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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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

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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