Isotope dependent system matrices for high resolution PET imaging

Measuring and incorporating scanner specific point spread functions (PSFs) within image reconstruction has been shown to improve spatial resolution in reconstructed PET images. However, due to the short half-life of the clinically used isotopes, other long-lived isotopes not used in clinical practice are chosen to perform the PSF measurements, consequently leading to over or under estimation of the true PSF width during reconstruction, due to the difference in positron range. In high resolution brain and preclinical imaging, this effect is of particular importance since the resolution becomes more positron range limited and isotope-specific PSFs can help maximizing the performance benefit from using resolution recovery image reconstruction algorithms. In this work, we use a printing technique to simultaneously measure multiple point sources and demonstrate the feasibility of deriving isotope-dependent system matrices on the High Resolution Research Tomograph (HRRT) by measuring spatially-variant and isotope-specific PSFs using Fluorine-18 and Carbon-11. Initial results based on these 2 isotopes illustrate that even small differences in positron range can result in different PSF maps. The difference is more distinct in the centre of the field of view (FOV) where the full width at half maximum (FWHM) from the positron range has a larger contribution in the overall FWHM compared to the edge of the FOV, where the parallax error dominates the overall FWHM. Further PSF measurements are underway to evaluate clinically used isotopes with larger positron ranges and significantly shorter half-lives. These measurements could be used to create a database of isotope-dependent system matrices to be used within image reconstruction.