June/5/2008 1
Electronics development for fast-timing PET detectors:
The multi-threshold discriminator Time of Flight PET system
Contents1. Introduction2. Experimental Setup3. Results4. Summary
Heejong Kim3, Jialie Lin1, Octavia Biris1, Chin-Tu Chen3, Woon-Seng Choong4,Henry Frisch1,2, Chien-Min Kao3, William Moses4, Fukun Tang2, Qingguo Xie3, Lin Zhou2
1. Department of Physics, University of Chicago, IL2. Enrico Fermi Institute, University of Chicago, IL3. Department of Radiology, University of Chicago, IL4. Lawrence Berkeley National Laboratory, Berkeley, CA
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1. Introduction• Digitize PET event waveform and apply digital-signal processing (DSP).• Potential advantages:
• allow sophisticated information processing• easy to upgrade• inexpensive, high-performance digital components are common • A promising multi-threshold method is in
vestigated by using digitized event waveforms obtained at a 20GHz sampling rate
Q. Xie, etal, “Potential advantages of digitally sampling scintillation pulses in timing determination in PET,” NSS/MIC 2007, pp. 4271-4274, 2007.
cf. PMT signal polarity is reversed for display purpose.Multi-threshold method
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Multi-threshold discriminator board
8 channels/board (4 channels installed):• ADCMP582 SiGe voltage comparator,
37ps rise/fall time• programmable threshold levels for
individual comparators, adjustable from 0~-700mV and controlled by UMDDA-08HC DAC
• Each channel provides differential LVDS outputs (high=1.26V, low=0.94V)
2 boards are implemented
ThresholdSetting
Input
Differential outputs
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2. Experimental Setup• Three discriminator thresholds: -100, -200, -300mV.• Tektronix TDS6154C oscilloscope: 4 input channels 15 GHz bandwidth 20GS/s sampling.• Trigger for the oscilloscope Lecroy 623B discriminator, Lecroy 365AL coincidence.
PMT Holder from LBL
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Light Shied Box
LSO
Na22
4 ch disc. board
Tektronix TDS6154Coscilloscope
R9800PMTTrigger by coincidence
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234
123
Light Shied Box
LSO
Na22
4 ch disc. board
Tektronix TDS6154Coscilloscope
R9800PMTTrigger by coincidence
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Light Shied Box
LSO
Na22
4 ch disc. board
Tektronix TDS6154Coscilloscope
R9800PMTTrigger by coincidence
1
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Block diagram for result B&C
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3. ResultsA. Time offset and resolution of discriminator
boardsB. Pulse shape reconstruction Energy resolution Scintillation decay constantC. Timing uncertainty of the multi-threshold
methodD. Coincidence timing resolutionF. Time readout using TDC (HPTDC)
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A. Time offset and resolution ofthe discriminator board
Top : comparator outputsBottom: Zoom in of the leading edge
Comparator 1 Comparator 2
Pulse Generator
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12
Disc. board
TDS6154Coscilloscope
Pulse Generator
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12
Disc. board
TDS6154Coscilloscope
Block diagram for result A
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A. Time offset and resolution ofthe discriminator board (Cont’d)
board#1 t (ps) (t)ch1-ch2 194.9 7.2ch1-ch3 404.8 7.6ch1-ch4 613.6 8.4
board#2ch1-ch2 209.5 7.1ch1-ch3 414.8 7.8ch1-ch4 630.4 8.0
Ch4 is the closest to the inputT( ch1-ch2)
• Measurements used to correct time offset between channels• Time resolution of single channel was estimated to be ~13ps (FWHM)
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B. Pulse shape reconstruction
use three thresholds: -100, -200 and -300mV time-offset correction applied reconstruct pulse shape using 6 time measurements
• linear fit on the leading• exponential fit on the falling
Blue: PMT signalRed: multi-threshold timing, time-offset correctedBottom : Zoom in of the leading part
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B. Energy resolution Pulse height:
20Gs/S waveform: sample sum multi-threshold: area under reconstructed curve
Energy resolution:• multi-threshold: 18%• 20Gs/S waveform: 13%
Integrated charge
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B. Decay constant
exponential fit on the tail gives the decay time constant
Results: mean: 43ns vs. 47ns
width (FWHM): 14ns vs. 4ns
Decay constant
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C. Timing uncertainty of the multi-threshold method
49ps FWHM
t(1-2)
(1) (2)
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D. Coincidence timing resolution
The differential time has a 332ps FWHMcf. Timing resolution of ~300ps using CFD discriminator (Q. Xie et al. NSS/MIC 2007, p4271)
Differential time
332ps FWHM
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D. Time readout using HPTDC
4 thresholds: -100, -200, -300, -400mV time-offset applied consistent with above results
HPTDC board (LBL)Sample output from HPTDC
multi-threshold board
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Summary Multi-threshold discriminator boards with adjustable thresholds develope
d and evaluated Time offset between comparator channels measured and corrected; timi
ng resolution of a single comparator: ~13ps (FWHM) Tested with LSO+R9800PMT and Na-22
• reconstruct event pulse using time outputs of the discriminators• 18% energy resolution at 511keV obtained by using 3 thresholds (-1
00mV, -200mV and -300mV), compared to 13% by using the full PMT waveform
• the use of the multi-threshold method with three thresholds estimated to contribute a timing uncertainty of ~49ps FWHM.
A 332ps FWHM coincidence timing resolution obtained by using 2 thresholds (-100mV and -300mV), compared to ~300ps when using a CFD
Initial successful results with HPTDC obtained
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