Improved contact design for the SiC photo-switch used in high power applications

The entire thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file; a non-technical public abstract appears in the public.pdf file.Title from PDF of title page (University of Missouri--Columbia, viewed September 10, 2009).Thesis advisor: William Nunnally.Includes bibliographical references.M.S. University of Missouri--Columbia 2007.Dissertations, Academic -- University of Missouri--Columbia -- Electrical engineering.Silicon Carbide (SiC) has a bulk dielectric strength of 3000 kV/cm, thermal conductivity of 4.9 W/(cm-K), and high tensile strength. It is considered the most promising photo-switching material that can enable the fielding of the most compact pulse power systems. The SiC photoswitch as an advantage over other high power switches when it is operated at a high blocking electric field. Presently, the field blocking performance of the SiC photo-switch has fallen short of the theoretical expectations. Breakdown occurs prematurely at an applied electric field of ̃300 kV/cm not at the expected 3000 kV/cm. Breakdown is not due to the SiC bulk material but to electric field enhancement caused by the switch packaging. No packaging method exists to effectively address electric field crowding at the point where the electrode leaves the SiC bulk. At this triple point junction, the induced electric field exceeds the bulk dielectric strength of the SiC. Without an improved package, the potential of the SiC photo-switch may not be realized. Reported in this paper is a novel concept of inserting contoured electrodes into the SiC bulk to minimize field crowding in order to improve the breakdown characteristics. The concept is simulated, a fabrication process is designed, and the first steps to the fabrication process are tested

3-D Monte Carlo-Based Scatter Compensation in Quantitative I-131 SPECT Reconstruction

We have implemented highly accurate Monte Carlo based scatter modeling (MCS) with 3-D ordered subsets expectation maximization (OSEM) reconstruction for I-131 single photon emission computed tomography (SPECT). The scatter is included in the statistical model as an additive term and attenuation and detector response are included in the forward/backprojector. In the present implementation of MCS, a simple multiple window-based estimate is used for the initial iterations and in the later iterations the Monte Carlo estimate is used for several iterations before it is updated. For I-131, MCS was evaluated and compared with triple energy window (TEW) scatter compensation using simulation studies of a mathematical phantom and a clinically realistic voxel-phantom. Even after just two Monte Carlo updates, excellent agreement was found between the MCS estimate and the true scatter distribution. Accuracy and noise of the reconstructed images were superior with MCS compared to TEW. However, the improvement was not large, and in some cases may not justify the large computational requirements of MCS. Furthermore, it was shown that the TEW correction could be improved for most of the targets investigated here by applying a suitably chosen scaling factor to the scatter estimate. Finally clinical application of MCS was demonstrated by applying the method to an I-131 radioimmunotherapy (RIT) patient study.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/85854/1/Fessler47.pd

Incorporation of System Resolution Compensation (RC) in the Ordered-Subset Transmission (OSTR) Algorithm for Transmission Imaging in SPECT

In order to reconstruct attenuation maps with improved spatial resolution and quantitative accuracy, we developed an approximate method of incorporating system resolution compensation (RC) in the ordered-subset transmission (OSTR) algorithm for transmission reconstruction. Our method approximately models the blur caused by the finite intrinsic detector resolution, the nonideal source collimation and detector collimation. We derived the formulation using the optimization transfer principle as in the derivation of the OSTR algorithm. The formulation includes one forward-blur step and one back-blur step, which do not severely slow down reconstruction. The formulation could be applicable to various transmission geometries, such as point-source, line-source, and sheet-source systems. Through computer simulations of the MCAT phantom and transmission measurements of the air-filled Data Spectrum Deluxe single photo emission computed tomography (SPECT) Phantom on a system which employed a cone-beam geometry and a system which employed a scanning-line-source geometry, we showed that incorporation of RC increased spatial resolution and improved the quantitative accuracy of reconstruction. In simulation studies, attenuation maps reconstructed with RC correction improved the quantitative accuracy of emission reconstruction.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/86030/1/Fessler42.pd

3-D Monte Carlo-based Scatter Compensation in Quantitative I-131 SPECT Reconstruction

We have implemented highly accurate Monte Carlo based scatter modeling (MCS) with 3-D ordered subsets expectation maximization (OSEM) reconstruction. The scatter is included in the statistical model as an additive term and attenuation and detector response are included in the forward/backprojector. In the present implementation of MCS, a simple multiple window-based estimate is used for the initial iterations and in the later iterations the Monte Carlo estimate is used for several iterations before it is updated. For I-131, MCS was evaluated and compared with triple energy window (TEW) scatter compensation using simulation studies of a mathematical phantom and a clinically realistic voxel-phantom. Even after just two Monte Carlo runs, excellent agreement was found between the MCS estimate and the true scatter distribution. Accuracy and noise of the reconstructed images were superior with MCS compared to TEW. However, the improvement was not large, and in some cases may not justify the large computational requirements of MCS. Finally clinical application of MCS was demonstrated by applying the method to a radioimmunotherapy (RIT) patient study.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/85865/1/Fessler201.pd