Microwave imaging creates images of electrical property distributions in tissue, and has promise for breast tumor detection due to the contrast in electrical properties of normal and malignant breast tissues and the accessibility of the breast for imaging. Confocal microwave imaging (CMI) is a recently introduced technique that avoids limitations associated with classical microwave imaging. CMI detects areas of increased scatter (e.g. tumors) by scanning the synthetic focus of an array of antennas through the breast. As the object is illuminated with ultra-wideband signals, this corresponds to computing time delays to the focal point, resulting in simple image reconstruction algorithms. Additionally, the resolution is determined primarily by the bandwidth of the illuminating signal, allowing for detection of small tumors with appropriate selection of this bandwidth. CMI appears to be a simple and effective technique for breast tumor detection. The development and evaluation of a new approach to confocal microwave imaging is the contribution of this thesis. CMI was only very recently introduced, and many key issues need to be addressed. Most importantly, the CMI system must be designed for physical compatibility with the breast examination. The previously introduced CMI system is planar, and involves placing an array of antennas directly on the naturally flattened breast (of a woman who is lying on her back). In this thesis, a cylindrical CMI configuration is developed. A woman lies on her stomach, the breast extends through a hole in the examination table, and is immersed in a low-loss material. The breast is encircled by an array of antennas, which is placed at a distance from the skin. The cylindrical configuration is likely more appropriate for clinical implementation. The development of cylindrical CMI involves design of appropriate sensing elements and development of image reconstruction algorithms. Construction of appropriate models and simulations of the system are required to test the feasibility of the proposed sensors and algorithms. The finite difference time domain (FDTD) method is well suited to these feasibility studies, as ultra-wideband signals are efficiently simulated in the time domain. In this thesis, four alternative antenna designs are characterized with measures appropriate for ultra-wideband radiation and this specific imaging application. The selected antenna is scanned in a circle around the breast and at a distance from the skin. This is repeated for a number of rows at different heights in order to synthesize a cylindrical or conical array. The returns recorded at each antenna location are processed to reduce clutter, then synthetically focussed at points in the domain of interest. Results indicate that the proposed antenna and algorithms provide the capability to detect and localize (in three dimensions) small spherical tumors at reasonable depths in the breast models. The detection capability achieved with the cylindrical system is comparable to that obtained with the previously introduced planar system. / Graduate

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.

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

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

Thesis (Ph. D.)--Oregon State University, 2011. / Printout. Includes bibliographical references (leaves 182-187). Also available on the World Wide Web.

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