Metrology for Quantitative Imaging and Molecular Radiotherapy
Creators: name: Wevrett, Jill L
Publisher: name: University of Surrey
Year: 2020

description: This thesis investigated quantitative SPECT/CT imaging of lutetium-177 (177Lu): assessing and quantifying variation in reported activity between clinical sites to help inform researchers attempting to compare quantitative imaging and molecular radiotherapy dosimetry performed at different clinical sites and to provide guidance on which methods and parameters to use in order to minimise activity uncertainty. The initial work involved assisting in the development of a practical, consensus-based calibration protocol for quantitative SPECT/CT imaging, followed by a pan-European hospital inter-comparison exercise in which participants were required to quantify the activity in a 177Lu source using local clinical protocols. The calibration protocol was also tested at each site. Finally, all calibration, imaging, reconstruction and segmentation methods and parameters used in the local clinical protocols were replicated on a single gamma camera to assess the impact of each method and parameter on activity quantification and make recommendations as to which methods and parameters to use for quantitative SPECT/CT imaging of 177Lu. The activities measured by the participants (seven clinical sites) differed from the true activity by up to 70% for an active volume surrounded by a less active shell and by up to 316% for the less active shell. Use of the calibration protocol (five clinical sites) reduced the difference between true and measured activity from a maximum of 19% to 12% for the active volume but increased the difference for the less active shell, from a maximum of 117% to 235%. The methods comparison work compared and identified methods and parameters that increased the accuracy of the measured activity volumes and values. In particular, the preferable calibration method was demonstrated to be one that uses objects of a similar size and geometry to the regions of interest for which quantification is required. In situations where this is not feasible, the use of planar Petri dishes performed best as a generic calibration method. Large volume dispersed sources performing best for quantifying areas of low activity located adjacent to areas of high activity. The use of the 208 keV photopeak only is recommended, as inclusion of the 113 keV photopeak decreases accuracy of quantitation. Careful use of post-reconstruction filters has been shown to have a beneficial effect on quantitative accuracy. Segmentation of image data should be performed using the CT data rather than SPECT data for volume delineation where possible, with thresholding based on the SPECT data followed by adjustments using the CT data giving improved results for activity quantification. This work revealed a general tendency to overestimate the accuracy with which volumes and activities can be reported. Uncertainties were underestimated by several of the participants in the inter-comparison exercise and the calibration protocol that was tested also gave underestimated uncertainties. Finally, the conclusions, recommendations and clinical impact of this work is presented, along with details of further work that could be performed to contribute to the body of research in this area.