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Browsing Medical Physics by Author "Du Raan, H."
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Item Open Access Accuracy of iodine-131 activity quantification and dosimetry for three-dimensional patient-specific models(University of the Free State, 2019-03) Ejeh, John Enyi; Van Staden, J. A.; Du Raan, H.Iodine-131 (131I) therapy of thyroid related and other diseases is limited by critical organ toxicity. Therefore, accurate activity quantification and dose calculation are important to optimise dose to tumours while limiting dose to critical organs. The aim of this study was to evaluate the accuracy of 131I activity quantification and dosimetry for three-dimensional (3-D) patient-specific models. Retrospective patient Computed Tomography (CT) data were segmented to create clinically realistic patient 3-D voxel-based models. These were used to simulate Single Photon Emission Computed Tomography (SPECT) data with a Monte Carlo (MC) simulation software, which was validated against physical measurements. The simulated SPECT data were reconstructed using an ordered-subsets expectation maximization (OS-EM) algorithm which includes scatter correction, CT-based attenuation correction, and 3-D collimator-detector response compensation. Predetermined recovery coefficients were used to compensate for partial volume effects. Image counts were converted to activity by using a predetermined calibration factor. The patients’ reconstructed activity maps and density maps were used to perform 3-D dosimetry with the MC program, LundADose. LundADose calculated mean tumour and organ absorbed doses were compared with OLINDA/EXM calculated mean absorbed doses using statistical analysis. Validation of the simulation software resulted in a percentage difference of -6.50 % between the measured and simulated extrinsic energy resolution at the 131I peak energy of 364 keV and - 18.57 % error for the measured and simulated intrinsic energy resolution. The measured and simulated FWHM and FWTM of the camera for system spatial resolution had percentage differences of -7.41% and -7.38 % and an error of -1.50 % and -2.6 % for system sensitivity and collimator septal penetration fraction. SPECT activity quantification was evaluated by comparing the true tumour activities defined for the patient models with the quantified activities obtained from the models’ reconstructed SPECT images. The quantification error for the studied patient models was < 9.0 % and < 5.1 % for 3.0 and 6.0 cm spherical tumours situated in the lungs (mean values were 3.9 ± 3.3 % and -1.6 ± 1.9 %). The error for the two tumours in the liver was < 11.2 % (mean values of 7.7 ± 3.9 % and 8.4 ± 2.9 %). The mean percentage differences between the mean absorbed doses calculated by LundADose and OLINDA/EXM for the left lung, right lung, liver, 3.0 cm ‘tumour’ and 6.0 cm ‘tumour’ were comparable. These mean percentage differences were -2.23 ± 1.98 %, -3.06 ± 1.67 %, 1.31 ± 4.15 %, -28.44 ± 18.36 %, and -5.10 ± 2.87% for the listed organs and tumours when the 3.0 cm tumour was located in the lung and the 6.0 cm tumour in the liver. For the scenario where the 3.0 cm tumour was positioned in the liver and the 6.0 cm tumour in the lung, the corresponding results were -2.84 ± 3.42 %, -1.49 ± 2.68 %, 3.97 ± 4.12 %, -28.80 ± 5.05 %, - 8.21 ± 17.06 %. The SIMIND MC model of the gamma camera was accurately validated with good agreement between results calculated from the physical measurements and simulation. Good accuracy of 131I activity quantification and 3-D dosimetry was found for 3-D patient-specific models. Statistical analysis of the results of the comparison of LundADose and OLINDA/EXM showed that the two dosimetry programs were strongly correlated with R2 values ranging from 0.85 to 1.00 for the mean absorbed dose in the various organs and tumours. Furthermore, the two (MC and MIRD) methods were found to agree well using Bland-Altman analysis of the dosimetry results. For 131I, activity quantification and dosimetric accuracy better than 10 % were achieved using state-of-the-art hybrid equipment and sophisticated correction methods for image degrading factors.Item Open Access Accuracy of patient-specific dosimetry using hybrid planar-SPECT/CT imaging: a Monte Carlo study(University of the Free State, 2021-07) Morphis, Michaella; Van Staden, J. A.; Du Raan, H.; Du Plessis, F. C. P.Introduction: Theragnostics is a precision medical discipline aiming to individualise patient targeted treatment. It aims at treating cancer by the systemic administration of a therapeutic radiopharmaceutical, which targets specific cells based on the labelling molecule. With the renewed interest in radiopharmaceutical therapy, the importance of accurate image quantification using iodine-123 (I-123) and iodine-131 (I-131), for dosimetry purposes, has been re-emphasised. Monte Carlo (MC) modelling techniques have been used extensively in Nuclear Medicine (NM), playing an essential role in modelling gamma cameras for the assessment of activity quantification accuracy, which is vital for accurate dosimetry. This thesis aimed to assess the accuracy of patient-specific I-131 dosimetry using hybrid whole-body (WB) planar-SPECT/CT imaging. The study was based on MC simulations of voxel-based digital phantoms, using the SIMIND MC code emulating the Siemens SymbiaTM T16 gamma camera. To achieve the aim, the thesis was divided into four objectives, (i) validating the accuracy of an energy resolution (ER) model, (ii) verifying the SIMIND setup for simulation of static, WB planar and SPECT images for I-123 with a low energy high resolution (LEHR) and a medium energy (ME) collimator and for I-131 with a high energy (HE) collimator, (iii) evaluating SPECT quantification accuracy for the three radionuclide- collimator combinations and (iv) assessing the accuracy of I-131 absorbed dose calculations for tumours and organs at risk, based on hybrid WB planar-SPECT/CT imaging. Methodology: The proposed ER model was fit to measured ER values (between 27.0 and 637.0 keV) as a function of photon energy. Measured and simulated energy spectra (in-air, in-scatter and a voxel-based digital patient phantom) were compared. The SIMIND setup was validated by comparing measured and simulated static and WB planar (extrinsic