Advances in medical imaging and gamma ray spectroscopy

Abstract

This thesis describes some results from the research by the author in his three year PhD period. The work presented can be divided into two major areas--developing detectors for Positron Emission Tomography and the application of gamma ray spectroscopy based on scintillation detectors. In recent years, great effort has been made to improve the performance of PET instrumentation. As a result, some new concepts were introduced in the design of modem PET systems. Several small and dedicated PET systems have been developed which are aimed to provide better cost effectiveness and image quality. Detectors with Depth-of-interaction capability helped to reduce the degradation of PET image quality due to the parallax error. In order to overcome some intrinsic limitations of PET, such as the relatively poor spatial resolution, PET systems compatible with other imaging modalities like MRI, CT and SPECT enables one to combine the quantitative information provided by PET and the structural (anatomical) information from MRI or CT. A Positron Emission Mammography system was designed for breast cancer imaging application. It can be placed inside an existing MR imaging system currently used in Royal Marsden Hospital. The PET detector was designed to work simultaneously with the MRI system. The Electromagnetic Interference (EMI) between the two imaging systems can be minimised by placing the conducting material, such as the read-out electronics for PET, outside the MRI system. In this design, the scintillation crystals are connected to the remote photodetectors through fibre light-guides of 2m in length. A comprehensive feasibility study based on both Monte Carlo simulation and experimental measurements was carried out. For research applications such as small animal PET imaging, we developed a Depth-of-interaction (DOI) PET detector based on the Hybrid Photodiode (HPD). A prototype detector was constructed and tested. The results show a DOI resolution of 5-8mm and a timing resolution better than 5ns. The second part of this thesis presents the work in gamma ray spectroscopy. A number of algorithms for unconstrained spectrum de-convolution were compared using both simulated and measured data. The application of this technique on energy- loss spectra collected using a 3" Nal scintillation counter shows huge improvement in both energy-resolution and effective detector sensitivity. This makes it possible to use such a detector as a replacement for HPGe spectrometers for some applications. The work in gamma ray spectroscopy also led to the development of a range of large volume scintillation detectors, which provided much-improved energy-resolution compared with the standard 3" Nal detectors. These detectors were shown to be ideal detectors for use in conjunction with the spectrum de-convolution process.

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