Charles T. M. Choi

Optimal Design of CPW Slot Antennas Using Taguchi's Method

In this study, a one-element coplanar-waveguide (CPW) slot antenna and a two-element series aperiodic CPW slot antenna array are optimized by Taguchis method, in conjunction with a full-wave simulator to analyze the antennas, to achieve the desired goals. As a comparison, particle swarm optimization (PSO) is also used to design the two antennas. Optimization results show that the desired frequency responses of the antenna are successfully achieved by the two approaches. The optimization results from the Taguchis method significantly outperformed the PSO method in these two slot-antenna configurations.

Optimization of cochlear implant electrode array using genetic algorithms and computational neuroscience models

Finite-element (FE) analysis is used to compute the current distribution of the human cochlea during cochlear implant electrical stimulation. Genetic algorithms are then applied in conjunction with computational neuroscience models and FE analysis to optimize the shape and dimensions of cochlear implant electrode array. The goal is to improve the focus of electrical energy delivered to the auditory nerves in the human cochlea, thus reducing energy wasted and improve the efficiency and effectiveness of the cochlear implant system.

Conditions for Generating Virtual Channels in Cochlear Prosthesis Systems

Simultaneous electrical stimulation of neighboring electrodes in cochlear prosthesis systems generates channel interaction. However, intermediate channels, or virtual channels between the neighboring electrodes can be created through controlled channel interaction. This effect may be exploited for sending new information to the hearing nerves by stimulating in a suitable manner. The actual stimulation sites are therefore not limited to the number of electrodes. Clinical experiments, however, show that virtual channels are not always perceived. In this paper, electrical simulation with finite element analysis on a half turn human cochlea model is adopted to model the virtual channel effect, and the conditions for generating virtual channels are discussed. Five input current ratios (100/0, 70/30, 50/50, 30/70, 0/100) are applied to generate virtual channels. Three electrode arrays parameters are taken into consideration: distance between electrode contact and modiolus, spacing between adjacent electrode contacts and scale of electrode contact size. By observing the activating function contours, the virtual channel patterns and performances can be measured and examined. The results showed that a broad excitation pattern is necessary to produce the kind of electrode interaction that can form distinct virtual channels.

A new subgridding scheme for two-dimensional FDTD and FDTD (2,4) methods

While the available finite-difference time-domain (FDTD) subgridding schemes can improve the solution accuracy over those of the traditional FDTD, it is known to have instability problems. A new subgridding scheme combining FDTD or FDTD(2,4) method with FD-Laplacian interpolation is proposed. By applying subgridding scheme at fine components in a structure, the necessary field resolution can be obtained. In this scheme, FD-Laplacian interpolation is applied in the coarse main grids only or both the coarse main grids and the subgrids, and the error introduced and the floating point operations required can be reduced significantly. The accuracy of the scheme is tested by computing the resonant frequencies of two cavities and solving a scattering problem. The results are compared with solutions for uniformly mesh FDTD method and other FDTD subgridding schemes. The proposed method is stable after 100 000 time steps.

Numerical performance and applications of the envelope ADI-FDTD method

The numerical performance of the envelope alternating-direction-implicit-finite-difference time-domain (ADI-FDTD) method and its applications are studied in this paper. The ADI-FDTD method is independent of the Courant-Friedrich-Levy stability condition, but its numerical dispersion grows with the increase of the time-step size. By introducing the envelope technique in the ADI-FDTD method, the numerical accuracy can be improved efficiently. In this paper, the phase velocity error of a propagating Gaussian pulse was studied for the envelope ADI-FDTD and ADI-FDTD and conventional FDTD methods with different cell size and time-step increment, then two waveguide problems and a scattering problem were simulated with the envelope ADI-FDTD and ADI-FDTD methods in graded meshes and the conventional FDTD method in a uniform mesh. The simulation results show the superior performance of the envelope ADI-FDTD over the ADI-FDTD in numerical accuracy

Nonlinear Electrical Impedance Tomography Reconstruction Using Artificial Neural Networks and Particle Swarm Optimization

Electrical impedance tomography (EIT) is an imaging technology that offers the advantages of being noninvasive, and it does not generate ionizing radiation. The main difficulty in applying EIT is to solve an ill-posed nonlinear inverse problem. Given a set of electrical voltages measured at the surface of a volume conductor, the goal is to identify the materials that are present in the domain by determining their electrical conductivities. However, since EIT is a nonlinear problem, various algorithms proposed in the literature can only approximate real conductivity distributions. Nonlinear algorithms, especially artificial neural networks (ANNs), have been proposed to solve this inverse problem, but these algorithms are usually limited by slow convergence issues during the training phase. In this paper, the particle swarm optimization (PSO) method is used to train an ANN to solve the EIT problem. It has been found that, compared with the back-propagation algorithm, PSO is capable of generating both faster and higher convergence. This paper also shows that the proposed method is capable of dealing with noisy data and the imperfections in the finite-element discretization, an important source of errors in EIT imaging.

Performance of the improved PML for the envelope ADI-FDTD method in two-dimensional domain

An improvement of the split-field perfectly-matched layer (PML) medium for the envelope alternating-direction implicit (ADI) finite-difference time-domain (FDTD) method is proposed. Comparing with the traditional PML, the performance of the proposed PML for the envelope ADI-FDTD method maintains good absorption even for large temporal increment.

Modeling ECAP in Cochlear Implants Using the FEM and Equivalent Circuits

Evoked compound action potential (ECAP) has been used clinically to determine whether auditory nerves are responding to electrical stimulation in cochlear implant systems. In this paper, a novel scheme is proposed to model ECAP and its measurement using a generalized Schwarz-Eikhof-Frijns nerve model and equivalent circuits based on the finite-element (FE) method. A 3-D FE model of a cochlear implant is used in this scheme. The ECAP modeling result is validated with clinical ECAP measurement results.

Simulation of Axon Activation by Electrical Stimulation— Applying Alternating-Direction-Implicit Finite- Difference Time-Domain Method

In a typical approach to model electrical stimulation of an axon, a cable model equivalent to an axon was placed in a simple homogeneous medium. An electrode was used to induce an excitation to stimulate the cable model, and then the transmembrane potentials and the ionic currents in the cable model in temporal domain were observed. Unfortunately, this simulation approach is not realistic since inhomogeneous tissues near the axon is not considered. In this paper, the alternating-direction-implicit finite-difference time-domain (ADI-FDTD) method is coupled with the equivalent model of a membrane (the Hodgkin-Huxley model), and a novel simulation scheme is developed to predict axon activation. By testing axon activation with current excitation, the simulation results show the new method is useful for simulating axon activation.

Envelope ADI–FDTD Method and Its Application in Three-Dimensional Nonuniform Meshes

The envelope alternating-direction-implicit finite difference time domain (ADI-FDTD) method in 3-D nonuniform meshes was proposed and studied. The phase velocity error for the envelope ADI-FDTD and ADI-FDTD methods in uniform and nonuniform meshes and different temporal increments were studied. A cavity problem was studied using the envelope ADI-FDTD and ADI-FDTD methods in graded meshes and the conventional FDTD method in a uniform mesh. The simulation results show that the envelope ADI-FDTD performs better than the ADI-FDTD in numerical accuracy