Parallax-free Total-Body PET using Monolithic Flat-Panel Detectors

Parallax error, caused by incorrect registration of lines of response (LORs) for oblique incidences, is an important factor limiting the spatial resolution of current (Total-Body) PET systems. We propose a novel concept for cost-effective, high-throughput and high-resolution Total-Body (TB-) PET: Walk-Through (WT) TB-PET. In this study, we investigate to what extent the parallax effect is present in the WT-PET, whose monolithic detector modules have intrinsic depth-of-interaction (DOI) decoding capabilities, allowing more accurate estimation of the LORs. To this end, the spatial resolution is evaluated and compared to that of the Biograph Vision Quadra, a non-DOI-capable clinical TB-PET device. Data is obtained in GATE by Monte Carlo simulation of point sources positioned along specific axes for both scanners. The WT-TB-PET shows approximately Gaussian behavior with equally narrow peaks (FWHM less than 2 mm) at all positions, proving that the device is (almost) unaffected by the parallax effect. The performance of the WT-TB-PET is superior compared to the Quadra, whose line profiles in the radial direction clearly show fingerprints of the parallax effect, characterized by broadening of the peaks and loss of Gaussian shape with increasing radial offsets. Cutting the maximal ring difference from 322 to 85 slightly improves the axial resolution. However, it does not reduce the radial parallax effect and leads to significant sensitivity losses (a factor 2.1).