Showing papers in "Electromagnetics in 2001"
TL;DR: In this paper, a theory is presented to estimate the effective conductivity of human tissue, which is defined as the difference in electrical conductivity between different layers of a human tissue.
Abstract: Human tissue consists of cells suspended in an aqueous medium. As a consequence, the electrical conductivity of a tissue is inhomogeneous on a microscopic scale. On a macroscopic scale the conductivity of a tissue can be considered to be homogeneous, and this conductivity is called the effective conductivity. In this paper a theory is presented to estimate this effective conductivity. On a cellular scale, the tissue will be modelled as a suspension of ellipsoidal cells. On a somewhat larger scale, it will be modelled as a layered structure, as most tissues can be described by layers that differ in conductivity. For both scales, upper and lower bounds for the effective conductivity will be estimated. The more information on the shape, orientation, conductivities, and distribution of the cells or layers is available, the more strict these limits are. The effective conductivity will be evaluated for several tissues, such as the cerebral cortex, the liver, and blood.
70 citations
TL;DR: In this paper, a fast algorithm is presented for solving a time-domain electric field integral equation pertinent to the analysis of scattering from uniformly meshed, perfectly conducting structures, which is accelerated by using the fast Fourier transform to perform spatial convolutions.
Abstract: A fast algorithm is presented for solving a time-domain electric field integral equation (EFIE) pertinent to the analysis of scattering from uniformly meshed, perfectly conducting structures The marching-on-in-time (MOT) scheme that results from discretizing this EFIE is accelerated by using the fast Fourier transform to perform spatial convolutions The computational cost and storage requirements of this algorithm scale as O(NtNs 15) and O(Ns 15), respectively, as opposed to O(NtNs 2) and O(Ns 2) for classical MOT methods Simulation results demonstrate the accuracy and efficiency of the approach and suggestions for extending the technique are proffered
40 citations
TL;DR: In this paper, a homogenized composite medium (HCM) that is biaxial with respect to its dielectric constitutive properties can arise even though its components have electromagnetic properties less complex than BIAxial.
Abstract: A homogenized composite medium (HCM) that is biaxial with respect to its dielectric constitutive properties can arise even though its components have electromagnetic properties less complex than biaxial. We investigate a variety of means of realizing biaxial HCMs and consider how their biaxial nature depends on the geometry, orientation, and composition of their constituents. We demonstrate that biaxiality in a composite medium may be achieved when the component mediums collectively present two noncollinear distinguished axes; these axes can have either an electromagnetic or topological origin.
25 citations
TL;DR: In this article, a high-order technique based on a locally-corrected Nystrom scheme is applied to the solution of electromagnetic scattering problems via the volume electric field integral equation.
Abstract: In this paper, a high-order technique based on a locally-corrected Nystrom scheme is applied to the solution of electromagnetic scattering problems via the volume electric field integral equation. The locally corrected Nystrom method is detailed for the solution of the vector polarization currents. It is demonstrated that this scheme converges in a high-order manner for the scattering cross section of a dielectric cylinder of arbitrary cross section, and is computationally efficient.
24 citations
TL;DR: An efficient implementation of an iterative method that includes fast mode transformations (FMTs) that is capable of analyzing larger bodies than other classical techniques and justifies the design procedure and validates the present analysis approach.
Abstract: This article presents an efficient implementation of an iterative method that includes fast mode transformations (FMTs). This method has the advantage of simplicity in that it does not involve basis functions and inversion of matrices, as used in other calculation methods. Therefore, it is capable of analyzing larger bodies than other classical techniques. An implementation of the iterative calculation is shown for the extraction of S parameters of unbounded multiport microstrip passive circuits. The good agreement between simulation results and experimental data (Zhu & Wu, 1997) justifies the design procedure and validates the present analysis approach.
24 citations
TL;DR: The Bruggeman approach is implemented to homogenize a collection of identical, similarly oriented, electrically small, ellipsoidal particles with intensity-dependent refractive index that are dispersed randomly and homogeneously in an isotropic, linear, dielectric host material as mentioned in this paper.
