PhD - Science / Screening of asymptomatic women using X-ray mammography technique is very common in many parts of the world in view of the prevalence of breast cancer among women.
Mammography X-ray procedures are well established; with radiation dose measurements
usually carried out using air ionisation chamber despite its inherent disadvantages. In this
study, the various parameters necessary for accurate dose calculations from mammography
X-ray energies and their effect on the calculated dose values, the relationship between dose, breast size, image quality and X-ray tube parameters as well as an alternative method for
dose measurements, were systematically investigated.
A method is presented for calculating accurately the mass attenuation and mass-energy coefficients for any energy bin of interest in the photon energy region 1-20 keV from existing mass attenuation and mass-energy coefficients data. Data fitting procedure was used for the study using an established equation. The results of the study showed that when data points containing high and low energies such as 1 - 200 keV are fit together with a single set of parameters, an overestimation of about 20% at the lower energies with far greater deviations at higher energies can result. It has been shown that grouping data into smaller energy regions when fitting would lead to accurate calculations of the mass attenuation or mass energy-absorption data. This is especially important if the data were to be used in low energy photon calculations such as would be the case for mammography beams.
An investigation into the effect of pressure, temperature and humidity in air on photon fluence at a typical mammography, low bremsstrahlung energy (25 kVp), has been carried
out. The results of the investigation showed that air kerma values from an X-ray spectrum
that has significant lower energy components is likely to be more sensitive to changes in
pressure, temperature and humidity than the air kerma from an X-ray spectrum with lower energy components less pronounced.
Mean glandular dose (MGD) values had been calculated for various tube potentials and tube loadings (TL) using direct measurements of the incident entrance air kerma (ESAK) at the surface of a standard breast phantom and also from spectral measurements acquired with a solid-state detector. Detailed presentations of dose measurements from direct measurements and also from X-ray spectral data employing the established methods are given. Comparisons of the MGD values thus derived are presented and the relationship between MGD, phantom thickness, image quality and tube operating parameters is discussed.
The possibility of evaluating radiation dose from mammography X-ray beams using constructed probes with diamond as the active radiation sensing material has been studied.
Diamond has been used in the conduction mode whereby electrodes are connected to it and the resulting current from the interaction of the ionizing radiation with the diamond detected. Single crystal diamonds produced under high pressure and temperature (HPHT), as well as polycrystalline diamonds manufactured by the chemical vapour deposition (CVD) method were used. Suitable diamond stones were carefully selected for the study using various techniques. The probe was constructed entirely using tissue equivalent materials. In current practice diamond in the form of thin plates are used in the “flat-on” geometry, where the radiation beam to be monitored or measured, impinges on the flat face of the diamond. In this work it was found that using diamond plates in a side-on, or “edgeon”, geometry improves the collection efficiency of the diamond.
The probe has been designed for radiation detection in both “edge-on” and “flat-on” sensor
geometry profiles without having firstly to unseat the diamond sensor element from its original position within the probe housing before taking measurements. The study has shown that with the “edge-on” geometry configuration, radiation from impinging photons with energies below 30 keV can be made to deposit almost all (about 90%) of their energy into the sensor. The probe was designed for use in combination with commercially available electrometer systems. The response of the diamond probe to changes in radiation dose correlated well with that obtained from a secondary standard ionization chamber at the same X-ray tube settings.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/161 |
| Date | 08 February 2006 |
| Creators | Assiamah, Mary |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | 153024 bytes, 73836 bytes, 215310 bytes, 161829 bytes, 107676 bytes, 6974 bytes, 345563 bytes, 11566 bytes, 361886 bytes, 329178 bytes, 132809 bytes, 249518 bytes, 14958 bytes, 23809 bytes, application/pdf, application/pdf, application/pdf, application/pdf, application/pdf, application/pdf, application/pdf, application/pdf, application/pdf, application/pdf, application/pdf, application/pdf, application/pdf, application/pdf, application/pdf, application/pdf, application/pdf, application/pdf, application/pdf, application/pdf, application/pdf, application/pdf, application/pdf, application/pdf, application/pdf, application/pdf, application/pdf, application/pdf |
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