Thermoacoustic emissions induced by 2450 MHz microwave pulses in water, tissue-simulating phantoms and dog kidneys have been detected. The analytic signal magnitude has been employed in generating 'A-mode' images with excellent depth resolution. Thermoacoustic emissions have also been detected from the dose-gradient at the beam edges of a 4 MeV x-ray beam in water. These results establish the feasibility of employing thermoacoustic signals in generating diagnostic images, and in locating x-ray beam edges during radiation therapy. A theoretical model for thermoacoustic imaging using a directional transducer has been developed, which may be used in the design of future thermoacoustic imaging system, and in facilitating comparisons with other types of imaging systems. A method of characterizing biological tissues has been proposed, which relates the power spectrum of the detected thermoacoustic signals to the autocorrelation function of the thermoacoustic source distribution in the tissues. The temperature dependence of acoustic signals induced by microwave pulses in water has been investigated. The signal amplitudes vary with temperature as the thermal expansion of water, except near 4°C. The signal waveforms show a gradual phase change as the temperature changes from below 4° to above 4°C. This anomaly is due to the presence of a nonthermal component detected near 4°C, whose waveform is similar to the derivative of the room temperature signal. The results are compared to a model based on a nonequilibrium relaxation mechanism proposed by Pierce and Hsieh. The relaxation time was found to be (0.20±0.02) ns and (0.13±0.02) ns for 200 ns and 400 ns microwave pulse widths, respectively. A microwave-induced thermoacoustic source capable of launching large aperture, unipolar ultrasonic plane wave pulses in water has been constructed. This source consists of a thin water layer trapped between two dielectric media. Due to the large mismatch in the dielectric constants, the incident microwaves undergo multiple reflections between the dielectric boundaries trapping the water, resulting in an enhanced specific microwave absorption in the thin water layer. This source may be useful in ultrasonic scattering and attenuation experiments.

This dissertation describes research conducted on the development and improvement of microwave tomography algorithms for imaging the bulk-electrical parameters of unknown objects. The full derivation of a new inversion algorithm based on the state-of-the-art contrast source inversion (CSI) algorithm coupled to a finite-element method (FEM) discretization of the Helmholtz differential operator formulation for the scattered electromagnetic field is presented. The algorithm is applied to two-dimensional (2D) scalar and vectorial configurations, as well as three-dimensional (3D) full-vectorial problems. The unknown electrical properties of the object are distributed on the elements of arbitrary meshes with varying densities. The use of FEM to represent the Helmholtz operator allows for the flexibility of having an inhomogeneous background medium, as well as the ability to accurately model any boundary shape or type: both conducting and absorbing. The CSI algorithm is used in conjunction with multiplicative regularization (MR), as it is typical in most implementations of CSI. Due to the use of arbitrary meshes in the present implementation, new techniques are introduced to perform the necessary spatial gradient and divergence operators of MR. The approach is different from other MR-CSI implementations where the unknown variables are located on a uniform grid of rectangular cells and represented using pulse basis functions; with rectangular cells finite-difference operators can be used, but this becomes unwieldy in FEM-CSI. Furthermore, an improvement for MR is proposed that accounts for the imbalance between the real and imaginary parts of the electrical properties of the unknown objects. The proposed method is not restricted to any particular formulation of the contrast source inversion. The functionality of the new inversion algorithm with the different enhancements is tested using a wide range of synthetic datasets, as well as experimental data collected by the University of Manitoba electromagnetic imaging group and research centers in Spain and France.

