First Positronium Lifetime Imaging with Scandium-44 on a Long Axial Field-of-view PET/CT

Lorenzo Mercolli^1,2*William M Steinberger³Pascal Grundler^4,5Anzhelika Moiseeva^4,5Saverio Braccini⁶Maurizio Conti³Paweł Moskal^7,8Narendra Rathod^1,2Axel Rominger²Hasan Sari^1,2,9Roger Schibli^4,5Robert Seifert^1,2Kuangyu Shi^1,2Ewa L. Stepien^7,8Nicholas P. van der Meulen^4,5

• ¹Universitat Bern ARTORG Center for Biomedical Engineering Research, Bern, Switzerland
• ²Inselspital Universitatsspital Bern Universitatsklinik fur Nuklearmedizin, Bern, Switzerland
• ³Siemens Medical Solutions USA, Inc., Knoxville TN, USA, Knoxville, United States
• ⁴Center for Radiopharmaceutical Sciences ETH-PSI-USZ, Paul Scherrer Institute, Villigen, Switzerland, Villigen, Switzerland
• ⁵Laboratory of Radiochemistry, Paul Scherrer Institute, Villigen, Switzerland, Villigen, Switzerland
• ⁶Universitat Bern Laboratorium fur Hochenergiephysik, Bern, Switzerland
• ⁷Uniwersytet Jagiellonski w Krakowie Wydzial Fizyki Astronomii i Informatyki Stosowanej, Kraków, Poland
• ⁸Uniwersytet Jagiellonski w Krakowie Center for Theranostics, Kraków, Poland
• ⁹Siemens Healthineers International AG, Zürich, Switzerland, Zürich, Switzerland

Purpose: 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. Method: 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 4x4x4 mm3 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 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.