Abstract: The Bruggeman approach is implemented to homogenize a collection of identical, similarly oriented, electrically small, ellipsoidal particles with intensity-dependent refractive index that are dispersed randomly and homogeneously in an isotropic, linear, dielectric host material. The homogenized composite medium is assumed to be cubically nonlinear and anisotropic, in the simplest approximation. The anisotropy of nonlinearity may be considerably different from the anisotropy of the linearity. The possible enhancement of nonlinearity is also anisotropic, with the most desirable performance along the shortest principal axis of the ellipsoidal particles.
22 citations
TL;DR: With reference to circular arrays of antennas, a method for synthesizing a desired pattern in the presence of both null constraints and excitation constraints is presented, allowing us to reduce both the dynamic range ratio of the excitations and the pattern degradation due to the quantization of theexcitations.
Abstract: With reference to circular arrays of antennas, we present a method for synthesizing a desired pattern in the presence of both null constraints and excitation constraints. Starting from the desired pattern, we carry out a sequence of projections onto the sets ¡ and « containing the array patterns that satisfy the null constraints and the excitation constraints, respectively. In « this generates a sequence fPng of patterns which are closer and closer to ¡. An approximate solution to the problem is a pattern Pn suYciently close to ¡. With a suitable choice of « the method allows us to reduce both the dynamic range ratio of the excitations and the pattern degradation due to the quantization of the excitations. Numerical results show the eVectiveness of the method.
19 citations
TL;DR: In this feasibility study, Laplacian electrocardiograms were utilized to generate body surface moment-of-activation (MOA) maps and a partial activation cycle of atrial flutter was mapped.
Abstract: In this feasibility study, Laplacian electrocardiograms were utilized to generate body surface moment-of-activation (MOA) maps. The data were obtained with Laplacian sensors from humans. Inverse distance interpolation was applied to improve the image quality. For normal subjects, the MOA maps showed certain common features. A partial activation cycle of atrial flutter was mapped and is presented. Further studies must be conducted to determine the diagnostic potential of such patterns based on a larger number of subjects.
14 citations
TL;DR: A new, very simple expression for the wide aperture dimension of the horn is presented and used in the optimum-gain pyramidal horn design and the gain of a designed pyramides horn is computed with no path length error approximation.
Abstract: A new, very simple expression for the wide aperture dimension of the horn is presented and used in the optimum-gain pyramidal horn design. The design parameters are determined from the simple and explicit formulas. These formulas do not need the application of the iterative methods and are not restricted to the high-gain horn designs. The gain of a designed pyramidal horn is computed with no path length error approximation. Several design examples are given that illustrate the performance of the proposed design method. This proposed design method is useful for microwave antenna designers and the computer aided design (CAD) of optimum-gain pyramidal horns.
13 citations
TL;DR: In this paper, a methodology is described to calculate the static effective permittivity for a two-dimensional multiphase lattice composed of dielectric and/or conducting circular cylinders.
Abstract: A methodology is described to calculate the static effective permittivity for a two-dimensional multiphase lattice composed of dielectric and/or conducting circular cylinders. This methodology uses an accurate T-matrix method to determine the dipole moments of the cylinders immersed in a uniform electric field, and then computes the effective permittivity by relating the lattice to a macroscopic model. With this methodology, the multiple scattering solution for the infinite lattice is presented in a succinct matrix-vector notation and is valid for any lattice type. The static effective permittivity equation described in this work allows us to account for the effect of all mutual interactions between the cylinders. This methodology is used to calculate the static effective permittivity for a two-phase lattice of metallic inclusions. These results are compared with the Maxwell Garnett formula and another formula presented by Kharadly and Jackson. Three additional examples are presented including two-phase d...
12 citations
TL;DR: In this article, wave reflection and transmission phenomena in slabs of artificial anisotropic chiral media, taking into account dielectric and magnetic properties, are theoretically considered, and a new device based on this effect is proposed.
