to epidemiological research include difficult exposure assessment, selection of appropriate controls, potential confounding bias, and validation of outcome. Scientists, community advocacy groups, and public health professionals must be equipped to critically analyze the emerging evidence within a benefit/risk assessment framework.

Numerical evaluation of 60 Hz magnetic induction in the human body in complex occupational environments.

Exposure to 60 Hz non-uniform magnetic fields is evaluated using realistic configurations of three-phase current-carrying conductors. Two specific scenarios are considered, one involving a seated worker performing cable maintenance in an underground vault with conductors carrying 500 A root-mean-square (rms) per phase, and the other involving a standing worker during inspection of a 700 MW generator with conductors carrying 20000 A (rms) per phase. Modelling is performed with a high-resolution (3.6 mm) voxel model of the human body using the scalar potential finite difference (SPFD) method. Very good correspondence is observed between various exposure-field measures, such as the maximum, average, rms and standard deviation values, and the associated induced field measures within the whole body and various organs. The exposure fields produced by the lower currents in the vault conductors result in correspondingly low current densities induced in human tissues. Average values are typically below 0.2 mA m(-2). On the other hand, the average exposure related to the inspection of the generator isophase buses is about 1.5 mT at a distance of 1.2 m from the conductors. This field induces organ average current densities in the range of 2-8 mA m(-2), and peak (maximum in voxel) values above 10 mA m(-2). A comparison with uniform field exposures indicates that induced fields in organs can be reasonably well estimated from the accurately computed exposure fields averaged over the organs and the organ dosimetric data for uniform magnetic fields. Furthermore, the non-uniform field exposures generally result in lower induced fields than those for the uniform fields of the same intensity.

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.

Modeling assemblies of biological cells exposed to electric fields.

Gap junctions are channels through the cell membrane that electrically connect the interiors of neighboring cells. Most cells are connected by gap junctions, and gaps play an important role in local intercellular communication by allowing for the exchange of certain substances between cells. Gap communication has been observed to change when cells are exposed to electromagnetic (EM) fields. In this work, we examine the behavior of cells connected by gap junctions when exposed to electric fields, in order to better understand the influence of the presence of gap junctions on cell behavior. This may provide insights into the interactions between biological cells and weak, low-frequency EM fields. Specifically, we model gaps in greater detail than is usually the case, and use the finite element method (FEM) to solve the resulting geometrically complex cell models. The responses of gap-connected cell configurations to both dc and time harmonic fields are investigated and compared with those of similarly shaped (equivalent) cells. To further assess the influence of the gap junctions, properties such as gap size, shape, and conductivity are varied. Our findings indicate that simple models, such as equivalent cells, are sufficient for describing the behavior of small gap-connected cell configurations exposed to dc electric fields. With larger configurations, some adjustments to the simple models are necessary to account for the presence of the gaps. The gap junctions complicate the frequency behavior of gap-connected cell assemblies. An equivalent cell exhibits low-pass behavior. Gaps effectively add a bandstop filter in series with the low-pass behavior, thus lowering the relaxation frequency. The characteristics of this bandstop filter change with changes to gap properties. Comparison of the FEM results to those obtained with simple models indicates that more complex models are required to represent gap-connected cells.

Biomedical concerns in wireless communications.

The last decade witnessed rapid development of new communication technologies and their broad acceptance at large. Digital wireless telephones are the most popular example of these technologies. There are two aspects of these technologies that are related to human health and therefore biomedical engineering. First, antennas of some devices are in close proximity to the user's head, thus possibly producing locally excessive energy deposition. Second, radiofrequency (RF) signals emitted are amplitude modulated at extremely low frequencies, potentially eliciting different biological effects from those of unmodulated RF radiation. Recent progress in addressing these two issues is reviewed in this article. Another area of research and concern not covered here is electromagnetic interference (EMI) with medical devices. Considerable research has been conducted on the development of a new method for numerical and experimental evaluation of the spatial distribution of the power deposition in tissue. Improved implantable electric field probes and automated scanning systems are presently available. With respect to numerical evaluation of electric fields in tissue, the finite difference time domain (FDTD) technique has proven to be a useful and accurate tool. These developments also are critical in view of the regulatory requirements now imposed on mobile/portable transmitters. Similarly, significant research effort on biological effects of modulated fields has been undertaken. Most of the studies are still in progress, and further research agendas have been proposed.

