Three-Dimensional Diffraction Tomography Using Filtered Backpropagation and Multiple Illumination Planes

In this paper, a three-dimensional (3-D) extension of the well-known filtered-backpropagation (FBP) algorithm is presented with the aim of taking into account scattered-field-data measurements obtained using incident directions not restricted in a single plane. The FBP algorithm has been extensively used to solve the two-dimensional inverse-scattering problem under the first-order Born and Rytov approximations for weak scatterers. The extension of this algorithm in three dimensions is not straightforward, because the task of collecting the data needed to obtain a low-pass filtered version of the scattering object, taking into account all spatial frequencies within a radius of radic2k₀, and of incorporating these data to the FBP algorithm, needs to be addressed. A simple extension using incident field directions restricted to a single plane (illumination plane) leaves a region of spatial frequencies of the sphere of radius radic2k ₀ undetermined. The locus of these spatial frequencies may be crucial for the accurate reconstruction of objects which do not vary slowly along the axis perpendicular to the illumination plane. The proposed 3-D FBP algorithm presented here is able to incorporate the data collected from more than one illumination plane and to ensure the reliability of the reconstruction results     

Analysis of the power coupling from a waveguide hyperthermiaapplicator into a three-layered tissue model

The power deposition from a rectangular-aperture flanged waveguide into a three-layered stratified tissue medium is analyzed theoretically. The fields inside the tissue layers are expressed in terms of Fourier integrals satisfying the corresponding wave equations, while the fields inside the waveguide are expanded in terms of the guided and evanescent normal modes. An integral equation is derived on the aperture plane of the flanged waveguide by applying the continuity of the tangential electric and magnetic fields. This integral equation is solved by expressing the unknown electric field in terms of the waveguide mode fields and by applying a Galerkin procedure. The electromagnetic fields inside the tissue medium are then determined and patterns of the deposited power at frequencies of 432 MHz and 144 MHz for apertures of 5.6×2.8 cm² and 16.5×8.3 cm², respectively, are computed and presented     

Estimation of temperature distribution inside tissues heated byinterstitial RF electrode hyperthermia systems

The computational method presented relies on a semi-infinite tissue model. The needle-shaped RF electrodes are modeled with elongated spheroids. The heat transfer problem is treated in three dimensions. The localized current fields set up inside the tissue from the discrete implants are computed by using electrostatic methods, and the bioheat diffusion equation under a steady-state condition is solved to determine the temperature distributions inside superficial tissues. A Green's-function technique is applied to solve the bioheat transfer equation. The heat removal due to blood circulation is also taken into account. Analytical techniques are used to treat the singularities in the vicinity of implanted electrodes. Numerical results are presented for several electrode configurations     

Dielectric properties of modified montmorillonites suspensions in polydimethylsiloxane

The dielectric characteristics of various modified montmorillonites suspensions in polydimethylsiloxane were investigated. Such materials are promising candidates as electrorheological fluids. The effect of small water content and temperature change in the dielectric spectra of suspensions was studied. The electrical conductivity of suspensions rises with filler concentration. Conductivity also increases with frequency for all samples by 5–6 orders of magnitude. The frequency dependence of permittivity changes significantly with temperature and strongly depends on the type of modifier. The positions of relaxation transitions peaks observed in dielectric loss curves substantially depend on the type of filler and water content. Dielectric loss peaks shift to higher frequencies with temperature. Relaxation mechanism is related to filler and described by capacitor model. Based on X‐ray data a model of relaxation transitions is proposed. Also, the activation energy of dielectric relaxation is estimated. The activation energy is independent of filler concentration but is determined by the structural features of fillers in polymer medium. The prospects of dielectric spectroscopy for analyzing the layered nanosilicates structure in polymer solution are demonstrated. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46614.     

Robustness of linear time‐varying systems with relaxed excitation

It is a well‐known fact that linear time‐varying systems with a persistently excited state matrix are exponentially converging and input‐to‐state stable with respect to additive perturbations. Recently, several relaxed conditions of persistent excitation have been presented, which ensure an asymptotic convergence rate in the system. In the present work, it is shown that these conditions are similar and that, under such a relaxed excitation, only nonuniform in time input‐to‐state stability and integral input‐to‐state stability properties can be obtained. The results are illustrated by simulations for a problem of estimation in the linear regression model.     

Analysis of focusing of pulsed baseband signals inside a layeredtissue medium

The derivation and application of a method designed to investigate the focusing properties of pulsed baseband signals of short pulsewidth (~1 ns) in biological tissue media are reported. To this end, sources fed from TEM waveguides, concentrically placed at the periphery of a three-layer cylindrical lossy model, are assumed. A Fourier-series-based methodology, appropriate for a useful class of pulse train incident signals, is presented and utilized to study the dynamics of pulse propagation inside the tissue medium. The propagation of each spectral component of the incident field within the tissue medium is analyzed by applying an integral-equation technique and a Fourier-series representation is used in order to obtain the time dependence of the electromagnetic fields produced at any point within tissue due to the pulsed excitation of the array elements. Numerical results are computed and presented at several points in a three-layer geometry, 20 cm in diameter, irradiated by an eight-element waveguide array. Focusing at a specific point of interest within tissue is achieved by properly adjusting the time delay of the signals injected to the individual applicators of the array     

Improving class separability using extended pixel planes: a comparative study

In this work we explored class separability in feature spaces built on extended representations of pixel planes (EPP) produced using scale pyramid, subband pyramid, and image transforms. The image transforms included Chebyshev, Fourier, wavelets, gradient, and Laplacian; we also utilized transform combinations, including Fourier, Chebyshev, and wavelets of the gradient transform, as well as Fourier of the Laplacian transform. We demonstrate that all three types of EPP promote class separation. We also explored the effect of EPP on suboptimal feature libraries, using only textural features in one case and only Haralick features in another. The effect of EPP was especially clear for these suboptimal libraries, where the transform-based representations were found to increase separability to a greater extent than scale or subband pyramids. EPP can be particularly useful in new applications where optimal features have not yet been developed.