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

The physical properties of ⁴⁴Sc, 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 ⁴⁴Sc on a commercial long-axial field-of-view (LAFOV) PET/CT.

A NEMA image quality phantom was filled with 41.7 MBq of ⁴⁴Sc 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 $4\times 4\times 4$ mm³ voxels.

All measured oPs lifetimes are compatible, within the uncertainties, with the literature values for water. The oPs lifetime is $2.65\pm 0.50$ , $1.39\pm 0.20$ and $1.76\pm 0.18$ ns in the three smallest spheres of the phantom and $1.79\pm 0.57$ 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.

Despite the favorable physical properties of ⁴⁴Sc, 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.