Abstract: In this paper plane wave reflection and transmission phenomena in slabs of artificial anisotropic chiral media, taking into account dielectric and magnetic properties, are theoretically considered The artificial medium is a superlattice of chiral layers and anisotropic magnetic layers Biaxial symmetry of the medium is defined by the direction of spiral inclusions and the direction normal to the interfaces between layers Normal incidence is assumed The effect of compensation of magnetic and dielectric anisotropy is discussed A new device based on this effect is proposed
TL;DR: In this paper, two approaches are proposed to determine the diffraction coefficients for a nonpenetrable wedge with the faces characterized by anisotropic impedance boundary conditions when it is illuminated at oblique incidence by an electromagnetic plane wave.
Abstract: Two approaches are proposed to determine the diffraction coefficients for a nonpenetrable wedge with the faces characterized by anisotropic impedance boundary conditions when it is illuminated at oblique incidence by an electromagnetic plane wave. In the more general case the anisotropic impedance tensors associated with the faces have their principal axes not coincident with the edge of the wedge. The first solution is obtained by using a physical optics approximation for the surface current densities on the wedge and by performing a uniform asymptotic evaluation of the radiation integral (uniform asymptotic physical optics solution). The second solution is derived by following a heuristic approach. The effectiveness of both solutions is demonstrated by comparing them with analytical solutions available for particular cases.
TL;DR: In this paper, a prediction method based on a hybridization between the standard parabolic equation method and the plane wave spectrum scattering technique was developed to perform the apparent radar cross section (RCS) of a target in a naval environment, taking into account the phenomena acting on the radio wave propagation between the radar and the target.
Abstract: To perform the apparent radar cross section (RCS) of a target in a naval environment, it is necessary to take into account the phenomena acting on the radio wave propagation between the radar and the target. These effects, like forward scattering on the rough sea surface and atmospheric refraction, can have a strong influence on the backscattering process. To take these propagation effects into account, a prediction method has been developed based on a hybridization between the standard parabolic equation method and the plane wave spectrum scattering technique. The reciprocity principle is used to avoid the return path computation. This new approach is a coherent one, considering the problem in a global view: indeed the propagation effects, the antenna pattern, and the target characteristics are taken into account simultaneously. Numerical results obtained using this hybridization approach are compared with a classical approach and with measurements on a trihedral corner reflector in a naval environment.
TL;DR: Recognition and exploitation of sequential broken symmetries inherent in the epicardial potential imaging formulation and the role of topological features of depolarization in the cardiac activation imaging formulation allow more efficient processing of the available information into images of cardiac electrical activity.
Abstract: Although superb technologies exist for noninvasive depiction of tissue anatomy and acquisition of bioelectromagnetic signals, uniting these inputs to effect noninvasive imaging of cardiac electrophysiology is confounded by the severe ill-posedness of the problem. However, recognition and exploitation of sequential broken symmetries inherent in the epicardial potential imaging formulation and the role of topological features of depolarization in the cardiac activation imaging formulation allow more efficient processing of the available information into images of cardiac electrical activity. This paper presents resulting preferred algorithmic structures for imaging epicardial potential and myocardial activation using noninvasive electromagnetic measurements.
TL;DR: In this paper, the boundary-element field equations for both approaches were developed, and the discretization and matrix deflation techniques were used with them, and, using a three-concentric-spheres head model, finally evaluated their performance for two widely different skull conductivities.
Abstract: The conventional approach to forward problem solutions in electroencephalography entails computing the surface potentials starting from source current dipoles. The reciprocal approach, on the other hand, first solves for the electric field at the source dipole location when the surface electrodes are reciprocally energized with a unit current. A scalar product of this electric field with the source dipole then yields the surface potential. Using a common weighted-residual formulation, this paper initially develops the boundary-element field equations for both approaches, next describes the discretization and matrix deflation techniques to be used with them, and, using a three-concentric-spheres head model, finally evaluates their performance for two widely different skull conductivities. It was found that while a conventional vertex method, in which the desired potentials are calculated at the vertices as opposed to the centroids of the discretization triangles, in general yields the most accurate forward...
