Masters Degrees (Medical Physics)
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Browsing Masters Degrees (Medical Physics) by Author "Du Raan, H."
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Item Open Access The effect of tumour geometry on the quantification accuracy of 99mTc and 123I in planar phantom images(University of the Free State, 2014-08) Ramonaheng, Keamogetswe; Van Staden, J. A.; Du Raan, H.English: Accurate activity quantification is important for its application in radiation dosimetry. Planar image quantification plays an important role in the quantification of whole body images which provide a full assessment of bio-distribution from radionuclide administrations. In the Department of Nuclear Medicine at Universitas Hospital, 123I meta-iodobenzylguanidine [123I]-MIBG quantification of neuroendocrine tumours is performed prior to therapeutic radionuclide treatment. The bio-distribution of activity in these studies is mostly in the abdominal region. Factors influencing quantification include scatter, attenuation, background activity and close proximity of organs with radioactivity uptake. The aim of this study was to evaluate the effect of tumour geometry on the quantification accuracy of 99mTc and 123I in planar phantom images, by applying scatter and attenuation corrections, with the focus on neuroendocrine tumours. The tumour geometry investigated included: various tumour sizes, various tumour-liver distances and two tumour-background ratios (0.5 % and 1.0 %). The quantification technique was first developed with the readily available 99mTc and subsequently applied to the more costly 123I used for imaging neuroendocrine tumours. Adjustments were necessary due to the difference in physical properties between the two isotopes. An in-house manufactured abdominal phantom was developed to mimic the clinical geometries under investigation. The phantom was equipped with cylindrical inserts used to simulate tumours (diameters of the tumours were 63 mm, 45 mm, 34 mm and 23 mm) and a slider to vary the tumour-liver distance. The processing technique incorporated the use of the geometric mean method with corrections for scatter and attenuation performed on image counts. Scatter correction was performed using a modified triple energy window scatter correction technique for 99mTc and 123I, according to gamma camera manufacturer specifications. Attenuation correction was performed using transmission images obtained with an uncollimated 99mTc printed source. Scatter contribution from the abdominal phantom and transmission source combination was limited by setting the detector transmission source distance to 73 cm. A system calibration factor, processed in the same manner as the tumour quantified data was used to convert the image counts to units of radioactivity. Partial volume effect (PVE), was compensated for by the manner in which regions for tumour activity distribution were defined. The activity measured in the dose calibrators served as a reference for determining the accuracy of the quantification. The largest percentage deviation was obtained for the smallest tumours. The average activity underestimations were 29.2 ± 1.3 % and 34.6 ±1.2 % for 99mTc and 123I respectively. These large underestimations observed for the smallest tumours were attributed to PVE, which diminished with increasing tumour sizes. Better quantification accuracy was observed for the largest tumour with overestimations of 3.3 ± 2.6 % and 3.1 ± 3.0 % for 99mTc and 123I respectively. PVE compensation resulted in improved quantification accuracy for all tumour sizes yielding accuracies of better than 9.1 % and 12.4 % for 99mTc and 123I respectively. Scatter contribution to the tumours from the liver had minimal effect on the quantification accuracy at tumour-liver distances larger than 3 cm. An increased tumour-background ratio resulted in an increase in the quantification results of up to 16.6 % for calculations without PVE compensation. This contribution was increased to 26.3 % when PVE were compensated for, using larger regions. The literature often report accurate planar quantification results, however, this study shows that it is important to consider the specific tumour geometry for the study. It remains the responsibility of the user to evaluate the clinical available software and implement it in a responsible manner. When applying all relevant corrections for scatter, attenuation and PVE without significant background, quantification accuracy within 12 % was obtained. This study has demonstrated successful implementation of a practical technique to obtain planar quantitative information.Item Open Access Evaluation 99mTc and 123I quantification using SPECT/CT(University of the Free State, 2015-02) Mongane, Modisenyane Simon; Van Staden, J. A.; Du Raan, H.English: A review of Single Photon Emission Tomography (SPECT) quantification shows that different protocols and phantoms are available to evaluate SPECT quantification accuracy. This study was necessitated by the lack of standardized protocols and the widespread use of a variety of non-standard phantoms. The aim of this work was to evaluate the influence of the geometry of a radionuclide distribution on SPECT quantification accuracy for 99mTc and 123I isotopes in an abdominal phantom. In order to achieve the aim, the following steps were taken: The preparatory phase of the study was to design and construct an abdominal phantom, verify the accuracy of the attenuation coefficients obtained with the Computed Tomography (CT) scanner, determine the accuracy of the source calibrator used in this study and then obtain a calibration factor in order to convert image counts to activity. During the quantification phase SPECT data were acquired, the influence of not applying scatter correction explicitly was evaluated and the final quantification was performed using the proposed standard clinical reconstruction protocols. The influence of different tumour sizes and locations in the abdominal phantom relative to a high uptake organ on the quantification accuracy was evaluated. Finally, parameters