Comparison of two light reflector patterns designed for PMT-Quadrant-Sharing (PQS) Time-of-Flight PET detectors

In this paper we compared both the timing resolution and the 2-dimension (2D) decoding map of two different reflector patterns of PMT-Quadrant-Sharing (PQS) block to obtain an optimized detector design for Time-of-Flight (TOF) PET application. Light reflectors (ESR mirror film from 3M Co.) in a PQS scintillation crystal block control the light-sharing ratio and light spread angle to the PMTs that determine the quality of the 2D decoding map. The objective of this study is to investigate the timing affected by reflector length. We built two 13×13 LYSO PQS blocks (4×4×20 mm³) with different reflector patterns (mask #1 and #2). Mask #1 is a decoding-optimized reflector pattern developed by our lab. Mask #2 is 2 mm in average shorter than the mask #1. A 4×4×20 mm³ monolithic LYSO coupled to fast Hamamatsu PMT R9779 was used as a reference for the timing measurement which had a resolution of 214 ps (FHWM). Signals from PMTs were captured by oscilloscope and analyzed with Labview. Linear fitting and extrapolation to the baseline of the pulse rising edge was used to get the pulse arrival time. The result shows the block with shorter mask #2 provided an improvement of 6.2% in timing resolution and 5.1% higher in energy than that with longer mask #1 for the crystals in the block center with a trade-off of 8% shrinking in 2D decoding map, which decreases the average peak-valley ratio from 3.3 to 3.0.