  Positron Emission Tomography (PET) using time-of-flight information, which
can greatly improve the quality of the reconstructed image, has recently become
an exciting topic. In this work, 3.2mm*3.2mm*25mm finger-like LYSO crystals
were used to make a 5*5 array, coupled to the Hamamatsu H8500 photomultiplier
tube (PMT) as a detector. A fast leading-edge discriminator was designed for
the LYSO-H8500 detector. Average coincidence time resolution FWHM of 330 ps was
obtained for the LYSO detector with a reference BaF2 detector, whose time
resolution for 511 keV {\gamma}-rays was FWHM 150 ps. Time resolution FWHM of
294 ps was calculated for the LYSO detector, and coincidence time resolution of
FHWM 415 ps can be expected for two identical LYSO detectors.
