Comparative timing measurements of LYSO and LFS to achieve the best time resolution for TOF-PET

There are research reports available presenting a Coincidence Time Resolution (CTR) below 150 ps; however often these measurements are made with short crystals of 3 or 5 mm in length. Such crystals have limited efficiency for the detection of the 511 keV gammas resulting from a positron annihilation, therefore are not really practical for a TOF-PET imaging device. We present our setup and measurements using 15 mm 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 two types of crystal, LFS (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 crystals (15 mm long) mounted on each side of a 22Na radioactive source with the HFF-MPPCs at 3.3 V over-voltage. This is in comparison to a CTR of 180 ps measured with the LYSO crystals.