in the Ordered Subset Expectation Maximization (OSEM) reconstruction protocol were altered in order to investigate the influence of number of subsets and iterations on the quantified data. The non-standard Density Phantom with five different compounds was used for the verification of the 99mTc and 123I attenuation coefficients. The percentage difference between the measured and theoretical attenuation coefficients values were < 3%, except for Polystyrene (85% and 65% respectively). The SPECT calibration factor was determined for both 99mTc (11.0 ± 1.3 cpm/kBq) and 123I (10.8 ± 0.3 cpm/kBq) using the Cylindrical Phantom. The in-house built Abdominal Phantom was used to evaluate the tumour activity quantification accuracy. The quantification accuracy of 99mTc and 123I was found to change significantly (p < 0.05) as a function of tumour size after corrections for “spill out” counts due to the partial volume effect, scatter and attenuation were applied. On the other hand, there was no significant difference in the quantification accuracy (p > 0.05) for each tumour at different tumour-liver distances when appropriate scatter and attenuation corrections were applied. The influence of OSEM parameters showed no dependence on the tumour-liver distance and no significant difference (p > 0.05) between quantification with background activity as compared to no background activity. In conclusion, the study showed that the quantification accuracy for 99mTc and 123I was comparable to other published studies. It was found that the tumour quantification accuracy is not influenced by proximity of high uptake organs when appropriate correction factors were applied. Tumour size influenced the accuracy of SPECT quantification for both radionuclides. The results of this study also showed that at least 128 Maximum Likelihood Expectation Maximization (MLEM) equivalent iterations were needed during iterative reconstruction to achieve convergence and consistent SPECT quantification accuracy. Finally, it is recommended that the evaluated quantification protocol may be used in our nuclear medicine clinic for 99mTc and 123I quantification.Item 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 Quantification accuracy for I-123 SPECT/CT studies using LEHR and me collimators: a Monte Carlo study(University of the Free State, 2021) Richards, Anneray; Van Staden, J. A.; Du Raan, H.The accurate quantification of Nuclear Medicine single photon emission tomography (SPECT) plays an important part in radiopharmaceutical therapy. Accurately quantifying SPECT images of a diagnostic radionuclide such as I-123 is desirable, although not a straightforward process as it is hindered by the complex decay scheme. I-123 has low-energy primary emissions of 159 keV, and performing acquisitions with a low-energy resolution (LEHR) collimator result in images with high resolution. However, I-123 also has high-energy photon emissions which degrade image contrast and quantification accuracy. This degradation can be reduced by using medium-energy collimators (ME); however, at the expense of spatial resolution. Most clinical facilities have access to LEHR collimators, but not necessarily ME collimators. The aim of this study was to evaluate the quantification accuracy of I-123 LEHR and ME collimated SPECT images when an optimised OSEM reconstruction protocol is applied. To accomplish the aim three objectives were identified: 1) validation of a SIMIND modelled gamma camera fitted with LEHR and ME collimators, 2) optimisation of the iterative reconstruction algorithm in terms of equivalent iterations and SPECT corrections, and based on these results, 3) evaluation of the quantification accuracy of I-123 LEHR and ME SPECT images. The first objective of this study, to validate the SIMIND modelled gamma camera fitted with LEHR and ME collimators for I-123, involved comparing measured and simulated I-123 data. Results of measured and simulated planar performance tests (system energy resolution, system spatial resolution, and system sensitivity) were compared for both collimators. The validation included a visual comparison of reconstructed SPECT images of a quality control phantom in terms of uniformity, cold contrast, resolution, and linearity. The measured and simulated planar results for system energy resolution, system spatial resolution and system sensitivity differed by 3.4%, 6.4% and 5.3%, respectively. The visual comparison performed on the reconstructed SPECT images showed good agreement between the measured and simulated data. The second objective was to optimise the OSEM iterative reconstruction algorithm concerning the number of iterations and SPECT corrections. SPECT images of voxel-based phantoms of spherical objects and image quality phantoms were simulated and reconstructed with different numbers of effective iterations. The count density recovery, image noise, contrast and resolution were evaluated. The image quality phantom was also reconstructed with different corrections (attenuation, scatter and collimator-detector response (CDR)) and compared. The optimal number of equivalent iterations was selected as 64 and the contribution of the different corrections was appreciated. When septal penetration and scatter was compensated for as part of the CDR correction, the LEHR collimator results were comparable to that obtained with the ME collimator. This led to the aim of the final objective: to determine the quantification accuracy of I-123 SPECT studies in patient phantoms acquired with LEHR and ME collimators. Using voxel based patient phantoms, the quantification accuracy was assessed for LEHR and ME SPECT images of spherical objects. Quantification errors smaller than 3.8% were recorded for both the LEHR and ME collimators when attenuation, scatter and CDR (including septal penetration and scatter) corrections were applied. Therefore, to conclude, when appropriate SPECT corrections were applied during the reconstruction of I 123 LEHR and ME SPECT images, the image quality between the collimators were comparable and quantification accuracy of up to 3.8% was achievable.