A novel equivalent circuit model for gap-connected cells.

Gap junctions connect neighbouring cells, providing the intercellular communication that is essential for cell growth regulation, for example. There is some evidence that gap communication changes upon exposure to electromagnetic (EM) fields. In previous work, we performed detailed finite element method (FEM) modelling of gap junction connected cells exposed to EM fields. For cell configurations, the presence of gap junctions influences the transmembrane potential and its frequency behaviour. The relaxation frequency cannot be accurately predicted by previously developed simplified models. We present a novel equivalent circuit model (ECM) that incorporates more detailed models of the gaps, and compare results obtained with this ECM to finite element and leaky cable (LC) model results. Our ECM provides more accurate estimates of the frequency behaviour of cells than the leaky cable model. Also, our ECM results suggest limitations of the application of simple models to gap-connected cells: with higher gap resistivity, the current flow in the cell interiors becomes increasingly complex and is not well represented by simple models. In this case, techniques such as the finite element method are required to model accurately cell behaviour.

Biological cells with gap junctions in low-frequency electric fields.

Biological effects have been observed from weak, low-frequency magnetic fields. It has been suggested that the observed effects are due to the induced currents and electric fields. The behavior of cells exposed to an electric field is investigated in this paper. The induced transmembrane potential (TMP) is examined in geometrically complex models of various cell configurations. The TMP is evaluated using the finite element method (FEM), a numerical technique that is well suited to complicated geometries. Because displacement currents can be neglected at very low frequencies, a FEM solver that considers only material conductivity is used. Therefore, our results apply only well below the relaxation frequency. Chains and clusters of gap-connected cells of various sizes are modeled. The conductivity and size of the gap junctions in the cell configurations are also varied. The results for small configurations are compared to models of ellipsoidal cells with shapes similar to those of the configurations. FEM estimates of TMP's in long, cylindrical cell chains are compared to the predictions of the leaky cable model. The FEM approach confirms that gap-junction-connected cells can be treated as a single similarly shaped cell. Gaps influence the potential in the interior of cell configurations, and these effects increase with gap size and conductivity. For configurations to which approximations such as the leaky cable model do not apply, the FEM approach can be used to estimate the TMP, if the model is adapted to fit within computational memory limits.

Effects of skeletal muscle anisotropy on human organ dosimetry under 60 Hz uniform magnetic field exposure.

The recent development of anatomically derived high-resolution voxel-based models of the human body suitable for electromagnetic modelling, and of effective methods for computing the associated induction, has resulted in numerical estimates of organ-specific dosimetry for human exposure to low-frequency magnetic fields. However, these estimates have used an isotropic conductivity model for all body components. More realistic estimates should account for the anisotropy of certain tissues, particularly skeletal muscle. In this work, high-resolution finite-difference computations of induced fields are used to estimate the effects of several extremal realizations of skeletal muscle anisotropy on field levels in various organs. It is shown that, under the present assumptions (anisotropy ratios up to 3.5:1), the resulting dosimetric values can vary by factors of between two or three for tissues other than muscle and up to 5.4 for muscle, despite the unchanged nature of the conductivity model used for all other body components.

Influence of human model resolution on computed currents induced in organs by 60-Hz magnetic fields.

The effects of human body model resolution on computed electric fields induced by 60 Hz uniform magnetic fields are investigated. A recently-developed scalar potential finite difference code for low-frequency electromagnetic computations is used to model induction in two anatomically realistic human body models. The first model consists of 204290 cubic voxels with 7.2-mm edges, while the second comprises 1639146 cubic voxels with 3.6-mm edges. Calculations on the lower-resolution model using, for example, the finite difference time domain or impedance methods, push the capabilities of workstations. The scalar method, in contrast, can handle the higher-resolution model using comparable resources. The results are given in terms of average and maximum electric field intensities and current density magnitudes in selected tissues and organs. Although the lower-resolution model provides generally acceptable results, there are important differences that make the added computational burden of the higher-resolution calculations worthwhile. In particular, the higher-resolution modelling generally predicts peak electric fields intensities and current density magnitudes that are slightly higher than those computed using the lower-resolution modelling. The differences can be quite large for small organs such as glands.