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UBC Theses and Dissertations

Crystal design simulation for a high resolution depth encoding pet tomograph Astakhov, Vadim

Abstract

Position Emission Tomography is a functional imaging modality where positron labeled radiotracers are used to investigate biological processes. The imaging process occurs via simultaneous detection of two 511keV gamma rays originating from positron annihilation. A PET camera is a γ detection apparatus. Reconstruction algorithms are used to reconstruct the original radioactivity source distribution in the camera field of view (FOV) from the simultaneous detection of y rays originating from the same annihilations. PET has been extensively used to investigate function in living organism, especially in human subjects. In order to make the detection process efficient and useful, PET camera designs strive for high detection sensitivity and high resolution. One of the factors influencing the resolution is the size of the detectors. Smaller detectors lead to a better spatial resolution. On the other hand sensitivity is affected by the detector crystal composition and by the solid angle subtended by the detection apparatus. An ideal tomograph design will therefore involve small, efficient detectors placed as close as possible to the object being scanned. The work described in this thesis examines various detector crystal configurations that would lead to an optimum tomograph performance. In order to make the results of this study immediately relevant to the PET community the overall tomograph geometry was constrained to that which is currently being built by a tomograph manufacturing company CTI. This design consists of an octagonal detector configuration where each detector head is built with two layers of detector material. Such a design allows for the identification of the y depth of interaction (DOI) in the detector assembly which in turn allows to minimize the effect of the parallax error and thus contributes to an increased resolution uniformity across the camera FOV. The studies presented here examine the effect of different crystal layer configuration on resolution and sensitivity. Octagonal HRRT geometry was also compared to circular detector geometry. As part of a system design optimization, several novel methods for crystal element identification were investigated: Genetic-algorithm, neural network algorithm and "simple" geometric algorithm were tested and showed relatively equal identification performance in identifying 64x64 crystal elements of each layer. A fuzzy-logic approach for estimation of depth-of-interaction (DOI) was investigated and compared with the decay time discrimination approach. The simulation results were used to generate a Look-Up-Table (LUT) that is accessed during simulated data acquisition for an effective and quick crystal identification. A correct crystal identification also facilitated an improvement of the capability for accurate energy discrimination, since the detector gain and appropriate energy thresholds were considered on an element-by-element basis by accessing energy LUT. The final result of the work presented in this thesis is the determination of the effect of DOI correction on resolution uniformity for different crystal configuration. DOI correction was found to improve the resolution uniformity up to 67%.

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