TL;DR: In this paper, the scattering from a thin rectangular plate of a perfect conductor illuminated by an electromagnetic plane wave at glancing incidence is studied using a formula derived by the method of the Kobayashi potential, which is a form of Weber?Schafheitlin discontinuous integrals.
Abstract: The scattering from a thin rectangular plate of a perfect conductor illuminated by an electromagnetic plane wave at glancing incidence is studied using a formula derived by the method of the Kobayashi potential, which is a form of Weber?Schafheitlin discontinuous integrals. Numerical results for the current density distribution and far-field pattern are presented. A part of these results are compared with those obtained from the approximation expression, which is derived from the current density induced on the half-plane. The agreement is relatively good, particularly for a large plate. The backscattering cross sections (radar cross sections) are obtained as a function of the dimension of the plate for various values of incident angles; our results agree well with the experimental data measured by Ross and others. Numerical results of the total scattering cross sections are also presented.
TL;DR: Simulation studies of Laplacian electrocardiograms using a realistically shaped heart-torso model show that the BSLM can distinguish the left ventricular breakthrough with a special pattern, which is not distinguished with BSPM, and has better specificity in discriminating multiple accessory pathways than the BSPm.
Abstract: This paper presents simulation studies of Laplacian electrocardiograms using a realistically shaped heart-torso model. Body surface Laplacian maps (BSLMs) during normal excitation and abnormal excitation with the Wolff-ParkinsonWhite syndrome are examined in comparison with the body surface potential maps (BSPMs). It shows that the BSLM can distinguish the left ventricular breakthrough with a special pattern, which is not distinguished with BSPM. It also shows that the BSLM has better specificity in discriminating multiple accessory pathways than the BSPM. These simulations theatrically suggest the advantages of Laplacian electrocardiogram as a non-invasive method in exploring the excitation processes of the heart.
TL;DR: In this article, the surface fields of an impedance polygonal cylinder with surface impedance were evaluated for an arbitrary incident angle, including those for the glancing incidence, and it was found that these approximate expressions are very accurate even when an observation point was very close to the edge.
Abstract: We have developed a method of evaluation of the surface fields sctattered by an impedance polygonal cylinder when a plane wave is normally incident to the axis of the cylinder. The physical optics (PO) currents are corrected by adding the transtion currents near each edge, which are determined by solving the canonical impedance wedge problem. The uniform asymptotic solutions for the surface fields of the impedance wedge are derived for an arbitrary incident angle, including those for the glancing incidence. It is found that these approximate expressions are very accurate even when an observation point is very close to the edge. When the observation point is far from the edge, surface fields are found to decrease according to (krho)-( 3 / 2 ) and (krho)-( 1 / 2 ) for nonglancing and glancing incidence, respectively. The surface fields may be used to evaluate the near and far fields scattered by a polygonal cylinder with surface impedance through the PO radiation integral. Using the concept of the equivalen...
TL;DR: The proposed method provides an alternative to estimating the surface Laplacian from potentials over a realistic-geometry scalp surface and is tested in visual-evoked potentials recorded in a human subject.
Abstract: This paper presents a new method to estimate the scalp Laplacian EEG from scalp potentials on a realistic geometry scalp surface. The mathematical potential distribution on the realistically shaped scalp is constructed using a three-dimensional spline, and the mathematical scalp geometry model is constructed using a two-dimensional spline. The performance of the proposed method is evaluated by computer simulations using the three-concentric-sphere head model. The feasibility of the present method has also been tested in visual-evoked potentials recorded in a human subject. The present simulation and experimental data suggest that the proposed method provides an alternative to estimating the surface Laplacian from potentials over a realistic-geometry scalp.
