Search PubMed⌕ Search

Biomedical subjects

M A Stuchly

Publications and source records attributed to M A Stuchly.

At least 37 records · Page 2Linked to original sources
PubMed1997

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.

Data Interpretation, Statistical↗
PubMed1995

Magnetic field induced currents in a human head from use of portable appliances.

The induced currents in a model of the human head resulting from use of small hand-held appliances were computed. The source of the induced currents, i.e. the alternating (60 Hz) magnetic field was modelled as a magnetic dipole. The limits of applicability of this model are given. The model of the human head was obtained by translating MRI data into tissue conductivity data. Computations of the distribution of the induced currents were performed using the impedance method. The induced current density rapidly decreases with the distance from the appliance. The maximum, however, predictably is not at the surface, because of the geometry of the head. For typical appliances, the values of the maximum current densities are high as compared to those induced by a uniform magnetic field. Generalized graphs were obtained for the maximum induced current density normalized to the appliance magnetic moment, as a function of the distance from the appliance. These graphs are given for two representative locations of the appliance with respect to the head.

Electromagnetic Fields↗
PubMed1995

Cylindrical tissue model for magnetic field stimulation of neurons: effects of coil geometry.

An analytical solution for the electric field induced in a homogeneous cylindrical conductor under quasi-static conditions from current in a coil is used to model peripheral nerve stimulation with magnetic fields. A variety of coil geometries is analyzed in terms of the spatial derivative of the induced electric field along a long straight nerve, parallel to the axis, in a human-arm model. The results of these computations are found to be in general agreement with conclusions regarding the optimization of stimulating coils derived from analyses of the semi-infinite tissue model. The differences and their physical basis are pointed out.

Arm↗
PubMed1994

Design and fabrication of well confined uniform magnetic field exposure systems.

Exposure systems that provide good magnetic field uniformity, minimum stray fields, and minimal heating, vibration, and hum, as well as capability for true sham exposure in which current flows in the coils, are needed to determine rigorously the biological effects of weak magnetic fields. Designs based on acrylic polymer coil support structures and twisted pair bifilary coil windings were employed to fabricate several different systems for the exposure of laboratory animals and cell cultures to magnetic fields. These systems exhibit excellent performance characteristics in terms of exposure field uniformity, stray field containment, and exposure field cancellation in the sham exposure mode. A custom-written computer program was used to determine the best arrangement for coils with regard to field uniformity in the exposure volume and stray field containment. For in vivo exposures, modules were made up of four Merritt four-coil sets, built into a single structure and positioned to form an octapole with fields directed in the horizontal plane. For in vitro applications, two different coil configurations were selected to produce the vertical fields required. A quadrupole system, comprising modules consisting of two Merritt four-coil sets arranged side by side to limit stray fields, was built as a prototype. In the second configuration, one Merritt four-coil set was positioned inside the other to form a concentric coil set. In both in vitro systems, exposure chambers were connected to remote commercial incubators in order to reduce ambient magnetic fields in the exposure volume.(ABSTRACT TRUNCATED AT 250 WORDS)

Acrylates↗
PubMed1994

Modelling induced currents in biological cells exposed to low-frequency magnetic fields.

Interactions of low-frequency magnetic fields with biological systems have been a subject of intense scientific inquiry and public concern. Most research has been done at powerline frequencies of 50 Hz or 60 Hz. One of the key questions related to interactions of low-frequency magnetic fields with biological systems is which parameters of the exposure field are responsible for observed effects. Knowledge of the induced electric field and current in various experimental in vitro systems is important for this purpose. The 3D impedance method is used in this research to model spatial patterns of induced electric fields and current in two preparations of cells. A cell monolayer with a random distribution of cells and a confluent monolayer of cells with gap junctions are considered; because of the limitations of the computational method, biological cells are represented by cubes rather than more realistic shapes (e.g. spheres). The random model indicates that for higher cell densities the pattern of the induced current flow has a limited dependence on the size and shape of the container in which the cells are placed, it depends mostly on the actual cell placement. Gap junctions, not surprisingly, are shown to increase the current density, but only if their resistance is sufficiently low. The highest current density occurs in the gaps.

Animals↗
PubMed1994

Quasi-static electric field in a cylindrical volume conductor induced by external coils.

An expansion technique based on modified Bessel functions is used to obtain an analytical solution for the electric field induced in a homogeneous cylindrical volume conductor by an external coil. The current in the coil is assumed to be changing slowly so that quasi-static conditions can be justified. Valid for any coil type, this solution is ideal for fast computation of the induced electric field at a large number of points. Efficient implementation of this method in a computer code is described and numerical results are presented for a perpendicular circular coil and a tangential double-square coil.

Electric Conductivity↗
PubMed1994

Induced electric currents in models of man and rodents from 60 Hz magnetic fields.

