Medical Physics
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Browsing Medical Physics by Advisor "Van Aswegen, A."
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Item Open Access The assessment of potential radiation hazards from gold mines in the Free State Goldfields to members of the public(University of the Free State, 1998-11) Ellis, Jozua Francois; Botha, J. C.; Van Aswegen, A.The gold mines In the Free State Goldfields extract and process ore from underground, which contains naturally radioactive uranium and its associated decay products. This assessment aimed to cost effectively determine the major potential radiation hazards to the public from the gold mines in the area. The potential exposure sources from the mines are radon gas, radioactive dust, contaminated water and external gamma radiation. The assessment focussed mainly on the public's potential exposure to radon gas emanating from tailings dams, waste rock dumps and upcast shafts from underground workings. The rate of radon emanation from the dams was measured using several different techniques, and the potential dispersion of the radon was modelled using internationally accepted modelling codes and local weather data for the Free State Goldfields. A maximum potential contribution to the natural background radon levels of 6 Bq m-3 was calculated. This is a small increment to the background levels in the order of 25 to 35 Bq m-3". Environmental measurements of outdoor radon concentrations confirmed the modelling results to the extent that no significantly high radon concentration could be detected in the environment. Background radon levels in towns outside the Free State Goldfields are in the same order as those measured around the mines. Similar environmental measurements of airborne dust and water sources around the mines indicated relatively low levels of radiation. A conservative estimate of the total potential exposure of the public in the Free State Goldfields is in the order of 130 to 250 µSv/a. This can be interpreted as well within the internationally accepted public dose limit of 1000 µSv/a.Item Open Access Development and evaluation of a spect attenuation correction method using an open transmission source and scatter correction(University of the Free State, 2011-06) Van Staden, Johannes Abraham; Du Raan, H.; Van Aswegen, A.Abstract not availableItem Open Access An evaluation of the effect of scatter and attenuation correction of gamma photons on the reconstructed radionuclide distribution in the myocardial wall during spect imaging(University of the Free State, 2000-11) Mdletshe, Nhlakanipho; Van Aswegen, A.; Du Raan, H.The purpose of this study was firstly to evaluate the selection of reconstruction parameters (i.e. the number of subsets and the number of iterations) based on phantom studies. The second aim was to evaluate the effect of the non-uniform attenuation and scatter correction on myocardial perfusion studies performed on healthy volunteers as well as patients with proven inferior wall perfusion defects. The quality of the images from the phantom studies showed that 16 subsets with 2 iterations gave the best results if considering image noise and image resolution. These number of subsets and iterations were therefore used as reconstruction parameters in the patient studies The application of an attenuation correction to the emission data required that attenuation coefficient maps of the subjects were obtained from transmission images. I39Ce was chosen as the transmission source and used in conjunction with 99mTcas the emission source. The emission data were corrected for scatter according to the triple energy window method. In the healthy male and female volunteers, the attenuation and scatter corrected myocardial SPECT images showed an improvement in the homogeneity of the counts distribution compared to the uncorrected images. The counts distribution in the inferior region improved after the attenuation correction was applied, however it exceeded the counts in the anterior region. After applying a scatter as well as an attenuation correction to the emission data, the counts in the inferior region of the myocardium were slightly reduced. This was a result of the scatter correction eliminating scattered counts in the inferior region originating mainly from the liver. The apparent lower counts in the anterior region could be a result of too little compensation for scatter in the inferior wall, and needs to be investigated further. The defects in the three unhealthy patients, were not obscured after applying the scatter and attenuation correction to the emission data. The correction technique did not introduce false negative results in these patients. The application of scatter and attenuation correction techniques shows promising results for the interpretation of myocardial perfusion studies. These correction algorithms however need to be investigated thoroughly before being used in the routine clinical practice to avoid the introduction of artefacts.Item Open Access Kwantifisering van radionukliedverspreiding deur planare beelding met 'n sintillasiekamera(University of the Free State, 2000-06) Marais, Johan; Van Aswegen, A.English: In this study attenuation and scatter correction techniques in planar imaging were investigated as well as the practical implementation of these techniques in clinical studies. The attenuation correction technique that was investigated was the geometrical mean method where an iterative method was used to determine the attenuation coefficient. This adaptation made the attenuation correction method independent of depth. In this study the depth dependence of the geometrical mean method was solved which is an important contribution. The three-energy window scatter correction technique was investigated with Monte Carlo methods for planar imaging. These simulations showed that the scatter correction method under corrects by approximately 10%. The scatter correction technique cannot be performed on it's own but must be used in conjunction with the attenuation correction method. The overall effect of the quantitation (attenuation and scatter correction) was determined in two ways. First a known amount of activity was imaged. Then the correction was applied to the images. Subsequently the corrected activity obtained from the latter images was compared to the known value. The calculated activity was 98,6 % of the known activity value. Secondly clinical studies were performed where the bladder activity was quantitated in vivo and correlated to the in vitro measured value. In this case the correlation coefficient was 0,996. The quantitation technique was applied to the clinical determination of the glomerular filtration rate. The quantitation technique was compared to the routine method that requires handling of urine. The correlation coefficient was 0.96 and the linear regression line had a slope ofO.91 and a y-axis intercept of4.8l. The quantitation of the bladder activity leads to a simplification of the clinical study procedure since the physical handling of urine can be eliminated. The accuracy of this method depends on the conversion factor that compensates for the difference between the sensitivity of the camera and the well counter. This factor must be checked on a regular basis to ensure the accuracy of this method. The bladder as source organ was used in this study. The bladder is a relative isolated organ and therefor the influence of radioactive uptake in other organs is relatively small. If these quantitations are applied to organs that are not isolated, i.e. the heart where the liver can influence it, this problem has to be address. In this project a quantitative planar imaging technique was investigated that corrects for attenuation and scatter and thereby facilitates depth independent corrections.