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

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 vs. 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.