This dissertation describes research conducted on the development and improvement of microwave tomography algorithms for imaging the bulk-electrical parameters of unknown objects. The full derivation of a new inversion algorithm based on the state-of-the-art contrast source inversion (CSI) algorithm coupled to a finite-element method (FEM) discretization of the Helmholtz differential operator formulation for the scattered electromagnetic field is presented. The algorithm is applied to two-dimensional (2D) scalar and vectorial configurations, as well as three-dimensional (3D) full-vectorial problems. The unknown electrical properties of the object are distributed on the elements of arbitrary meshes with varying densities. The use of FEM to represent the Helmholtz operator allows for the flexibility of having an inhomogeneous background medium, as well as the ability to accurately model any boundary shape or type: both conducting and absorbing. The CSI algorithm is used in conjunction with multiplicative regularization (MR), as it is typical in most implementations of CSI. Due to the use of arbitrary meshes in the present implementation, new techniques are introduced to perform the necessary spatial gradient and divergence operators of MR. The approach is different from other MR-CSI implementations where the unknown variables are located on a uniform grid of rectangular cells and represented using pulse basis functions; with rectangular cells finite-difference operators can be used, but this becomes unwieldy in FEM-CSI. Furthermore, an improvement for MR is proposed that accounts for the imbalance between the real and imaginary parts of the electrical properties of the unknown objects. The proposed method is not restricted to any particular formulation of the contrast source inversion. The functionality of the new inversion algorithm with the different enhancements is tested using a wide range of synthetic datasets, as well as experimental data collected by the University of Manitoba electromagnetic imaging group and research centers in Spain and France.

Thesis (M.S.)--University of Missouri-Columbia, 2007. / The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. Title from title screen of research.pdf file (viewed on January 11, 2008) Includes bibliographical references.

Thesis (Ph.D.)--Drexel University, 2009. / Includes abstract and vita. Includes bibliographical references (leaves 104-115).

Thesis (M.S.)--Michigan State University. Electrical Engineering, 2008. / Title from PDF t.p. (viewed on July 27, 2009) Includes bibliographical references (p. 127-131). Also issued in print.

Thesis (M.S.)--Baylor University, 2006. / Includes bibliographical references (p. 56 [i.e. 55]).

This thesis contributes to the methods for evaluating the hardware performance of microwave imaging systems. Different aspects of this performance study are systematically described and evaluated through existing and novel performance metrics. The dynamic range of two vector network analyzers and their noise floor are explored. The effects of low noise amplifiers on the dynamic range of the microwave system are also studied. To increase the dynamic range and sensitivity of the system, a new high efficiency ultra wide band TEM horn antenna is introduced. The new antenna significantly blocks the electromagnetic interference from the surrounding medium. The parasitic coupling between the transmitting and receiving antennas has been reduced to a minimum. To estimate the efficiency of the antenna used in tissue imaging more realistically, a measurement based method is also proposed. The method models the two-port network formed by the antennas and the medium between them by signal flow graph. The medium’s electrical properties mimic those of tissue and can vary according to the application. Also, a new performance metric for microwave systems namely, the physical contrast sensitivity is proposed. Methodologies are proposed for its evaluation through measurements and through simulations. This enables the estimation of the smallest detectable target permittivity contrast or size for the system under evaluation. / Thesis / Doctor of Philosophy (PhD)

An array of electronically-switched circular patch antennas is designed for breast tissue imaging for the purposes of cancer screening. Each array element is connected to a dedicated heterodyne radio, which performs amplification and downconversion to a single, low intermediate frequency for improved signal recovery. Data acquisition is accomplished with a VNA-based architecture. Details of the design are provided and are accompanied by simulated and experimental data. A receiver sensitivity of at least -120 dBm is expected, and an imaging sensor switching dynamic range between ON and OFF states of at least 127 dB is demonstrated based on experimental measurements. / Thesis / Master of Applied Science (MASc)

Yes / A compact microstrip antenna design to be used in breast cancer detection is presented. The antenna consists of a radiating patch mounted on two vertical plates, fed by coaxial cable. A study is carried out on different parameters of the antenna. Simulation results show that the antenna possesses a wide bandwidth and this is confirmed experimentally. In experiments, a homogeneous dielectric box, having similar properties to human tissue is used to study the interaction of the antenna with tissue. Even without added matching medium or lumped loads there is good matching when the antenna is in contact with the tissue. Finally a two-element antenna array is investigated numerically, with promising results. / MSCRC