Optimization of energy window and multiple event acceptance policy for PETbox4, a high sensitivity preclinical imaging tomograph

PETBox4 is a preclinical system dedicated to imaging mice. This system is composed by four detector panels, each made by a 24 × 50 array of 1.825 × 1.825 × 7 mm BGO crystals. The face to face crystal separation of the detectors is 5 cm, generating a 4.5 × 4.5 × 9.4 cm field of view (FOV). The result is a tomograph with a large detection solid angle, which in combination with a wide open energy window achieves high peak detection efficiency (~17%). Despite the small size of the typical imaged subjects, these design features also increase the fraction of accepted crystal and object scattered events, which reduce the overall image signal to noise ratio. In this work, we investigated the system acquisition configuration settings necessary to optimize the NEC (Noise equivalent Counts) and image quality. A Monte Carlo simulation software package was used (GATE) to investigate the different types of events detected as a function of energy window settings and multiple event acceptance policy. This was done for a realistic distribution of activity and attenuation coefficients in the PETBox4 FOV, based on emission data from an in-vivo preclinical PET image from an average sized mouse (30g). Based on the simulations, the NEC rate was optimized for a dual energy window that accepts both low (50-175 keY) and high (410-650 keY) energy events. This indicates that low energy events that are composed mostly from single crystal scatter contribute useful image information, while events in the middle of the energy spectrum (175keV-410keV), tend to include large fractions of crystal backscatter as well as object scatter and do not contribute significantly in data signal to noise ratio. As a result of the same simulations, a new policy for the acceptance of multiple events was introduced, implementing a "KiIlAIl" multiple policy, further improving the NEe. Overall, these two optimization parameters improved the NEC rate by 56%, providing a significant advantage in image signal to noise ratio.