Thesis (Ph. D.)--Ohio State University, 1986. / Includes bibliographical references (leaves 297-301). Available online via OhioLINK's ETD Center

Thesis (Ph. D.)--University of Washington, 2000. / Vita. Includes bibliographical references (leaves 134-141).

La capacité de pénétration des ondes électromagnétiques (OEM), généralement non destructive, dans les matériaux ou tissus permet de sonder les milieux étudiés. En termes de santé publique, améliorer la qualité de vie est devenu un objectif majeur de la société actuelle. Des applications de sondage par OEM de divers milieux ou tissus dans le domaine biologique présentent un intérêt majeur dans les opérations de diagnostic et dans les opérations thérapeutiques. Les travaux de cette thèse abordent le vaste domaine des applications biomédicales, des interactions micro-ondes/radiofréquences et de la matière. Ces travaux de thèse se focalisent sur l’univers émergeant de l’Imagerie par Résonnance Magnétique (IRM) à Ultra-Haut Champ. Le premier objectif est de développer un fantôme anthropomorphique de tête humaine ou il faut prendre en compte les propriétés électromagnétiques et les temps de relaxation caractéristiques de chaque tissu. Cela permet d’obtenir une estimation précise des niveaux de DAS pour l’être humain et une notion des formations de hotspots lors d’un examen IRM haut-champ.Le deuxième objectif dans le domaine des IRM à très haut champ est l’étude et la fabrication des dispositifs d’homogénéisation de champ permettant ainsi d’éclaircir les zones d’ombres. Ce processus, dénommé Dielectric Shimming, est basé entre autre sur l’utilisation d’éléments discrets à hautes permittivités appelés pads. Ces pads sont composés de matériaux diélectriques à fortes permittivités, comme des solutions aqueuses de Titanate de Baryum, afin de focaliser le champ dans les zones initialement sombres de l’image de l’IRM (dans cas du cerveau : le cervelet et les lobes temporaux). / The penetration capacity of the electromagnetic (EM) waves in matter or biological tissues allows exploring media non-destructively. Concerning the public health sector, improving the quality of life has become one of the greatest concerns of nowadays society. EM wave research on different media and biological tissues shows a great potential for diagnostic applications and eventually for therapeutically applications. In this doctoral thesis, we focus on the vast domain of the biomedical applications of wave-matter interactions, based on the knowledge of the electromagnetic properties of matter, the complex permittivity and the conductivity. On a first instance, we address the emerging domain of ultra-high field MRI (Magnetic Resonance Imaging), which nowadays puts effort into the clinical implementation of 7T devices. Firstly our purpose is to produce an anthropomorphic head model, composed of the brain’s different layers, and taking into account the electromagnetic properties and the proton relaxation times inherent to each tissue. These realistic head models allow to evaluate the newly developed protocols for these ultra-high field devices. Secondly, we have studied and developed field homogenization devices, which allow brightening the shadow areas displayed in some MRI images, such as the cerebellum and the temporal lobes in brain imaging at 7T. This procedure, named Passive Shimming, is based on the use of high permittivity dielectric pads composed of Barium Titanate, which focalize the field to the areas where normally the wavelength in insufficient to generate a homogeneous signal distribution.

This work is a study of the scattering of electromagnetic waves from random media of discrete scatterers. The object is primarily the investigation of existing general discrete-scatterer theories and the development of more accurate ones, the technique of Monte Carlo computer simulation being employed to provide "exact" experimental results for comparison with theoretical data. A one-dimensional model of randomly-positioned planar scatterers is used as a tool in the investigation and as a means of providing insight into the physical and statistical characteristics of discrete-scatterer media. The limitations of the one-dimensional forms of Twersky's theories for the coherent field are illustrated by a presentation of results for a wide range of scattering parameters, and requirements necessary for the approximate validity of these theories are given. Accurate series expressions for several average field functions of interest in the problem of plane-wave scattering from distributions of uniformly-random planar scatterers are presented and verified from simulation results. The asymptotic scattering behavior for a low average density of scatterers is emphasized; a modification to the one-dimensional form of Twersky's free-space theory for the coherent transmitted field to give exact asymptotic behavior is shown to be a considerable improvement for higher average densities also. The relation of the one-dimensional model theory and results to more complex three-dimensional models is discussed where possible. Simulation methods for the generation of a non-uniform distribution of planar-scatterer configurations weighted towards "periodicity" are presented. Based on the scattering results obtained, criteria for the assumption of a uniform distribution are given. The physical conditions necessary for the approximate validity of the bivariate Gaussian distribution in describing the total field statistics of the one-dimensional model are discussed and quantitative results based on the third and fourth field moments given. Also presented is a new physical model of a random medium of discrete spherical scatterers for use in controlled laboratory experiments at millimeter-wave frequencies. The main feature of this model is that the scatterer statistics are directly controlled by an application of the Monte Carlo method. The results of an experimental investigation into the suitability of the model are given. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

Most solutions for electromagnetic wave diffraction by obstacles and apertures assume plane wave incidence or omnidirectional local sources. Solutions to diffraction problems for local directive sources are needed. The complex source point representation of directive beams together with uniform solutions to high frequency diffraction problems is a powerful combination for this. Here the method is applied to beam diffraction by planar structures with edges, such as the half-plane, slit, strip, wedge and circular aperture. Previously used restrictions to very narrow beams and paraxial regions, are removed here and the range of validity increased. Also it is shown that the complex source point method can give a better approximation to broad antenna beams than the Gaussian function. The solution derived for the half-plane problem is uniform, accurate and valid for all beam orientations. This solution can be used as a reference solution for other uniform or asymptotic solutions and is used to solve for the wide slit and complementary strip problems. Uniform solutions for omidirectional sources are developed and extended analytically to become solutions for directive beams. The uniform theory of diffraction is used to obtain uniform solutions where there are no simple exact solutions, such as for the wedge and circular aperture. Otherwise rigorously correct solutions at high frequencies for singly diffracted far fields are used, such as for the half-plane, slit and strip. The geometrical theory of diffraction and equivalent line currents are used to include interaction between edges. Extensive numerical results including the limiting cases; e.g. plane wave incidence, line and point sources are given. These solutions are compared with previous solutions, wherever possible and good agreement is evident Beam diffraction by a wedge with its edge on the beam axis is analysed. This solution completes a previous asymptotic solution which is infinite on the shadow boundaries and inaccurate in the transition regions. Finally, the diffraction by a circular aperture illuminated by normally incident acoustic beam, is derived and the singularity along the axial caustic is removed using Bessel functions and a closed form expression for multiple diffraction is derived. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

No description available.

No description available.

No description available.

No description available.

No description available.