TL;DR: In this article, advanced methods for the modeling of human cortical activity from combined high-resolution electroencephalography (EEG) and functional magnetic resonance imaging (fMRI) data are reviewed.
Abstract: In this paper, advanced methods for the modeling of human cortical activity from combined high-resolution electroencephalography (EEG) and functional magnetic resonance imaging (fMRI) data are reviewed. These methods include a subject's multicompartment head model (scalp, skull, dura mater, cortex) constructed from magnetic resonance images, multidipole source model, and regularized linear inverse source estimate based on boundary element mathematics. Furthermore, determination of the priors in the resolution of the linear inverse problem was performed with the use of information from the hemodynamic responses of the cortical areas as revealed by block-designed (strength of activated voxels) and event-related (coupling of activated voxels) fMRI. Linear inverse source estimates of cortical activity were regularized by taking into account the covariance of background EEG sensor noise. As an example, these methods were applied to EEG (128 electrodes) and fMRI data, which were recorded in separate sessions wh...
TL;DR: In this paper, the capacitance matrix of a pair of cylinders of unequal radii and dielectric width above a ground plane is evaluated using the moment method using Fourier harmonic functions as the basis functions for the charge distribution.
Abstract: This paper presents a method of evaluation of a capacitance matrix of a pair of cylinders of unequal radii and dielectric width above a ground plane. The moment method using Fourier harmonic functions as the basis functions for the charge distribution is employed for the derivation of the four sets of simultaneous equations which lead to a partitioned matrix equation using point matching for testing. It is shown that the use of point matching for testing does not lead to a singular matrix when the cylinder pair is located at a finite height above a ground plane. For the special case of height equal to infinity, the moment matrix is found using Galerkin's technique and conformal transformation. Numerical data are presented.
TL;DR: In this paper, it was shown that one can have a generalized TEM plane wave propagating in the u3 direction in u1, u2, u3 orthogonal curvilinary coordinates.
Abstract: Previous results have shown that one can have a generalized Transverse Electromagnetic (TEM) plane wave propagating in the u3 direction in u1, u2, u3 orthogonal curvilinary coordinates. The formal fields are functions of u1 and u2 only and have components in both these directions. The medium is inhomogeneous but isotropic, with formal propagation speed (with respect to the u3 coordinate) a function of only u3. In this case the u3 surfaces can only be planes or spheres. The case of constant φ surfaces for u3 gave a class of TEM waves propagating in the φ
TL;DR: In this paper, three kinds of electric-circuit models for calculating the effective permittivity of a dielectric mixture and a general formula were presented, and the average value of three correction factors based on three different electric circuit models happens to be the value of the correction factor of Maxwell Garnett's formula.
Abstract: This paper presents three kinds of electric-circuit models for calculating the effective permittivity of a dielectric mixture and a general formula. The average value of three correction factors based on three kinds of electric-circuit models happens to be the value of the correction factor of Maxwell Garnett's formula. These new models can be spontaneously extended to a dielectric mixture with multiple-sized spherical inclusions. Model permittivities are compared with Doyle-Jacobs's model permittivity and iterative model permittivity. The results show that the two other models are the limits of the electric-circuit model for the dielectric mixture with multiple-sized spherical inclusions.
TL;DR: In this article, the effect of the absolute refractory period variation at the postmembrane of a neuromuscular junction is theoretically analyzed for the input stimulus with exponentially and normally distributed interpulse intervals (IPIs).
Abstract: The variation in refractory period of muscle fibers has not been studied systematically in electromyography (EMG) forward modeling, although changes in cell excitability are observed experimentally in some myopathies. The effect of the absolute refractory period variation at the postmembrane of a neuromuscular junction is theoretically analyzed in this work for the input stimulus with exponentially and normally distributed interpulse intervals (IPIs). Mathematical expressions on the probability density function (p.d.f.) of the output IPIs are carried out for both cases. The comparison is made between the theoretical and simulated results of output IPI probabilities and mean firing rate for different values of absolute refractory period. It has been demonstrated that the theoretical results are consistent with simulated ones. The output mean firing rates will be reduced when increasing the value of absolute refractory period at the postmembrane for exponential cases, while for Gaussian cases, it will keep ...
