This study analyzed the influence of electrode geometry, tissue-electrode angle, and blood flow on current density and temperature distribution, lesion size, and power requirements during radio-frequency ablation. We used validated three-dimensional finite element models to perform these analyses. We found that the use of an electrically insulating layer over the junction between electrode and catheter body reduced the chances of charring and coagulation. The use of a thermistor at the tip of the ablation electrodes did not affect the current density decreased more slowly with distance from the electrode surface. We analyzed the effects of three tissue-electrode angles: 0, 45, and 90 degrees. More power was needed to reach a maximal tissue temperature of 95 degrees C after 120 s when the electrode-tissue angle was 45 degrees. Consequently, the lesions were larger and deeper for a tissue-electrode angle of 45 degrees than for 0 and 90 degrees. The lesion depth, volume, and required power increased with blood flow rate regardless of the tissue-electrode angle. The significant changes in power with the tissue-electrode angle suggest that it is safer and more efficient to ablate using temperature-controlled RF generators. The maximal temperature was reached at locations within the tissue, a fraction of a millimeter away from the electrode surface. These locations did not always coincide with the local current density maxima. The locations of these hottest spots and the difference between their temperature and the temperature read by a sensor placed at the electrode tip changed with blood flow rate and tissue-electrode angle.

In this animal model, impedance is a bulk electrical property of tissue that varies with the evolution of myocardial infarction. Impedance mapping revealed significantly different values for normal, ischemic, and infarcted tissue and may prove useful in better defining the electrophysiological characteristics of such tissue.

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We apply boundary integrals to the analysis of diffraction from both conductive and dielectric diffractive optical elements. Boundary integral analysis uses the integral form of the wave equation to describe the induced surface distributions over the boundary of a diffractive element. The surface distributions are used to determine the diffracted fields anywhere in space. In contrast to other vector analysis techniques, boundary integral methods are not restricted to the analysis of infinitely periodic structures but extend to finite aperiodic structures as well. We apply the boundary element method to solve the boundary integral equations and validate its implementation by comparing with analytical solutions our results for the diffractive analysis of a circular conducting cylinder and a dielectric cylinder. We also present the diffractive analysis of a conducting plate, a conducting linear grating, an eight-level off-axis conducting lens, an eight-level on-axis dielectric lens, and a binary dielectric lens that has subwavelength features.

In this paper, we present a numerical model for evaluating tissue heating during magnetic resonance imaging (MRI). Our method, which included a detailed anatomical model of a human head, calculated both the electromagnetic power deposition and the associated temperature elevations during an MRI head examination. Numerical studies were conducted using a realistic birdcage coil excited at frequencies ranging from 63 to 500 MHz. The model was validated both experimentally and analytically. The experimental validation was performed at the MR test facility located at the Food and Drug Administration's Center for Devices and Radiological Health.

We present an optimization-based synthesis algorithm for the design of diffractive optical elements (DOE's) that are finite in extent, have subwavelength features, and are aperiodic. The subwavelength nature of the DOE's precludes the use of scalar diffraction theory, and their finite extent and aperiodic nature prevents the use of coupled-wave analysis. To overcome these limitations, we apply the boundary element method (BEM) as the propagation model in the synthesis algorithm. However, the computational costs associated with the conventional implementation of the BEM prevent the design of realistic DOE's in reasonable time frames. Consequently, an alternative formulation of the BEM that exploits DOE symmetry is developed and implemented on a parallel computer. Designs of finite extent, subwavelength, and aperiodic DOE's, such as a lens and a focusing beam splitter, are presented.

A new method for synthesizing broadband antireflective (AR) surfaces at microwave and millimeter wave frequencies is demonstrated. The AR surface, we call an inverse motheye, was formed by machining a multi-layer grating of subwavelength circular holes into a non-absorptive dielectric. This created low reflected energies ( 30 dB) over reasonably large bandwidths and incidence angles. An optimization algorithm, based on a direct pattern search, integrated with a rigorous electromagnetic model was used to design the grating geometries. Experimental results are provided at Ka-band demonstrating the validity of the method.

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