Doctoral Degrees (Medical Physics)

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Browsing Doctoral Degrees (Medical Physics) by Author "Visvikis, D."

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    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.