Evaluation of scatter fraction and count rate performance of two small-animal PET scanners using dedicated phantoms

Positron Emission Tomography (PET) image quality deteriorates as the object size increases owing to increased detection of scattered and random events. The characterization of the scatter component in small animal PET imaging has received little attention owing to the small scatter fraction when imaging rodents. The purpose of this study is first to design and fabricate a dedicated cone-shaped phantom which can be used for measurement of object size dependent SF and noise equivalent count (NEC) rates and second, to evaluate those parameters for two small animal PET scanners, namely the X-PET™ and LabPET™-8 as function of radial offset, object size and lower energy threshold. Both scanners were modeled as realistically as possible using GATE Monte Carlo simulation platform. The simulation models were validated against experimental measurements in terms of sensitivity, SF and noise equivalent count rate (NECR). The dedicated phantom was designed and fabricated in-house using high-density polyethylene. The optimized dimensions of the cone-shaped phantom are 150 mm (length), 20 mm (minimum diameter), 70 mm (maximum diameter) and taper angle of 9°.The relative difference between simulated and experimental results for the LabPET™-8 scanner varied between 0.66% and 10% except for few results where it was below 16%. Depending on the radial offset from the axial centre for a central field of view (3-6 cm diameter), the SF for the cone-shaped phantom varied from 26.3 to 18.2% (X-PET™); 34.4 to 26.9% (LabPET™-8), 18.6 to 13.1% (X-PET™); 19.1 to 17.0% (LabPET™-8); 10.1 to 7.6% (X-PET™) and 9.1 to 7.3% (LabPET™-8) for lower energy thresholds of 250, 350 and 425 keV, respectively. The SF increases as the radial offset decreases, lower energy threshold (LET) decreases and object size increases. The SF values are higher for the LabPET™-8 compared to the X-PET™. The NECR increases as the- radial offset increases and object size decreases. Maximum NECR was obtained at a LET of 350 keV for LabPET™-8 whereas 250 keV for X-PET™. High correlation coefficients (R²) for SF and NECR were observed between the cone-shaped phantom and an equivalent volume cylindrical (EVC) phantom for the three considered axial fields-of-view. A single cone-shaped phantom enables the assessment of effects of three factors, namely radial offset, energy threshold and object size on small animal PET imaging characteristics like SF and NECR. Hence, a cone-shaped phantom may be more suited for evaluation of object size-dependent SF and NECR instead of using various discrete phantoms of different size.