High-resolution Positronium Lifetime Imaging on the PennPET Explorer

Positronium lifetime imaging (PLI) has received growing interest because the lifetime of orthopositronium (o-Ps) can be affected by tissue microstructure and the concentration of certain molecules including O2. Previously we have developed several model-based lifetime image reconstruction methods and demonstrated their ability of reconstructing high-resolution lifetime images using point sources and small objects. In this study we leverage the high-performance of the PennPET Explorer scanner and its flexibility in singles data acquisition to perform highresolution PLI using ${ }^{82} \mathrm{Rb}$ on a large flow phantom and a cylinder phantom. The latter has a $6-\mathrm{mm}-$ thick polycarbonate insert to create lifetime contrast. List-mode data were acquired in singles mode with an extended energy window $[429,1533] \mathrm{keV}$. A software-based method was used to identify triple coincidence events with an annihilation photon energy window of [430, 560] keV and a prompt gamma energy window of $[700,835] \mathrm{keV}$ to capture the 777 keV gamma from ${ }^{82} \mathrm{Rb}$. The average positron lifetime images were reconstructed using the SIMPLE method. The reconstructed lifetime image of the flow phantom showed a uniform lifetime distribution inside the phantom, despite the very high randoms-to-trues (R/T) ratio. The cylinder phantom reconstruction showed the $6-\mathrm{mm}$ thick polycarbonate insert can be resolved in the lifetime image. These results clearly demonstrate the feasibility of performing high-resolution PLI for extended phantoms at activity levels relevant to clinical human imaging. In future work, we will extend the study to other radiotracers and perform in vivo animal and human imaging.