A finite element model for radiofrequency ablation of themyocardium

A finite element model was developed to simulate the temperature distributions produced by radiofrequency catheter ablation. This model incorporated blood, myocardium and torso tissues. The Laplace equation was solved to determine the steady-state electric field. The heat generation in the tissues was then computed from the power density distribution and the bioheat equation was solved to determine the time-varying temperature distribution, taking into account the convective energy exchange at the blood-myocardium and torso-air interfaces. This model was used to predict the lesion depth and to evaluate the effects of electrode location, changes of the electrical and thermal conductivities, and the electrode radius on the thermally induced damage to the myocardium. Temperature distributions induced by radiofrequency ablation were found to be: i) not very sensitive to the reference electrode location, ii) more sensitive to electrical conductivity changes than to thermal conductivity changes, and iii) larger electrodes allow a current distribution at higher level of power with reducing the chance of impedance rise     

Time-harmonic impedance tomography using the T-matrix method

A time-harmonic formulation for the electrical impedance tomography (EIT) inverse problem accounting for electrodynamic effects is derived. This work abandons the standard electrostatic impedance model for a full-wave T-matrix model. The advantage of this method is an accurate physical model that includes finite frequency effects, such as diffusion phenomena, and electrode contact impedance effects. This model offers the potential for increased resolution and larger invertible contrast objects than other methods when used on experimental data, because it may represent a more realistic physical model. Also, an accurate gradient matrix is used in the Newton iterative method so the image reconstruction converges in a few iterations. These advantages are realized with no increase in the computational complexity of this algorithm, compared to the static finite element model. A calibration technique is suggested for measurement systems, to test the validity of a theoretical model that includes electrode contact impedance effects.     

Three-dimensional electrical impedance tomography: a topology optimization approach.

Electrical impedance tomography is a technique to estimate the impedance distribution within a domain, based on measurements on its boundary. In other words, given the mathematical model of the domain, its geometry and boundary conditions, a nonlinear inverse problem of estimating the electric impedance distribution can be solved. Several impedance estimation algorithms have been proposed to solve this problem. In this paper, we present a three-dimensional algorithm, based on the topology optimization method, as an alternative. A sequence of linear programming problems, allowing for constraints, is solved utilizing this method. In each iteration, the finite element method provides the electric potential field within the model of the domain. An electrode model is also proposed (thus, increasing the accuracy of the finite element results). The algorithm is tested using numerically simulated data and also experimental data, and absolute resistivity values are obtained. These results, corresponding to phantoms with two different conductive materials, exhibit relatively well-defined boundaries between them, and show that this is a practical and potentially useful technique to be applied to monitor lung aeration, including the possibility of imaging a pneumothorax.     

交叉指型电极压电振子的谐振特性研究

根据压电振子的动力学方程,采用有限元软件ANSYS对交叉指型电极压电振子进行了谐响应分析,得到了压电振子的横向振动响应,获得了分支电极参数对交叉指型电极压电振子谐振频率的影响规律.利用阻抗分析仪对交叉指型电极压电振子的谐振频率进行了实验研究,并与仿真结果进行了对比,两者基本吻合,验证了数值模型的可靠性.此外,研究结果还表明,采用较小的电极周期,可以明显降低振子的谐振频率,从而在相同的激励下,获得较大的作动应变.     

Modeling of an Active Nerve Fiber in a Finite Volume Conductor and Its Application to the Calculation of Surface Action Potentials

Potentials within and on the surface of a finite cylindrical volume conductor due to a single active nerve fiber along its center have been calculated by solving Laplace's equation using a relaxation model. The results have enabled the variation of the potential that would be recorded from a surface electrode to be estimated for differing nerve depths and conduction velocities.     

Simple Low-Cost Hysteretic Controller for Single-Phase Synchronous Buck Converters

This paper presents a simple low-cost control architecture for low-voltage hysteretic regulators supplying loads with low-to-medium current consumption. Only two sensed voltages, a passive filter network and a hysteretic comparator are required to implement the main control functions: the output voltage regulation and the adaptive voltage positioning. This paper also shows that other previously reported low-cost control solutions have an important design tradeoff between load transient response and efficiency. In the proposed controller, the closed-loop output impedance can be designed to be resistive and the switching frequency can be adjusted to be independent of the output impedance requirement so that the load transient response and the efficiency can be optimized separately. Experimental results validate the performance of the proposed controller (i.e., a load transient response with insignificant output voltage overshoot and selectable switching frequency independent of the output impedance requirement).     

Extracellular Potential Recordings by Means of a Field Effect Transistor Without Gate Metal, Called OSFET

The design and operation of a new semiconductor element, based upon the field effect principle is discussed. The device, which is called an OSFET, can be used as a measuring electrode for bioelectric activity. The OSFET consists of an MOS-transistor configuration, where the gate metal has been omitted and can be used directly in the extracellular fluid as an active probe. The operation of the OSFET is described by a physical-mathematical model, by means of which it is possible to explain the results of electrophysiological measurements. These results are compared with the results of a computer simulation, in which the Rosenfalck equation for extracellular potential distributions serves as an input for the OSFET model stated. A special electronic circuit is used to perform low impedance measurements in a simple way with the OSFET electrode. It appears that extracellular potentials measured with an OSFET-electrode have an extended frequency range, with respect to conventional potential measurements (0 ?< f ?< 45 K c/s).     

