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An improved numerical model for calculating the ultrasonic power deposition in layered medium was developed and experimentally verified. The new model takes into account the ultrasound wave reflection and refraction at irregular tissue interfaces thereby providing improved accuracy in ultrasound hyperthermia treatment planning. The model was compared with a simplified model to evaluate when the tissue interfaces could be ignored in the hyperthermia treatment planning and evaluation. The effects of variations in water and tissue temperatures, the fat layer thicknesses, the bone-tissue interface, and the beam entrance angles were also investigated to establish guidelines for treatment execution. It was found that in most cases the effects of the soft tissue interfaces can be ignored. However, in some instances the acoustic focus may be shifted several millimeters off axis in layered medium. This is important when sharply focused transducers are used for ultrasound surgery or under the condition of pulsed, high temperature hyperthermia treatments.

Charged particle beam source problems are difficult to simulate because of the extremely large and anisotropic scattering cross sections, and the associated singularities in angle and space. To overcome these difficulties the Extended First Collision Source method and the SMART (Simulation of Many Accumulative Rutherford Trajectories) scattering method have been developed. In this study, the extended first collision source calculation was improved in the SN code using the SMART scattering method which produces smoother and smaller effective cross sections. To determine the uncollided flux which was necessary for the first collision source calculation, the SMART scattering cross sections were used instead of screened Rutherford scattering cross sections. After the SN code was modified, it produced results that approached those obtained using the benchmark Monte Carlo code.

Our goal is to develop a single-photon emission computed tomography (SPECT) system for dynamic cardiac imaging so that heart disease may be more accurately evaluated. We have developed multiple, stationary, modular scintillation cameras that allow for dynamic imaging because of large detector area, large collection efficiency, high count-rate capability, and no motion of detector, collimator, or aperture. We make use of coded-aperture pinhole arrays because they increase photon-collection efficiency. The coded apertures allow for overlapping projections or multiplexing of an object onto the detector face. We have designed a novel collimation system that allows for an increased number of pinhole projections without substantial multiplexing. This new method is called "subslicing". We verified the subslice concept both in computer simulation and with our 16-module ring imaging system. Comparison of results with and without subslicing shows that the new approach substantially reduces artifacts in the image reconstruction. (Abstract shortened with permission of author.)

Consideration is given to the premise that certain electron paramagnetic resonance signals in natural quartz are the result of age-indicative accumulations of alpha recoil-induced lattice damage. The point defects potentially responsible for the observed EPR signals are reviewed, as a basis for consideration of point defect production by ionizing radiation of and alpha recoil fragment transport in quartz. The production of oxygen vacancies in the quartz lattice by individual alpha recoil fragments in both the $\sp{238}$U and $\sp{232}$Th decay chains is calculated using two different methods. The Monte Carlo transport method provides a detailed collision history for each of the recoil fragments. The EPR experiments focus on the size of the E' and oxygen hole-related center (OHRC) signals in five different types of quartz: pegmatitic, unmetamorphosed plutonic, metamorphosed plutonic, hydrothermal vein and volcanic. The effects of gamma and neutron irradiation on the EPR spectra are also considered. The concentrations of $\sp{238}$U, $\sp{232}$Th and $\sp{147}$Sm are determined using neutron activation analysis in many of the quartzes that were extracted from rocks (or were intimately associated with minerals) of known age. Significant correlations between the age of the quartz and the size of both the E' and OHRC signals are initially established, and improved correlation is observed after the signal sizes are normalized to the respective total alpha activity of each sample (based on its $\sp{238}$U, $\sp{232}$Th and $\sp{147}$Sm concentration). These correlations establish the basis for a potential quartz geochronometer. / Theoretical considerations involve factors related to (1) recoil damage efficiency, (2) paramagnetic fractions of precursor defect populations, (3) overlap of alpha recoil track volumes and (4) fluid inclusion-hosted alpha activity. It is hypothesized that spherical, alpha recoil-induced lattice damage halos may develop around alpha-active fluid inclusions. A method is proposed for quartz age determinations, and suggestions are made toward overcoming the present limitations. It is concluded that approximate age determinations ($\pm$15-20%) may eventually be possible, and that maximum age determinations might also eventually be possible. / Source: Dissertation Abstracts International, Volume: 51-07, Section: B, page: 3289. / Major Professors: Arthur Leroy Odom; George W. Devore. / Thesis (Ph.D.)--The Florida State University, 1990.

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