A first study on the timing performance of PMT-quadrant-sharing LYSO detector array for time-of-flight PET

Time-of-flight (TOF) information can improve the imaging quality of positron emission tomography (PET) camera by calculating the original positron-electron annihilation position. In this work, we measured the preliminary TOF resolution of non-TOF-optimized LYSO arrays for TOF PET using our PMT-Quadrant-Sharing (PQS) technology which has higher light collection efficiency (no external light guide). We investigated digital signal processing methods that may achieve better time resolution. Digital oscilloscope Tektronix DPO7054 was used to capture the waveforms. Three timing methods, Linear Fitting (LF) to the pulse rising time, Constant Fraction Discriminator (CFD) and Leading edge Discriminator (LD), were studied. A new Time-Energy (T-E) correction using Multivariate Least Mean Square Fit was applied to minimize time walk caused by the variation of signal shape. The time resolution as a function of the depth of interaction was also studied. The average block-to-block coincident timing resolution of 13x13 (4x4x20 mm³ detector pixel) array with fast PMT R9779 (51-mm diameter) was measured to be 432 ps (FWHM). Another 13x13 crystal array with ultrahigh spatial resolution 1.4x1.4x10 mm³ detector pixels coupling to small 19-mm diameter poor-timing XP1912 PMTs was also tested and achieved 551 ps. This study shows that our regular PQS detector blocks (without optimization for TOF) have good TOF time resolution suitable for TOF PET systems, in addition to higher spatial resolution and reducing PMT cost.