Estimation of Electron Density from joint Reconstructions of Activity and Attenuation of ToF-PET Scans for Particle Therapy

The aim of the study is to provide a novel type of computed tomography (CT) image set for the dose calculation in particle therapy. These gamma-ray attenuation images (γCTs) are derived from time-of-flight PET (ToF-PET) scans utilizing the joint reconstruction of activity and attenuation, particularly the Maximum Likelihood estimation of Attenuation and Activity (MLAA). As the γCTs are established from 511 keV photons, there is an inherent linearity of the attenuation values of the voxels and the electron density. The latter is the most important material characteristics, which determines the slowing down of particles in matter. A phantom was built, which mimics the lower part of a human head. Inserts of varying electron density were immersed in FDG filled water solution within a cylindrical container. The grayscale levels of the reconstructed images were analyzed and compared with the material specification provided by the vendors of the inserts. A model to convert the material properties from the CT numbers was developed. The data from the images were in agreement with a linear model. In future investigations, the influence on the reduction of uncertainties in proton therapy treatment planning will be derived from the results.