First positronium lifetime imaging with scandium-44 on a long axial field-of-view PET/CT

Purpose The physical properties of 44 Sc, combined with its imminent clinical application, position it as a prime candidate for in vivo positronium lifetime imaging. In this study, we investigate the count statistics for ortho-positronium (oPs) measurements with 44 Sc on a commercial long-axial field-of-view (LAFOV) PET/CT. Method A NEMA image quality phantom was filled with 41.7 MBq of 44 Sc dissolved in water and scanned on a LAFOV PET/CT. Three-photon events were identified using a prototype feature of the scanner and dedicated software. The lifetime of oPs was determined in the phantom spheres and in 4 × 4 × 4 mm 3 voxels. Results All measured oPs lifetimes are compatible, within the uncertainties, with the literature values for water. The oPs lifetime is 2.65 ± 0.50 , 1.39 ± 0.20 and 1.76 ± 0.18 ns in the three smallest spheres of the phantom and 1.79 ± 0.57 ns for a single voxel in the central region of the largest sphere. The relative standard deviation in the background regions of the time difference distributions, i.e., for time differences smaller than − 2.7 ns, is above 20%—even for voxels inside the phantom spheres. Conclusions Despite the favorable physical properties of 44 Sc, the count statistics of three-photon events remains a challenge. The high prompt-photon energy causes a significant amount of random three-photon coincidences with the given methodology and, therefore, increases the statistical uncertainties on the measured oPs lifetime.