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Positron emission tomography (PET)-based on time-of-flight (TOF) is desired to enhance the image quality by improving the noise variance in the reconstruction. This benchtop study evaluates the timing performance of the current Hi-Rez detector which is based on a 13×13 LSO array of 4 mm×4 mm×20 mm crystals and not optimized for TOF PET applications. A Hamamatsu R9779, fast, 51-mm-diameter PMT was used for some experiments in this study as a reference PMT and its time resolution was measured to be 160 ps with a plastic scintillator. The average crystal time resolution of two Hi-Rez detectors, coupled to a 2×2 array of Hamamatsu 25-mm-diameter R8619 PMTs at +1050 V, in coincidence was measured to be 733 ps. The average crystal energy resolutions for two detectors were measured 13.1% and 13.2%. The average crystal time resolution of two Hi-Rez detectors improved from 733 ps to 642 ps when the PMT voltage was increased to +1300 V. The Hi-Rez detector was originally optimized for good energy resolution to reduce scatter and high packing fraction to obtain high sensitivity and improved spatial resolution for clinical PET.