Abstract
The best Coincidence Time Resolution (CTR) obtained so far – with very short crystals of 3–5mm in length – reach values between 100 and 150ps. Such crystals are not really practical for a TOF PET imaging device, since the sensitivity is quite small for the detection of the 511keV gammas resulting from a positron annihilation. We present our setup and measurements using 15mm length crystals; a length we regard as reasonable for a TOF-PET scanner. We have used a new series of Silicon Photo-Multipliers (SiPM) manufactured by Hamamatsu. These are the High Fill Factor (HFF) and Low Cross-Talk (LCT) Multi-Pixel Photon Counters (MPPC). We have compared three different crystals, LFS-3 (supplied by Zecotek) and two samples of LYSO (manufactured by Saint Gobain and CPI). We have obtained an excellent value of 148 ps for the Coincidence Time Resolution (CTR) with two LFS-3 crystals (15mm long) mounted on each side of a 22Na radioactive source with the HFF-MPPCs at 3.3V over-voltage. Our results are148ps obtained with LFS-3 and 170ps with LYSO crystals using identical SiPMs and electronics.
Highlights
The precise measurements of the time and the energy of 511 keV gammas interacting in scintillating crystals are nowadays the major challenge for TOF-PET scanners
The decay time of the LFS-3 is shorter than the two types of LYSO; this implies that the LFS-3 emits more photons at the beginning of the light pulse, and this aids the time resolution
A critical parameter concerning the timing is the number of photoelectrons that we observe in the Multi-Pixel Photon Counters (MPPC) corresponding to the 511 keV photopeak
Summary
The precise measurements of the time and the energy of 511 keV gammas interacting in scintillating crystals are nowadays the major challenge for TOF-PET scanners. The decay time of the LFS-3 is shorter than the two types of LYSO; this implies that the LFS-3 emits more photons at the beginning of the light pulse, and this aids the time resolution. K. Doroud et al / Nuclear Instruments and Methods in Physics Research A 793 (2015) 57–61 crystals with very short decay times may suffer from lower PDE. Doroud et al / Nuclear Instruments and Methods in Physics Research A 793 (2015) 57–61 crystals with very short decay times may suffer from lower PDE In this case the wavelength of the emitted light from both types of crystal is close to the maximum of the PDE of the MPPCs. mounting the two crystals on each side of a 22Na source; and the number of SPADs that fire corresponding to the light generated by a 511 keV gamma interaction (photopeak).
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