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Application of a pixellated detector to energy dispersive x-ray diffraction, and investigation of materials for breast tissue diffraction evaluation

Christodoulou, C; (2016) Application of a pixellated detector to energy dispersive x-ray diffraction, and investigation of materials for breast tissue diffraction evaluation. Doctoral thesis , UCL (University College London). Green open access

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Abstract

Two pixellated application specific integrated circuits (ASICs) bump bonded to 1 mm thick Cadmium Telluride sensors were characterised and compared for the purpose of application in a combined energy dispersive and angular dispersive x-ray diffraction system. The energy resolution of the ASICs, the energy detection efficiency at 60 keV, the radiation responses, the number of dead pixels, and the charge sharing effects on the detection efficiency and energy resolution were considered. The parameters of the field programmable array (FPGA) used to acquire the data were also explored based on the number of frames detected, and the counting efficiency of the ASIC, which is not optimised to photon count, but to energy resolve for spectroscopic applications using a peak hold read-out. The selected ASIC was incorporated into a diffraction system which included a microfocal source. This system was characterised for comparison with other future systems, as well as to establish important parameters such as the divergence of the system and the size of the beam at the sample, as this was necessary to define the voxel of the sample being considered in diffraction scanning.The obtained data from the 2D array were investigated, in terms of utilising the detector array as a whole, i.e. all energies and detection angles, compared to only selecting angles and energies of interest. The obtained diffraction signals for caffeine, lard and Poly(methyl methacrylate) (PMMA) were investigated with respect to the sample thickness. Parameters utilised in the literature in diffraction studies, including the intensity of the diffraction peak, the full width half maximum (FWHM) of the peak and their ratios with respect to the area of the diffraction peak were also considered. These signals were shown to vary with the sample thickness, however this relationship was affected by the post–processing method used. A High Purity Germanium system (HpGe) Energy Dispersive X-ray Diffraction(EDXRD) system was used to acquire diffraction spectra for a materials library and to establish potential materials for the design of anthropomorphic phantoms for breast tissue diffraction studies. In order to obtain a phantom with a realistic shape, materials could be melted and set into a tumour shape. Materials included plastics, such as polymorph, and biodegradable materials such as gelatine and agar. Polymorph, agar and gelatine showed, in terms of their diffraction spectra, density and their attenuation coefficients, the potential to be used in phantoms as breast tissue equivalent materials. The diffraction system was used to scan a PMMA phantom created using a laser etching method. The phantom creation method was developed in this research and is characterised using transmission imaging to establish the relationship of the speed setting of the laser etching system, the greyscale value obtained from the images, and the etch depths of the laser system. This method of characterisation leads to recreation of the phantom based on the etch depths. The phantoms area created from an 8-bit image of a tumour. The phantom was scanned in the system both as is and also with water as an infilling material.

Type: Thesis (Doctoral)
Title: Application of a pixellated detector to energy dispersive x-ray diffraction, and investigation of materials for breast tissue diffraction evaluation
Open access status: An open access version is available from UCL Discovery
Language: English
Keywords: EDXRD, energy dispersive x-ray diffraction, CdTe, tissue equivalent materials, laser system, EDXRD phantom
UCL classification: UCL > Provost and Vice Provost Offices
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science > Dept of Med Phys and Biomedical Eng
URI: https://discovery.ucl.ac.uk/id/eprint/1482172
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