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Detectors based on LYSO and LGSO scintillators in a phoswich arrangement coupled to an avalanche photodiode are used in the LabPET™, an all-digital positron emission tomography (PET) scanner for small animal imaging developed in Sherbrooke. A Wiener filter based crystal identification (CI) algorithm achieving excellent discrimination accuracy ( 98%) was recently proposed for crystal identification of LYSO-LGSO phoswich detectors [1]. This algorithm was based on estimating parameters describing the scintillation decay time constant and the light yield of events sampled at 45 MSPS. The CI process was performed by applying a threshold on the scintillation decay parameter of events. The light yield was not considered in the CI process even if it should be. We propose a 2-fold faster CI approach which takes both the scintillation decay and light yield coefficients of each crystal into consideration. The new algorithm uses the previous Wiener filter based algorithm as a calibration process in order to evaluate the model of each individual crystal. The DAQ chain model as a priori knowledge is then incorporated into the model of each crystal and the output signal is estimated. The CI is performed by evaluating a single parameter representing the percentage contribution of each crystal characteristics in the event signal. The CI algorithm demonstrated a discrimination rate accuracy 98 5% for LYSO-LGSO LabPET detectors and 99% for LSO-GSO crystals in phoswich arrangement for 511 keV events. Although a calibration is required, the real-time implementation of the new CI algorithm needs 2 times less direct operations. An FPGA clocked at 400 MHz can process up to 25 M events/sec with such an algorithm. Index Terms-Crystal identification, digital signal processing, phoswich, positron emission tomography (PET), Wiener filter.

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Crystal identification (CI) of phoswich detectors is a technique used in positron emission tomography (PET) for improving spatial resolution through depth-of-interaction determination or higher pixelization. Digital algorithms using advanced digital signal processing techniques currently provide the most powerful approaches for CI of phoswich detectors made of crystals with only slightly different scintillation decay times. Such methods can be implemented in the all-digital architecture of LabPET, a small animal PET scanner developed in Sherbrooke, for fast and accurate real-time CI. In order to validate the new CI algorithms and assess their performance for different front-end electronics, a pulse generator simulator was developed to generate PET signals and investigate the effects of factors such as electronic noise, photon statistics and pulse shaping filter. The pulse generator was validated with LabPET-like pulses and CI results were compared with experimental data. The pulse simulator enables CI algorithms to be validated together with detector performance such as energy and timing resolution at an early stage of scanner design.

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