Time-over-threshold for pulse shape discrimination in a time-of-flight/depth of interaction phoswich PET detector

Abstract

We are developing a PET detector capable of measuring both time-of-fight (TOF) and depth-of-interaction (DOI) with a goal to improve resulting image quality and accuracy. Phoswich designs have been realized in PET detectors to measure DOI for more than a decade. However, PET detectors based on phoswich designs put great demand on the readout circuit, which has to differentiate the pulse shape produced by different crystal layers. A simple and practical pulse shape discrimination approach is required to realize the phoswich design in a clinical PET scanner, which often consists of thousands of crystal elements. In this work, we studied the energy, timing and DOI performance of a phoswich detector design comprising a 3 × 3 × 20 mm3 crystal made from a 3 × 3 × 10 mm3 LYSO:Ce crystal epoxied to a 3 × 3 × 10 mm3 calcium co-doped LSO:Ce,Ca(0.4%) crystal and read out by silicon photomultipliers (SiPM). A DOI accuracy of 97.2% was achieved for photopeak events by using time-over-threshold (ToT) as a pulse shape parameter. The measured energy resolution was 9.7 ± 0.2% and 11.3 ± 0.2% FWHM at 511 keV for the LYSO and LSO crystal layers, respectively. The coincidence timing resolution for photopeak events ranges from 164.6 ± 0.9 ps to 183.1 ± 4.2 ps FWHM, depending on the layer combinations between two identical, dual layer detector configurations set in coincidence. These results show promise of using ToT for pulse shape discrimination in a TOF-DOI phoswich-based detector since ToT measurement can be easily implemented in readout electronics.