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

⁴⁴Sc has been increasingly investigated as a potential alternative to ⁶⁸Ga in the development of tracers for positron emission tomography (PET). The lower mean positron energy of ⁴⁴Sc (0.63 MeV) compared to ⁶⁸Ga (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 ⁴⁴Sc decay, which can reduce image quality. Therefore, we investigated the impact of these physical properties and performed an unbiased performance evaluation of ⁴⁴Sc and ⁶⁸Ga 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 ⁴⁴Sc decay, a higher image resolution of small structures was observed with ⁴⁴Sc when compared to ⁶⁸Ga. 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 ⁴⁴Sc were significantly higher than those for ⁶⁸Ga. Despite the lower positron energy, ⁴⁴Sc-derived spillover ratio (SOR) values were similar or slightly higher to ⁶⁸Ga-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, ⁴⁴Sc appears to be a suitable alternative to ⁶⁸Ga. 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.