Finite element modeling of electrode-skin contact impedance inelectrical impedance tomography

In electrical impedance tomography (EIT), the measured voltages are sensitive to electrode-skin contact impedance because the contact impedance and the current density through it are both high. Large electrodes were used to provide a more uniform current distribution and reduce the contact impedance. A large electrode differs from a point electrode in that it has shunting and edge effects that cannot be modeled by a single resistor. The finite-element method (FEM) was used to study the electric field distributions underneath an electrode, and three models were developed: a FEM model, a simplified FEM model, and a weighted load model. The FEM models considered both shunting and edge effects and closely matched the experimental measurements. It is concluded that FEM models of electrodes can be used to improve the performance of an electrical impedance tomography reconstruction algorithm     

Variability analysis of impedance matching network

This paper investigates the L network impedance matching analytically. The equation describing the effect of load impedance variation on the impedance matching performance is derived. The deviation from perfect match is proportional to the variation in load reflection coefficient. However, if the load impedance is too large or too small, resulting in a large reflection coefficient, the solution to the impedance matching problem will have poorer quality in term of variability. The variability factor increases rapidly when the load reflection coefficient is larger than 0.7. A small variation in the load impedance will cause a large deviation from perfect match.     

A 3-D SAR model for current source interstitial hyperthermia

A three-dimensional (3-D) model is presented for the calculation of the specific absorption rate (SAR) in human tissue during current source interstitial hyperthermia. The model is capable of millimeter resolution and can cope with irregular implants in heterogeneous tissue. The SAR distribution is calculated from the electrical potential. The potential distribution is determined by the dielectric properties of the tissue and by the electrode configuration. The dielectric properties and the current injection of the electrodes are represented on a 3-D uniform grid. The calculated potential at an electrode current injection point is not the actual electrode potential at that point. To estimate this potential a grid independent representation of an electrode together with an analytical solution in the neighborhood of the electrode are used. The calculated potential on the electrode surface is used to estimate the electrode impedance. The tissue implementation is validated by comparing calculated distributions with analytical solutions. The electrode implementation is verified by comparing different discretizations of an electrode configuration and by comparing numerically calculated electrode impedances with analytically calculated impedances     

Three-dimensional electrical impedance tomography based on thecomplete electrode model

In electrical impedance tomography an approximation for the internal resistivity distribution is computed based on the knowledge of the injected currents and measured voltages on the surface of the body. It is often assumed that the injected currents are confined to the two-dimensional (2-D) electrode plane and the reconstruction is based on 2-D assumptions. However, the currents spread out in three dimensions and, therefore, off-plane structures have significant effect on the reconstructed images. In this paper we propose a finite element-based method for the reconstruction of three-dimensional resistivity distributions. The proposed method is based on the so-called complete electrode model that takes into account the presence of the electrodes and the contact impedances. Both the forward and the inverse problems are discussed and results from static and dynamic (difference) reconstructions with real measurement data are given. It is shown that in phantom experiments with accurate finite element computations it is possible to obtain static images that are comparable with difference images that are reconstructed from the same object with the empty (saline filled) tank as a reference.     

Body surface Laplacian ECG mapping

A new noninvasive approach has been developed to resolve spatially distributed cardiac electrical activity by measuring the surface Laplacian of the body surface potential. Computer simulations demonstrate the ability of the Laplacian map compared with the potential map to image spatially distributed dipole sources embedded in a semi-infinite volume conductor. Body surface Laplacian mapping has been implemented in human subjects utilizing dry bipolar Laplacian electrodes and compared with potential maps obtained using the central terminal of each bipolar Laplacian electrode. The body surface Laplacian ECG distribution was found to provide better spatial resolution than the body surface potential distribution. The body surface Laplacian map appears to resolve depolarization and repolarization of different regions of the heart. Further improvements of the body surface Laplacian mapping may permit noninvasive mapping of spatially distributed intracardiac events.     

Origins of the impedance change in impedance cardiography by athree-dimensional finite element model

A three-dimensional finite-element model of the thorax and neck using eight-node trilinear hexahedron elements was constructed to investigate the impedance change associated with various physiological events during systole. A three-dimensional finite-element code was developed to solve the generalized Laplace equation with Dirichlet and homogeneous Neumann boundary conditions. The current in each element as well as potential at each node was calculated. The results suggest that an approximately linear relationship exists between the impedance change and blood volume change in the aorta. This is a promising result since the relationship helps explain the correlation between impedance cardiography and invasive techniques. Impedance changes due to blood volume changes in the aorta and ventricles, the lung-resistivity change, and the blood-resistivity change were calculated for standard impedance electrode configurations     

The charge-density method of solving electrostatic field: Program and error analysis

