Studies in single photon emission imaging

2017-03-01T04:34:23Z (GMT) by Brown, Jeremy Michael Cooney
This thesis presents a number of studies into Single Photon Emission Imaging (SPEI) that can be grouped into three main sections: Monte Carlo Radiation Transport Modelling, Development of a Hybrid SPEI System and Fundamental Study of Advanced Compton Imaging. The first section of this thesis, Monte Carlo Radiation Transport Modelling, presents an overview of the Monte Carlo radiation transport modelling toolkit Geant4 and a newly derived low energy Compton scattering model, the Monash University Compton scattering model. This Compton scattering model was developed to address the limitation present in the majority of Monte Carlo bound atomic electron Compton scattering models: incorrect determination of the ejected direction of Compton electrons due to the non-zero momentum of the bound atomic electron. A theoretical foundation that ensures the conservation of energy and momentum in the relativistic impulse approximation was utilised to develop energy and directional algorithms for both the scattered photon and ejected Compton electron from first principles. Assessment of this model was undertaken in two steps: comparison with respect to two Compton scattering classes of Geant4 adapted from Ribberfors' work, and experimental comparison with respect to Compton electron kinetic energy spectra obtained from the Compton scattering of 662 keV photons off the K-shell of gold. It was shown that this new Compton scattering model was a viable replacement for the majority of computational models that have been adapted from Ribberfors' work. Additionally, this model was shown to be able to reproduce the Compton scattering triply differential cross-section Compton electron kinetic energy spectra of 662 keV photons K-shell scattering off of gold to within experimental uncertainty. The second section of this thesis, Development of a Hybrid SPEI System, presents the development of a novel hybrid collimated SPEI system: the Pixelated Emission Detector for RadiOisotopes (PEDRO). The PEDRO is a conceptual proof of principle hybrid SPEI system that was designed to explore and quantify the relationship between spatial resolution and sensitivity, inherent in SPEI, over the energy range of 30 keV to 511 keV. This system was originally intended to be constructed from a Compton camera stack located behind a coded mask composed of a mix of pinholes, slats and/or open areas. A total of three studies are presented in this section that outline the development of: 1) a Geant4 application to be used for optimisation of PEDRO with respect to a robust metric, 2) an automated routine for the optimisation of large-area slits in the outer regions of a coded mask for PEDRO which has a central region allocated for pinholes, and 3) a novel experimentally motivated image deblurring technique for multi-plane Gamma cameras such as PEDRO. These three studies illustrated two main points: 1) it may indeed be possible to overcome the trade-off between spatial resolution and sensitivity inherent in SPEI through hybrid Compton-mechanical collimation, and 2) the multi-plane nature of hybrid collimation systems can be capitilised on to reduce the level of image blurring associated with collimator opening geometry and, in turn, improve recovered image quality. Finally, the third section of this thesis, Fundamental Study of Advanced Compton Imaging, presents a preliminary study into the fundamental limits of recoil electron tracking enhanced Compton collimation, or Advanced Compton Imaging, for photon energies below 2 MeV. A custom Geant4 application of an idealised detection system was developed and four different detection materials were tested over a wide range of electron tracking resolutions. Increased image performance and point source convergence was observed with respect to standard Compton imaging regardless of recoil electron tracking accuracy in all tested materials across the investigated energy range. Additionally, the rate of point source convergence with respect to standard Compton imaging was discovered to be maximised for electron tracking accuracies of 45° to 60° Full Width at Tenth Maximum (FWTM) for photons of incident energy greater than 500 keV. Further study with more detailed simulation and image recovery frameworks is required to assess the validity of these observed trends.