Positron Emission Tomography (PET) serves a unique and important role in medical research because it permits non-invasive quantitative study of biological processes using radionuclides of naturally occuring elements. In the last decade, the imaging properties of PET have improved significantly because of better understanding of the design principles and introduction of novel concepts. One such development has been that of the 'block' detector system, consisting of an array of scintillation crystals coupled to a relatively small number of photomultiplier tubes (PMTs). Identification of the particular element in the block is made by comparing the outputs from the PMTs. The block provides the basic unit of the detector rings in modern PET cameras. The prototype block detector system employed in this study incorporates the CTI 831 detector module (49.47 mm wide by 53.36 mm tall by 30 mm deep). This is segmented into a matrix of 8 by 4 crystal elements, 5.62 mm(transaxially) by 12.86 mm (axially) and 30 mm (deep), and coupled to four square PMTs. The drive towards improvement of image quality in PET has prompted the development of even smaller crystals, promising 'high resolution' multiplane imaging. While these detectors have significant advantages over other detectors, the aim of this study was to investigate the physical performance of this specific block detector and to assess how its limitations will affect the information obtained from it. The system investigated offered a coincidence time resolution of 5.8 ±0.3 ns FWHM for a pair of block detectors, an individual crystal energy resolution of 19 % ±3 FWHM at 511 keV, maximum intrinsic efficiency of 45.7% ±0.5 and a column transaxial resolution of 4.2 ±0.4 mm FWHM, offering important immediate advantages. However, the drawback in the current implementation scheme is the nonuniformity across the detector face. The variation of efficiency, energy and spatial resolution for the individual detector crystals across the face of the detector block were investigated, the factors contributing to these variations were identified and suggestions for reducing their effects were made. For example inter-detector scattering was found to be a problem that leads to mispositioning of detected events. Different techniques for evaluating the amount, distribution and consequently the removal of inter-detector scattering were established. Finally these block detectors offer other possibilities like gamma-gamma coincidence imaging, attractive for adaption to neutron induced gamma ray emission tomography (NIGET). This would reduce the long scanning times presently required. However, the 'electronic collimation' provided by the coincidence detection of the two annihilation photons along the line of response between the opposing detectors is lost. Imaging of the cascade gamma rays necessitates the use of physical collimation in order to define a plane through the object. This will however reduce the absolute efficiency of the system from 2.28*10-2t 2*104to 7.3*10-6± 3"10-7w hen collimation is used on both sides but increases the spatial resolution from 5.7 ±0.2 to 2.4 ±0.2 mm. However, if a collimator is used on one side only, the spatial resolution (3.8 ±0.2 mm) obtained is comparable to that of a Ge detector with a1 mm diameter hole collimator and the absolute efficiency of the system (1.1 *10-4±3`10'5) is many times better.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:361872 |
| Date | January 1996 |
| Creators | Mesbahi, Mohammad Esmail |
| Publisher | University of Surrey |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://epubs.surrey.ac.uk/771943/ |
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