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Title: Study of 2-to-3 photon annihilation using hydrophilic material as hypoxic tumour phantom
Author: Aldousari, Hanan
Awarding Body: University of Surrey
Current Institution: University of Surrey
Date of Award: 2013
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Abstract:
The use of three gamma annihilation as a new PET molecular imaging modality which can predict tumour hypoxia was proposed by Kacperski and Spyrou in 2004. The positronium and its annihilation, could then serve as an oxygen-sensitive marker. The relative magnitude of three photon annihilation is increased in an oxygen deficient environment. The main questions addressed in this thesis are the potential use of the lanthanum bromide LaBr3: Ce (5%) scintillator detector for three-photon positron annihilation measurements. Also, the optimum source-to-detector distance that achieves good efficiency and sensitivity of the measurements. The other question is finding the best method to prepare in vitro three types of hypoxic samples (mineral water, defibrinated horse blood and semm). In addition, the suitability of the hydrophilic material is examined in terms of hydration behaviour, radiation and physical properties, for use as a phantom that represents the hypoxic tumour in the three-photon positron annihilation measurements. The main critical question is which the best method that could be used to measure the three-photon positron annihilation yields. Also, how does the 3y/2y ratio vary with different oxygen concentrations in different hydrophilic materials? Lanthanum bromide LaBr3: Ce (5%) scintillator detectors have good fast timing resolution, good stopping power and large light output. The LaBr3:Ce (5%) has very good scintillator characteristics, combining high effective Z and density, fast decay time, light emission wavelengths matching that of commonly available photon detectors and excellent energy resolution (~3% at 662 keY). However, it is highly hygroscopic in nature, making it difficult to produce, but its commercial availability has been gradually increasing in recent times. LaBr3: Ce (5%) has come to be accepted for the superior energy resolution it offers. Therefore, LaBr3: Ce (5%) detectors can be considered as the scintillators of choice for determining the yield of 2-and3-y positron annihilation. The characterisation of the detector used was can-ied out in terms of energy resolution and efficiency. The effect of amplifier parameters on the energy resolution was also studied. The 'geometrical solid angles subtended by the source-to-detector distances play an important role in the sensitivity and accuracy of the detector efficiency measurements. Therefore, the efficiency measurements were investigated at various distances to optimise the geometrical solid angle for the LaBr3: Ce (5%) detector for the three photon annihilation measurements. The experimental data were compared with the GATE simulated results obtained. The objective to study the factors that affect dissolved oxygen (DO) in three types of samples (mineral water, defibrinated horse blood and semm) was to investigate the 3y/2y ratio in hypoxic, nOlIDoxic and hyperoxic conditions of tissues for future application in oncology in the detection and quantification of tumour hypoxia. This was achieved by measuring the DO, pH and temperature before, during and after treating the samples with nitrogen, carbon dioxide and ascorbic acid (AnaeroGen). Two methods for the preparation of the in vitro hypoxic samples were investigated and evaluated. Carbon dioxide proved to be most effective for the reduction of DO in the samples. Blood tended to resist DO reduction since it decreased at a much slower rate than in water and semm. Together with the fact that the oxygen level in blood remained low after the end of exposure to the gases and ascorbic acid suggests that the solubility of oxygen in blood depends upon the concentration of haemoglobin as well as upon ligands such as CO2 • CO2 combines with haemoglobin affecting oxygen binding and fonns bicarbonate which further decreases the affinity of haemoglobin to oxygen Hydrophilic materials.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.616952  DOI: Not available
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