energy spectra, system sensitivity and system spatial resolution in-air and in-scatter), as well as SPECT (simple geometry sensitivity) results. Quantification accuracy was assessed in voxel- based digital simple and patient phantoms, using optimised OS-EM iterative reconstruction updates, calibration factor and recovery curves. Finally, using the true and quantitative activity data from I-123 and I-131 voxel-based digital patient phantoms, full MC radiation transport was performed, to determine the accuracy of the absorbed dose for I-131-mIBG radiopharmaceutical therapy. Results: The fitted ER model better simulated the energy response of the gamma camera, especially for high energy photopeaks, (I-123: 528.9 keV and I-131: 636.9 keV). The measured and simulated system energy spectra (differences ≤ 4.6 keV), system sensitivity (differences ≤ 6.9%), system spatial resolution (differences ≤ 6.4%) and SPECT validation results (difference ≤ 3.6%) compared well. Quantification errors less than 6.0% were obtained when appropriate corrections were applied. I-123 LEHR and I-123 ME quantification accuracies compared well (when corrections for septal scatter and penetration are applied), which can be useful in departments that perform I-123 studies and may not have access to ME collimators. Average I-131 absorbed doses of 2.0 ± 0.4 mGy/MBq (liver), 20.1 ± 4.0 mGy/MBq (3.0 cm tumour) and 22.6 ± 4.2 mGy/MBq (5.0 cm tumour) were obtained in simulated patient studies. When using a novel method of replacing the reconstructed activity distribution with a uniform activity distribution, eliminating the Gibbs artefact, the dosimetry accuracy was within 10.5%. Conclusion: Using the proposed fitted ER model, SIMIND could be used to accurately simulate static and WB planar and SPECT projection images of the Siemens SymbiaTM T16 SPECT/CT for both I-123 and I-131 with their respective collimators. Accurate quantification resulted in absorbed dose accuracies within 10.5%. The hybrid WB planar-SPECT/CT dosimetry method proved effective for personalised treatment planning of I-131 radiopharmaceutical therapy, with either I-123 or I-131 diagnostic imaging.Item Open Access Assessment of factors affecting accuracy of standardised uptake values in positron emission tomography(University of the Free State, 2015-01) Du Toit, Petrus Daniel; Du Raan, H.; Rae, W. I. D.; Visvikis, D.English: Positron emission tomography (PET) is an imaging method that uses tracers labelled with positron emitting isotopes for the monitoring and evaluation of in vivo molecular processes. Semi-quantitative determination of tracer uptake in a lesion is accomplished by calculating the standardised uptake value (SUV), an index that represents the amount of uptake in a given volume-of-interest (VoI) in relation to the average uptake throughout the body. The SUV is influenced by biological and physical factors that determine the uptake or detectability of the tracers which may result in false results. Changes in SUV of small lesions or lesions with low activity uptake cannot be determined with enough certainty and precision to be used for decision-making and it is therefore necessary to investigate the factors affecting the SUV. The aim of this study was to assess the relative importance of the physical factors that affect the accuracy of a single SUV measurement using Monte Carlo modelling. Phantom studies were performed to determine the influence of the partial volume effect due to spatial resolution using a PET scanner. Comparative Monte Carlo simulations were performed on a computer cluster using a voxelised version of the same phantom. The XCAT anthropomorphic phantom was used to assess the influences on SUV in a human-like configuration and was set-up to simulate movement in the thorax during breathing. SUVs were calculated using simulations of the phantom in 2D and 3D modes to assess the influence of the partial volume effect by variation of the size of the lesions, by variation of the contrast ratios and by placing the lesions in different areas in the lungs during. Influence of activity from outside the field-of-view (FoV) was also assessed as well as the impact the various coincidence types have. Statistical methods were used to compare the difference in data for statistical significance. It was found that the partial volume effect was present when evaluating the SUVs of the activity in the spheres of the phantom when scanned on a PET/CT scanner as well as when performing Monte Carlo simulations. Statistically there were no significant differences between the two scanning modes. The mean SUV increased as the voxel sizes became smaller. The choice of matrix influenced the amount of partial volume effect. The relative contributions of true-, scatter- and random coincidences demonstrated that the true coincidences were the major contributor when assessing the data from this phantom. The relative contribution of the trues-to-total coincidences decreased with a decrease in lesion size and contrast ratio whereas the relative contributions of the scattered- and random coincidences increased. The contributions of scatters and randoms increased during the 3D acquisition mode compared to 2D mode. The contribution of the trues-to-total coincidences decreased with an increase in VoI size and consequently caused a decrease in the mean SUV. The location of the lesion made a difference in SUV when the same size lesions are compared to each other. Apical lesions experienced the least amount of motion during breathing, were distorted less and had the least amount of variation in SUV. By moving the phantom partly outside the FoV, significant effects on the SUVs of objects still inside the FoV were found. An increase in the SUVs was observed when the true coincidences were used for the calculation. A decrease in true SUVs was found at the right basal lesion. In conclusion, partial volume effects play a significant role when determining the SUV of objects based on their size and contrast ratio; the location of pulmonary lesions affects SUV calculation during breathing; and activity outside the field-of-view of the scanner contributed to a change in SUV in particular to the central and basal regions of the lung.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 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.