An investigation of waveform sampling for improved signal processing in TOF PET

Abstract

Abstract-The development of fast photodetectors and of fast, high light output scintillation crystals has placed an increased emphasis on the need for fast readout electronics used for TimeOf-Flight (TOF) PET detectors. These improvements have paralleled developments in analog sampling technology that makes fast waveform digitizing of photodetector signals an attractive alternative to custom PCB and IC designs. Waveform digitization offers a flexible means for evaluating and implementing more complex signal processing algorithms. We used a commercial 1GHz 2Gs/s waveform digitizing system to acquire coincident pulses from LYSO and LaBr 3 (5%Ce) scintillator crystals coupled to a fast PMT. By measuring the time pickoff based on a linear fit to the rising edge of the pulse, we show an improvement in coincident timing resolution from ~200ps to ~80ps FWHM for LaBr 3 (5%Ce) coupled to Photonis XP20D0 PMTs, and from ~160ps to ~80ps Full Width at Half Maximum (FWHM) for LaBr 3 (5%Ce) coupled to Hamamatsu H4998 PMTs. Our results show a consistent improvement in timing resolution as well as reduced sensitivity to signal risetime as compared to leading edge (LE) pickoff techniques. Measurements with partial detector modules indicate that system timing resolution of <200ps is attainable with LaBr 3 (5%Ce) pixelated detectors designed for TOF PET.

The imaging performance of a LaBr3-based PET scanner

“…As has been shown by Kuhn et al (2004), smaller PMTs would eliminate much of this count rate dependence. Pulse shape analysis of digitized signals will, in principle, also reduce the pulse pile-up effects, and work is on-going in this area (Wiener et al 2008). …”

“…faster and more uniform convergence of iterative reconstructions) (Karp et al 2008), they have nonetheless been shown to be useful to predict global TOF performance gains (Surti et al 2006). Wong et al (1983) also predicted an increased signal-to-noise (SNR) ratio with improved timing resolution, again for uniform distributions and analytical reconstruction. To take further advantage of this TOF benefit, we have developed a TOF-PET system based on LaBr 3 crystals, which have a faster decay time and higher light output than LSO or LYSO; together, these characteristics improve the timing resolution of LaBr 3 over that of LSO or LYSO.…”

LaBr3:Ce and SiPMs for time-of-flight PET: achieving 100 ps coincidence resolving time

“…A simulation by Kuhn et al (2005) predicts a CRT of ∼100 ps FWHM for very small crystals. The dependence of CRT on the time pick-off method was tested by Wiener et al (2008) by comparing analogue to digital methods. Although it was originally reported that the CRT with digital waveform sampling was superior, further work has since demonstrated that these measurements are sensitive to the assumptions made about the shape of the signal rising edge, and that the timing resolutions obtained with digital and analogue methods are comparable (private communication).…”

Waveform-sampling electronics for time-of-flight PET scanner

“…In recent years, there has been considerable community interest in the use of waveform sampling for TOF PET [1], [2], [3], [4], [5], [6].…”