High-Resolution Positronium Lifetime Tomography at Clinical Activity Levels on the PennPET Explorer

The objective of this study is to demonstrate high-resolution positronium lifetime tomography using a 3-dimensional phantom with multiple radioisotopes at activity levels relevant to human imaging. Methods: A cylinder phantom was constructed with a 6-mm thick polycarbonate slab inserted at its center. The phantom was filled with radioactive solutions and scanned in the PennPET Explorer scanner, a long-axial-field-of-view scanner with high sensitivity. Four scans were conducted with 3 different radioisotopes at varying activity levels: ⁸²Rb (240 MBq), ⁶⁸Ga (110 MBq), and ⁴⁴Sc (7.4 and 40.7 MBq). The average positron lifetime images were reconstructed with correction for random events. Radial and axial resolutions were measured from lifetime profiles across a hole on the slab and through the slab, respectively. Results: The polycarbonate slab was resolvable in all reconstructed positron lifetime images and showed a lifetime longer than that in water. The lifetime images from ⁸²Rb and ⁶⁸Ga were noisy because of their low prompt γ yields and high random fractions, whereas noise was substantially reduced in the ⁴⁴Sc images. The average lifetime estimates were consistent across the 4 scans. The estimated radial and axial resolutions were 3.7 ± 1.8 mm and 3.9 ± 0.4 mm, respectively. Conclusion: This study successfully demonstrated the feasibility of high-resolution positronium lifetime tomography using ⁸²Rb, ⁶⁸Ga, and ⁴⁴Sc on the PennPET Explorer.