Abstract
In small animal positron emission tomography (PET) studies, given the spatial resolution of preclinical PET scanners, quantification in small regions can be challenging. Moreover, in scans where animals are placed away from the center of the field of view (CFOV), e.g. in simultaneous scans of multiple animals, quantification accuracy can be compromised due to the loss of spatial resolution towards the edge of the FOV. Here, we implemented a spatially variant resolution model to improve quantification in small regions and to allow simultaneous scanning of multiple animals without compromising quantification accuracy. The scanner’s point spread function (PSF) was characterized across the FOV and modelled using a spatially variant and asymmetric Gaussian function. The spatially variant PSF (SVPSF) was then used for resolution modelling in the iterative reconstruction. To assess the image quality, a line source phantom in a cold and warm background, as well as mouse brain [18F]FDG scans, were performed. The SVPSF and the vendor’s maximum a posteriori (MAP3D) reconstructions produced uniform spatial resolution across the scanner FOV, but MAP3D resulted in lower spatial resolution. The line sources recovery coefficient using SVPSF was similar at the CFOV and at the edge of the FOV. In contrast, the other tested reconstructions produced lower recovery coefficient at the edge of the FOV. In mouse brain reconstructions, less spill-over from hot regions to cold regions, as well as more symmetric regional brain uptake was observed using SVPSF. The contrast in brain images was the highest using SVPSF, in mice scanned at the CFOV and off-center. Incorporation of a spatially variant resolution model for small animal brain PET improves quantification accuracy in small regions and produces consistent image spatial resolution across the FOV. Therefore, simultaneous scanning of multiple animals can benefit by using spatially variant resolution modelling.
Highlights
To improve the spatial resolution of positron emission tomography (PET) images, knowledge about the system point spread function (PSF) can be used in the reconstruction process
A simple resolution model uses a spatially invariant and isotropic Gaussian, with a width approximating the PSF width at the center of the scanner field of view (FOV)
Radioactive point sources can be measured in several positions in the scanner FOV, and the PSF shape can be parametrized with respect to the spatial location
Summary
To improve the spatial resolution of positron emission tomography (PET) images, knowledge about the system point spread function (PSF) can be used in the reconstruction process. A simple resolution model uses a spatially invariant and isotropic Gaussian, with a width approximating the PSF width at the center of the scanner field of view (FOV). Since the PSF of PET scanners is usually spatially variant and its shape does not follow a symmetric Gaussian, the isotropic Gaussian PSF is an approximation. More realistic models, which capture the asymmetric and spatially variant nature of the PSF, have been proposed (Lee et al 2004, Cloquet et al 2010). Radioactive point sources can be measured in several positions in the scanner FOV, and the PSF shape can be parametrized with respect to the spatial location (Panin et al 2006, Cloquet et al 2010). Monte Carlo simulations and analytical calculations of the detection process can be performed to circumvent the experimental measurement of the PSF (Alessio et al 2006, Zhang et al 2010)
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