The Charge-Density method of solving electrostatic field has many advantages over the finite difference or finite element method, but its accuracy and fitness are still in question. By direct evaluating electrical potentials, the errors along surface of the electrode are plotted, which are the maximum errors for an electrostatic problem. It is shown that higher accuracy will be reached if more sub-regions are chosen at where the charge density is high or at the region near the area where the field is to be calculated. This will be helpful to understand and use the method efficiently. It is more convenient and accurate to use the chargedensity method when we deal with the calculation of an electrostatic system having a great difference in size between electrodes (such as the point emitter diode). Using the information of error on the surface of electrode, we can estimate the potential errors in each calculation or rearrange the sub-regions to improve the accuracy of the next calculation. A program is set up. The difference between the calculated data and that of E. Harting and F. H. Read (1976) is less than 1%.     

Enhanced skin effect for partial-element equivalent-circuit (PEEC)models

In this paper, a skin-effect modeling approach is presented that is suitable for all frequency regimes of interest and therefore is most appropriate for transient interconnect analysis. Yet, the new formulation lends itself to a model that can be abstracted for use in conjunction with surface integral and finite difference-based electromagnetic tools for interconnect modeling. While a volume filament technique is not computationally feasible at high frequencies, where a fine discretization is necessary, the formulation that is presented avoids this difficulty by carefully casting the behavior of a conductor into the form of a global surface impedance, thus requiring fewer unknowns. Several examples illustrating the ability of the proposed model to accurately capture proximity and skin-effect behaviors will be shown, Interconnect resistance and inductance per-unit-length results are given and compared with those obtained using different models     

Real-time discrimination of ventricular tachyarrhythmia withFourier-transform neural network

The authors have developed a method to discriminate life-threatening ventricular arrhythmias by observing the QRS complex of the electrocardiogram (ECG) in each heartbeat. Changes in QRS complexes due to rhythm origination and conduction path were observed with the Fourier transform, and three kinds of rhythms were discriminated by a neural network. In this paper, the potential of the authors' method for clinical uses and real-time detection was examined using human surface ECGs and intracardiac electrograms (EGMs). The method achieved high sensitivity and specificity (>0.98) in discrimination of supraventricular rhythms from ventricular ones. The authors also present a hardware implementation of the algorithm on a commercial single-chip CPU     

Threshold voltage model for deep-submicron fully depleted SOI MOSFETs with back gate substrate induced surface potential effects

A simple analytical threshold voltage model for short-channel fully depleted SOI MOSFETs has been derived. The model is based on the analytical solution of the two-dimensional potential distribution in the silicon film (front silicon), which is taken as the sum of the long-channel solution to the Poisson's equation and the short-channel solution to the Laplace equation, and the solution of the Poisson's equation in the silicon substrate (back silicon). The proposed model accounts for the effects of the back gate substrate induced surface potential at the buried oxide-substrate interface which contributed an additional 15–30% reduction in the threshold voltage for the devices used in this work. Conditions on the back gate supply voltage range are determined upon which the surface potential at the buried oxide-substrate interface is accumulated, depleted, or inverted. The short-channel associated drain induced barrier lowering effects are also included in the model. The model predications are in close agreement with PISCES simulation results. The equivalence between the present model and previously reported models is proven. The proposed model is suitable for use in circuit simulation tools such as Spice.     

Measurement of Ventricular Volume by Intracardiac Impedance: Theoretical and Empinrcal Approaches

The most widely used equation, V = pL2/R, is developed for the computation of ventricular volume from catheter based impedance measurements. The assumptions implicit in this derivation are examined and found to be generally invalid. An empirical discrete resistor model is described which includes the impedance of the myocardial tissue and the adjoining ventricular blood volume. Once the parameters of this model are determined for individual canine hearts, the model predicts stroke volume from measured impedances. Due to the difficulty involved in determining the parameters of the empirical model, a numerical model is developed which solves the equation V ?a V U + F = 0 in a three-dimensional volume. This model is then used to determine the effect of parallel tissue resistance, catheter position, and contraction of the other ventricle on volumes computed by intracardiac impedance. Parallel tissue resistance is found to have the greatest impact on absolute volume measurements. However, stroke volume computations are relatively unaffected by any of the three factors.     

A Direct Synthesis Approach for Microwave Filters With a Complex Load and Its Application to Direct Diplexer Design

This paper presents a direct synthesis approach for general Chebyshev filters terminated with a complex load. The new approach is based on the fact that the polynomial functions for synthesizing the filters are composed for any matched loads. By normalizing the polynomial functions with assumed complex matched load impedance by a real reference load impedance using power waves normalization, a set of new polynomial functions for the same filter, but with real load impedance, can be formulated, from which the coupling matrix for the physical filter design can be obtained using a standard direct filter synthesis approach. This new direct synthesis approach can find many applications. A practical application is the direct diplexer design with a realistic junction model being taken into account. With the diplexer design is concerned, a fast-converged iterative scheme is proposed. The effectiveness and the validation of the proposed scheme are demonstrated by two design examples