Study of PET scanner designs using clinical metrics to optimize the scanner axial FOV and crystal thickness

The aim of this study is to understand the trade-off between crystal thickness and scanner axial field-of-view FOV (AFOV) for clinical PET imaging. Clinical scanner design has evolved towards 20-25 mm thick crystals and 16-22 cm long scanner AFOV, as well as time-of-flight (TOF) imaging. While Monte Carlo studies demonstrate that longer AFOV and thicker crystals will lead to higher scanner sensitivity, cost has prohibited the building of commercial scanners with >22 cm AFOV. In this study, we performed a series of system simulations to optimize the use of a given amount of crystal material by evaluating the impact on system sensitivity and noise equivalent counts (NEC), as well as image quality in terms of lesion detectability. We evaluated two crystal types (LSO and LaBr3) and fixed the total crystal volume used for each type (8.2 L of LSO and 17.1 L of LaBr3) while varying the crystal thickness and scanner AFOV. In addition, all imaging times were normalized so that the total scan time needed to scan a 100 cm long object with multiple bed positions was kept constant. Our results show that the highest NEC cm-1 in a 35 cm diameter ×70 cm long line source cylinder is achieved for an LSO scanner with 10 mm long crystals and AFOV of 36 cm, while for LaBr3 scanners, the highest NEC cm-1 is obtained with 20 mm long crystals and an AFOV of 38 cm. Lesion phantom simulations show that the best lesion detection performance is achieved in scanners with long AFOV (≥36 cm) and using thin crystals (≤10 mm of LSO and ≤20 mm of LaBr3). This is due to a combination of improved NEC, as well as improved lesion contrast estimation due to better spatial resolution in thinner crystals. Alternatively, for lesion detection performance similar to that achieved in standard clinical scanner designs, the long AFOV scanners can be used to reduce the total scan time without increasing the amount of crystal used in the scanner. In addition, for LaBr3 based scanners, the reduced lesion contrast relative to LSO based scanners requires improved timing resolution and longer scan times in order to achieve lesion detectability similar to that achieved in an LSO scanner with similar NEC cm-1.