Doctoral Degrees (Medical Physics)

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Browsing Doctoral Degrees (Medical Physics) by Author "Du Raan, H."

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    ItemOpen 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.
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    ItemOpen 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.
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    ItemOpen 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.
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    ItemOpen 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 available