Respiratory synchronized CT image generation from 4D PET acquisitions

Respiratory motion reduces accuracy in image fusion from combined PET/CT systems. Solutions presented to date include respiratory synchronized PET and CT acquisitions. In order to increase the signal to noise ratio of the synchronized images the use of non-rigid transformations during the reconstruction process accounting for the respiratory motion has been proposed. In the majority of this work the 4D CT images have been used for the derivation of the necessary deformation maps. However, differences between acquired 4D PET and corresponding CT image series have been reported due to differences in respiration conditions during PET and CT acquisitions. In addition, the radiation dose burden resulting from a 4D CT acquisition may not be justifiable for every patient. In this paper we present a method for the generation of dynamic CT images from one reference CT image, based on deformation matrices obtained from the elastic registration of 4D non attenuation corrected PET images. Such an approach eliminates, on one hand the need for the acquisition of dynamic CT, while at the same time ensuring the good matching between CT and PET images allowing accurate attenuation correction to be performed for respiratory synchronized PET acquisitions. This method has been validated by generating 4D CT images for two patients and evaluated by comparison with the acquired CT images. Different levels of PET image statistical quality were also considered in order to investigate the impact of image noise in the derivation of the 4D CT series. Our results suggest that clinically relevant PET acquisition times can be used for the implementation of such an approach, making this an even more attractive solution considering the absence of extra dose given by a standard 4D CT acquisition.