TL;DR: Body surface Laplacian maps of the anterior MI patients showed a highly localized early negativity overlying the site of infarct, which was determined by echocardiography.
Abstract: The recent development of body surface Laplacian mapping has shown increased capability for resolving multiple epimyocardial electrical events. For the myocardial infarction (MI) patient, accurate localization of the site and extent of infarct remains a critical issue, and the body surface Laplacian map (BSLM) may serve as a useful tool for this purpose. One hundred and fifty lead electrocardiogram (ECG) recordings were taken over the anterolateral chest of five anterior MI patients. After 20-beat averaging of the potential ECGs, the Laplacian ECGs were calculated using a finite difference estimation algorithm. BSLMs of the anterior MI patients showed a highly localized early negativity overlying the site of infarct, which was determined by echocardiography. Compared to the corresponding body surface potential maps, the BSLMs showed a more localized area of activity corresponding to the anterior infarct site, as judged by echocardiographic interpretation.
TL;DR: In this article, the propagation characteristics of sinusoidally impedance-level modulated microstrip structures are investigated by solving a standard second-order differential equation using the WKB method (named after Wentzel, Kramers, and Brillouin).
Abstract: The propagation characteristics of sinusoidally impedance-level modulated microstrip structures are investigated by solving a standard second-order differential equation using the WKB method (named after Wentzel, Kramers, and Brillouin). The WKB method, when modified by the successive approximation technique, is found to remove all the drawbacks of the method (zero order) and is able to predict accurately the behaviour of the dispersion characteristics of the propagation phenomenon. In addition, the dependence of the wave equation solution-both magnitude and phase, on frequency is also explored. Furthermore, convergence tests and experimentation establish the validation of the proposed method. Investigation is carried out to explore the dependency of the propagation properties on the modulation parameters of the structure.
TL;DR: In this paper, a model for electromagnetic wave backscattering from a bare soil field is developed based on the integral equation model, which is used to interpret the measured data and to generate simulated data for developing retrieval algorithms when the data is scarce or does not exist at all.
Abstract: In the remote sensing area theoretical models of microwave scattering properties from a bare soil field are needed to interpret the measured data and to generate simulated data for developing retrieval algorithms when the measured data is scarce or does not exist at all. A model for electromagnetic wave backscattering from a bare soil field is developed in this paper. The model developed is based on the integral equation model. The ground-based measurement system measures brightness temperatures and backscattering coefficients at frequencies between 1 and 2 GHz. For evaluating the model availability, comparisons of model prediction with the measured data are shown. The unknown quantity in the model is the surface correlation length. The surface correlation length is chosen to fit the measured data. The parameters of soil moisture content, surface root mean square height, and surface correlation length will affect the scattering strength from bare soil. Further, the soil composition of sand, loam, and clay will also influence the scattering coefficient from the bare soil surface. It is seen that satisfactory agreement is obtained between the model prediction and measured data. The difference between the model prediction and measured data is less than a dB.
TL;DR: In this paper, the frequency dependence of the reflection coefficient is studied for the case when a plane wave is incident upon an inhomogeneous slab representing the plasma wavetrain of a special type, which consists of the singleextremum regular and multiextremum multiscale irregular parts.
Abstract: The frequency dependence of the reflection coefficient is studied for the case when a plane wave is incident upon an inhomogeneous slab representing the plasma wavetrain of a special type, which consists of the single-extremum regular and multiextremum multiscale irregular parts. A band-limited fractal function is invoked to model the irregular part of the permittivity profile that describes the multiscale inhomogeneities. A first-order nonlinear differential equation of the Riccati type is used for calculations. The obtained results demonstrate the effect of the parameters of the fractal function on the frequency-dependent reflection coefficient.