Studies of attenuation effects in two- and three-photon positronium decays in phantom models

<ns3:p>Objective: Positron Emission Tomography enables non-invasive imaging of metabolic processes. Standard PET reconstructs radiotracer distribution using two back-to-back photons from electron-positron annihilation. However, about 1% of annihilations results in creation of three photons. Three photons may be created in direct annihilation and in annihilation via formation of metastable ortho-positronium carrying additional information not used by conventional PET. The Jagiellonian-PET detector allows us to use this information by applying positronium imaging based on ortho-positronium lifetime or the 3γ/2γ decay ratio. Accurate tomographic images require attenuation correction. This study investigates photon absorption in phantom models to form a basis for future attenuation maps for three-gamma decays. Methods: Monte Carlo simulations in ROOT and GATE were performed for water sphere, cylinder, a simplified head model, and the mesh50_XCAT phantom. Both p-Ps and o-Ps decays were simulated as uniformly distributed sources. Absorption probabilities were calculated by checking whether any photon in a multiplet interacted within the phantom. Emission-point-specific absorption maps were generated for all models. Toy Monte Carlo and GATE simulations showed good overall agreement. Results: Photon triplets from o-Ps decays experienced higher absorption than photon pairs from p-Ps due to lower individual energies and higher attenuation. Absorption maps showed dependence of photon survival probability on the decay location. In the mesh50_XCAT phantom, 24.9% of p-Ps pairs and 10.3% of o-Ps triplets escaped without interaction. Conclusions: Gamma absorption depends strongly on positronium decay mode and location, with o-Ps events experiencing higher attenuation. The generated absorption maps provide the first step toward dedicated attenuation correction for three-gamma positronium imaging, enabling accurate reconstruction in novel 3γ/2γ positronium imaging technique. A Study presented indicates that the absorption of 3γ in the head is only about 2.5 times higher than for the 2γ, which is encouraging for further development of the 3γ/2γ rate ratio imaging.</ns3:p>