Advancing PET through Direct Imaging of Three-Photon Decay using Pure Positron Emitters

Positronium is abundantly formed inside the human body during positron emission tomography (PET). Its properties, such as lifetime and three-photon decay rate, are highly sensitive to the surrounding environment, particularly to the oxygen partial pressure and the size of free voids. Thus, positronium is expected to serve as a promising biomarker for a new imaging modality, complementary to the conventional PET system. In this study, we introduce a novel imaging technique based on the three-photon decay of positronium, achieved through the direct localization of three-photon annihilation events. The first PET-like image of the 3γ/2γ decay ratio was obtained using high-resolution GAGG scintillation crystal detectors and 18F-FDG as the positron source, and a 2.37(08)-fold increase in this ratio was observed when the source was mixed with XAD-4, a macroporous material. Notably, a spatial resolution of less than 1.1 cm was achieved in both experimental and simulated data, without relying on tomographic reconstruction or time-of-flight information. This technique is expected to provide additional information about tissue hypoxia or morphology in early diagnosis.