Induced electric currents in models of man, rat and mouse from 60 Hz magnetic fields are computed using the impedance method. The models all have realistic shapes, and in the case of rodents, a homogeneous average tissue conductivity is assumed. The model of man is analyzed for two cases, a homogeneous average tissue conductivity and a heterogeneous model, both consisting of 1.3 cm cubical tissue cells whose conductivities are representative of the tissue within the cube. The results for various models and species, as well as different orientations of the magnetic field, are compared. The data presented are useful as the first step in dosimetry for 60 Hz magnetic fields, and for interspecies scaling of biological interactions related to the tissue induced electric currents.

Animals↗
PubMed1994

Modeling of magnetic field stimulation of bent neurons.

We consider a simple model of magnetic stimulation of a long bent neuron located in a semi-infinite volume conductor with a planar interface. It is shown that the stimulating coil characteristics (size, shape and location) and the neuron shape affect the location of the stimulation. The activating function, defined as the electric field derivative along the neuron, has two components. One component depends on the derivative of the electric field along the straight section of the neuron, and the other on the field magnitude. The maximal stimulation point is at the bent part of the nerve and its position depends on the nerve shape and coil parameters. The analysis also has shown a better performance (a stronger stimulus) for a double-circular (figure eight) coil than for a double-square coil.

Electric Stimulation↗
PubMed1992

Modification of tumor promotion in the mouse skin by exposure to an alternating magnetic field.

Some epidemiological studies have suggested that exposure to an alternating magnetic field may increase the incidence of some cancers. Our earlier study of carcinogenesis in mouse skin, indicated that exposure to a magnetic field (MF) alone did not promote the growth of tumors. In the present experiment, the ability of a MF to act as a tumor copromoter was investigated. The dorsal skins of female SENCAR mice (6-7-weeks-old) were treated with 10 nmol of 7,12-dimethylbenzanthracene (DMBA) to initiate the carcinogenic process and then tumor development was promoted, for 23 weeks, by weekly applications of 4.9 nmol (0.3 microgram) of 12-0-tetradecanoylphorbal-13-acetate (TPA). One group of 48 mice were exposed to a 60-Hz magnetic field of 2 mT (20 Gauss) for 6 h/day 5 days/week, while a similar group (48 mice) were sham exposed. After week 12, the percentage of mice with tumors and the mean number of tumors per mouse, were higher for the group exposed to MF. At week 18, for example, where the differences between field and sham groups were statistically significant, the percentage of mice with tumors were, respectively, 25% and 8% (P less than 0.05, Fisher exact) and, the mean yield of tumors 1.9 +/- 0.69 and 0.65 +/- 0.46 (mean +/- S.E.M.) (P less than 0.05, Wilcoxon). At week 23 these differences were no longer statistically significant.

9,10-Dimethyl-1,2-benzanthracene↗
PubMed1992

Factors affecting neural stimulation with magnetic fields.

High amplitude magnetic field pulses produced by coils external to the body have been used for medical diagnosis since the mid-1980s to stimulate motor neurons in the brain cortex and peripheral nerves. While successful applications have since blossomed, it has only been during the last three years that quantitative dosimetric data have become available. The factors affecting neural stimulation can be divided into three categories broadly related to the characteristics of (i) the stimulus, (ii) the neuron, and (iii) the induced electric field as related to the configuration of the stimulating coil. The stimulus, in the case of magnetic field stimulation, has the form of an exponentially decaying pulse with a small overshoot of the opposite polarity. Physical and electrical properties of a neuron affect its electrical stimulation. Dosimetric considerations are limited to the linear model describing the threshold phenomena, where passive electrical properties and the cable model provide a reasonable approximation of neuron behavior. The electromagnetic variable responsible for stimulation is the spatial derivative of the induced electric field along the neuron axis. This paper examines the factors involved in eliciting threshold excitation of motor neurons by magnetic fields. The description of various factors is largely based on published data except for the analysis of the electromagnetic stimuli induced by various coils.

Animals↗
PubMed1992

Neural stimulation with magnetic fields: analysis of induced electric fields.

Spatial distributions of the derivative of the electric field induced in a planar semi-infinite tissue model by various current-carrying coils and their utility in neural stimulation are evaluated. Analytical expressions are obtained for the electric field and its spatial derivatives produced by an infinitely short current element. Fields and their derivatives for an arbitrarily shaped coil are then obtained by numerical summation of contributions from all the elements forming the coil. The simplicity of the solution and a very short computation time make this method particularly attractive for gaining a physical insight into the spatial behavior of the stimulating parameter and for the optimization of coils. Such analysis is useful as the first step before undertaking a more complex numerical analysis of a model more closely representing the tissue geometry and heterogeneity.

Computer Simulation↗
PubMed1991

Cancer promotion in a mouse-skin model by a 60-Hz magnetic field: I. Experimental design and exposure system.

