Image quality analysis of 44 Sc on two preclinical PET scanners: a comparison to 68 Ga

Sc has been increasingly investigated as a potential alternative to Ga in the development of tracers for positron emission tomography (PET). The lower mean positron energy of Sc (0.63 MeV) compared to Ga (0.83 MeV) can result in better spatial image resolutions. However, high-energy γ-rays (1157 keV) are emitted at high rates (99.9%) during Sc decay, which can reduce image quality. Therefore, we investigated the impact of these physical properties and performed an unbiased performance evaluation of Sc and Ga with different imaging phantoms (image quality phantom, Derenzo phantom, and three-rod phantom) on two preclinical PET scanners (Mediso nanoScan PET/MRI, Siemens microPET Focus 120). Despite the presence of high-energy γ-rays in Sc decay, a higher image resolution of small structures was observed with Sc when compared to Ga. Structures as small as 1.3 mm using the Mediso system, and as small as 1.0 mm using the Siemens system, could be visualized and analyzed by calculating full width at half maximum. Full widths at half maxima were similar for both isotopes. For image quality comparison, we calculated recovery coefficients in 1-5 mm rods and spillover ratios in either air, water, or bone-equivalent material (Teflon). Recovery coefficients for Sc were significantly higher than those for Ga. Despite the lower positron energy, Sc-derived spillover ratio (SOR) values were similar or slightly higher to Ga-derived SOR values. This may be attributed to the higher background caused by the additional γ-rays. On the Siemens system, an overestimation of scatter correction in the central part of the phantom was observed causing a virtual disappearance of spillover inside the three-rod phantom. Based on these findings, Sc appears to be a suitable alternative to Ga. The superior image resolution makes it an especially strong competitor in preclinical settings. The additional γ-emissions have a small impact on the imaging resolution but cause higher background noises and can effect an overestimation of scatter correction, depending on the PET system and phantom.