Scanning rodents on the High Resolution Research Tomograph (HRRT) with point spread function reconstruction: A feasibility study

The ECAT High Resolution Research Tomograph (HRRT) is a dedicated human brain PET camera with a 6% absolute sensitivity and a (2.3mm)³ spatial resolution, improving to (1.8mm)³ when point spread function (PSF) resolution recovery algorithms are used. These values are very close to those of the dedicated small animal PET camera microPET FOCUS 120 (F120). The larger axial and transaxial FoV of the HRRT compared to the F120 allows in principle for whole body imaging of several rats at the same time thus potentially reducing scanning costs and time. In this study we investigate the feasibility of using the HRRT for small animal brain studies by comparing the tissue input binding potentials (BP_ND) obtained from scans of the same rats imaged on the F120 and on the HRRT. The animal experiments are complemented by phantom studies aimed at investigating noise properties relevant to the size of typical regions of interest used in rat brain image analysis. Our investigations show that (i) without PSF modelling the BP_ND obtained from HRRT data are lower than those obtained on the F120 by 28%, (ii) with PSF modelling the BP_ND obtained from HRRT data are 15% higher than those obtained on the F120, (iii) the reproducibility on the HRRT is 81% when PSF modelling is not used, lower than the F120 reproducibility (92%), and decreases even further with PSF modelling to 73%. In addition Gibbs type artefacts are clearly visible when PSF modelling is used. In summary, while the resolution achieved with PSF modelling might be comparable to that obtained with the F120, the clumpy noise structure coupled with the introduction of Gibbs type artefacts in the images reconstructed with this technique currently preclude the use of the HRRT for reliable small animal brain imaging.