The rationale for selection of an animal model, the experimental design, and the design and evaluation of an exposure system used in studies of 60-Hz magnetic fields are described. The studies were conceived to assay development of cancer and immune responsiveness in mice exposed to magnetic fields. The exposure system utilized a quadrupole-coil configuration to minimize stray magnetic fields. Four square-wound coil provided a uniform field within a volume occupied by 16 animal cages. The magnetic field had a mean flux density of 2 mT that varied less than +/- 10% within the volume occupied by animals' cages. The flux density decreased to less than 0.1 microT at a distance of 2 m from the coils. In each exposure system 32 animals could be housed in plastic cages.

9,10-Dimethyl-1,2-benzanthracene↗
PubMed1991

Cancer promotion in a mouse-skin model by a 60-Hz magnetic field: II. Tumor development and immune response.

This paper describes preliminary findings on the influence of 60-Hz (2-mT) magnetic fields on tumor promotion and co-promotion in the skins of mice. The effect of magnetic fields on natural killer (NK) cell activity in spleen and blood was also examined. Groups of 32 juvenile female mice were exposed to the magnetic field as described in part I. The dorsal skin of all animals was treated with a subthreshold dose of the carcinogen 7,12-dimethyl-benz(a)anthracene (DMBA). One week after the treatment, two groups were sham exposed (group A) or field exposed at 2 mT (group B) 6 h/day for 21 weeks, to test whether the field would act as a tumor promoter. No tumors developed in these two groups of mice. To test whether the magnetic field would modify tumor development by directly affecting tumor growth or by suppressing immune surveillance, two additional groups of mice were treated weekly with the tumor promoter 12-0-tetradecanoylphorbol-13-acetate (TPA) and then either sham exposed (group C) or field exposed (group D). The time to appearance of tumors was shorter (but not statistically so) in the group exposed to magnetic fields and TPA. Some differences in NK cell activity and spleen size were observed between the sham- and field-exposed groups.

9,10-Dimethyl-1,2-benzanthracene↗
PubMed1991

Measurements of contact currents in radiofrequency fields.

Radiofrequency electromagnetic fields can affect human health not only by direct interactions but indirectly through induction of charges on isolated or poorly grounded conductive (metallic) objects located in these fields. A person who touches such an object may perceive a tingling or prickling sensation or heat, or experience pain or electric shock. For sufficiently large objects, these phenomena can occur at field strengths that are relatively low and below the health protection limits based on direct interactions. We describe a method and circuitry developed to evaluate steady-state contact currents that may flow through a person touching conductive objects and give a summary of experimental tests performed. The method is simple and viable for field tests aimed at preventing pain, shock, and burn hazards in radiofrequency electromagnetic fields except those related to spark discharges. The method is applicable up to about 30 MHz.

Electric Injuries↗
PubMed1989

Exposure to electromagnetic fields in arc welding.

Electric and magnetic fields at the power line frequency (60 Hz) and its harmonics have been measured around 22 arc welders. The electric field strengths in most cases were very low (1 V m-1) and only for one device reached 300 V m-1. Magnetic flux densities ranged from a few microT to a few hundred microT (a few A m-1 to a few hundred of A m-1). Several devices surveyed produced high magnetic flux densities (200-400 microT), which are rarely encountered in the environment and are representative of high occupational exposures.

Electromagnetic Fields↗
PubMed1989

Comparison of finite-difference time-domain SAR calculations with measurements in a heterogeneous model of man.

A finite-difference time-domain technique was used to calculate the specific absorption rate (SAR) at various sites in a heterogeneous block model of man. The block model represented a close approximation to a full-scale heterogeneous phantom model. Both models were comprised of a skeleton, brain, lungs, and muscle. Measurements were conducted in the phantom model using an implantable electric-field probe and a computer-controlled data acquisition system. The calculation and measurement of SAR distributions were compared primarily in the head (including the neck) and chest. To obtain the necessary spatial resolution with the computer model, the head and neck were modeled with approximately 105,000 cells, while 86,000 cells were used to configure the chest. Planewave fields, polarized in the E orientation, were utilized to irradiate the models at an exposure frequency of 350 MHz. Reasonable correlation existed between the calculations and measurements.

Absorption↗
PubMed1988

Teratological assessment of exposure to time-varying magnetic field.

A teratological assessment was performed using rats that were exposed to an alternating magnetic field. The magnetic field had a sawtooth waveform similar to that produced by video display terminals (VDTs). Female rats were exposed 2 weeks prior to and throughout pregnancy at a rate of 7 h/day. Three intensities of magnetic field (5.7, 23 or 66 microT) were used. All of these field intensities were much greater than those to which VDT users are exposed. A slight but statistically significant decrease in maternal lymphocyte count for the highest intensity field was found as compared with the control group. However, the lymphocyte count was within the normal range, and the observed changes in hematological parameters were considered mild. No other maternal or fetal parameters that were examined showed a significant difference for any of the three field intensities. Where minor variations in skeleton development were observed they were known to be the common "noise" that appears in every teratological evaluation.

Animals↗