Figure 5. Histogram of the decay constant. Black curves: raw histograms; Blue curve: Gaussian fit to the histogram from the 20GS/s waveform; Red curve: Gaussian fit to the histogram from multi-threshold sampling.

Figure 4. Pulse-height histogram. Black curves: raw histograms; Blue curve: Gaussian fit to the histogram from the 20GS/s waveform; Red curve: Gaussian fit to the histogram from multi-threshold sampling.

A Multi-Threshold Sampling Method for (TOF) PET Event Detection

1. Introduction 3. Results

c. Energy, coincidence timing, and decay constant

a. Time resolution of discriminator• Send square pulses to two channels and measure time difference of the rising edge of the output pulses from their waveforms measured by using TDS6154 oscilloscope• Time resolution of single channel: ~13.3ps FWHM

b. Pulse reconstruction• Select coincidence events that have at least two samples on the leading edges on both boards• Reconstruct pulse shape:

• Linear fit to sample points of 100mV and 200mV on the leading edge, determining event time• Exponential fit on the falling edge, determining event energy and decay constant

Idea of multi-threshold sampling method• Sampling pulse at pre-defined threshold levels• Output: digitized timings only (using TDC)• Pulse reconstruction from resulting samples• Benefits:

• Remove analog blocks (Pre-Amp, ADC, CFD)• DSP can be applied

Figure 1. Multi-threshold sampling for PET event detection. Black curve: An event waveform (measured by using Tektronix TDS6154 at 20GS/s); Blue circles: data points generated by multi-threshold sampling with 4 thresholds; Red curve: reconstructed pulse shape from the samples.

Reference

4. Summary• Multi-threshold sampling with 4 thresholds achieves ~350ps coincidence timing resolution, 18% of energy resolution, and 9ns resolution for the decay constant with an accurate mean.

• Using 20GS/s waveform, ~330ps coincidence timing resolution, 13% energy resolution, and 4ns resolution in decay constant are obtained.

• Multi-threshold sampling with 4 thresholds contributes ~20ps degradation to coincidence timing, 5% degradation to energy resolution, and 5ns degradation to decay-constant.

• Threshold settings and fitting methods can affect results.

• Current results indicate that multi-threshold sampling is practically useful for implementing digital PET DAQ.

Q. Xie et al, “Potential advantages of Digitally sampling scintillation pulses in timing determination in PET,” NSS/MIC, p. 4271, 2007.

20GS/s waveform Multi-threshold

Energy resolution 13% FWHM 18% FWHM

Decay constant 45ns (4ns FWHM) 44ns (9ns FWHM)

Figure 6. Histogram of the time difference in a coincidence event from multi-threshold sampling.Black curve: raw histogram; Red curve: Gaussian fit.

20GS/s waveform: 330ps FWHMMulti-threshold sampling: 350ps FWHM

Coincidence timing resolution

TDC board

4chs outputs (LVDS)

Figure 2. Multi-threshold board (left) connected to TDC board (right).

PMT InputThreshold control

Prototype implementation• Multi-threshold discriminator boards:

• 2 boards with 4 discriminator channels implemented (can be extended to 8 channels)• programmable threshold levels from 0 to 700mV• use ADCMP582 fast comparators

• TDC board (from Lawrence Berkeley National Laboratory):• use HPTDC (CERN) chip, providing 8 TDC channels at 25ps/bit

Figure 3 Experimental setup.

2. Experimental Setup• Two Hamamatsu R9800 PMTs (HV = -1300V), each coupled to a 6.25x6.25x25mm3 LSO crystal through the long side of the crystal. Two units separated by 5cm. A Na-22 source placed midway.

• Multi-threshold sampling: Each PMT output is fed to a multi-threshold discriminator board with the 50, 100, 200, 300mV thresholds. The digital time of the rise and falling edges of the discriminator logic outputs are determined by the HPTDC chip.

• 20GS/s waveform: PMT output is sampled at 20GS/s by using Tektronix TDS6154

Heejong Kim1, Chien-Min Kao1, Qingguo Xie1, Chin-Tu Chen1,Octavia Biris2, Jialin Lin2, Fukun Tang2, Lin Zhou2, Henry Frisch2,

Woon-Seng Choong3, William Moses3

1. Department of Radiology, University of Chicago, IL2. Enrico Fermi Institute, University of Chicago, IL

3. Lawrence Berkeley National Laboratory, Berkeley, CA

/ HPTDC