Simulation Study of Clinical PET Scanners With Different Geometries, Including TOF and DOI Capabilities

Abstract

Total body positron emission tomography (TB-PET) scanners provide high-quality images due to the large sensitivity. Our motivation is to design a TB-PET system with up to 70 cm axial coverage that mitigates the parallax error degradation by using a detector concept based on semi-monolithic LYSO crystals. Furthermore, this detector approach allows to simultaneously reach an accurate coincidence time resolution (CTR) to enhance the image quality by means of time-of-flight (TOF) reconstruction algorithms. We have simulated and compared two positron emission tomography (PET) prototypes with about 70 cm but a different number of detector rings (7 versus 5). The NEMA NU 2 2018 protocol has been implemented. By correcting the parallax error with the depth-of-interaction (DOI) information, the spatial resolution remains homogeneous and below 3 mm in the entire field of view (FOV), differently from designs based on pixelated crystals. The sensitivity reaches values of 58 and 115 cps/kBq, for the 5 and 7 rings configurations, respectively. The noise equivalent count rate (NECR) was found at 563 kcps/mL. This value is lower than other systems, most likely due to the requirement to process a larger number of channels to characterize the DOI. Percent contrasts obtained for two different phantoms are in general beyond 80% for the largest spheres, nearly 100% for the 7 rings configuration once TOF is applied during the reconstruction process. In conclusion, although the sensitivity and NECR results for the 5-rings configuration are lower compared to the 7-rings approach, its overall performance is enhanced by the addition of TOF and parallax error correction, improving that of conventional Whole Body PET scanners (axial length: 20–30 cm) in terms of image quality.