Experimental investigation of irregular motion impact on 4D PET based particle therapy monitoring

Particle therapy positron emission tomography (PT-PET) is an in vivo and non-invasive imaging technique to monitor treatment delivery in particle therapy. The inevitable patient respiratory motion during irradiation causes artefacts and inaccurate activity distribution in PET images. 4D maximum likelihood expectation maximization (4D MLEM) allows for a compensation of these effects, but has up to now been restricted to regular motion for PT-PET investigations. However, intra-fractional motion during treatment might differ from that during acquisition of the 4D-planning CT (e.g. amplitude variation, baseline drift) and, therefore, might induce inaccurate 4D PET reconstruction results. This study investigates the impact of different irregular analytical motion patterns on PT-PET imaging by means of experiments with radioactive source and irradiated moving phantoms. Three sorting methods, namely phase sorting, equal amplitude sorting and event-based amplitude sorting, were applied to manage PET list-mode data. The inuence of these sorting methods on the motion compensating algorithm has analysed. The event-based amplitude sorting presented superior performance, although the 4D PT-PET reconstructions still suffered partly from the inexact deformation specifications due to the irregular motion.