Waveform-Sampling Electronics for TOF PET...
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Waveform-Sampling Electronics for TOF PET Scanner W.J. Ashmanskas, B.C. LeGeyt, F.M. Newcomer, R. Van Berg, R.I. Wiener, J.S. Karp Radiology Department (Physics & Instrumentation Group) and Physics & Astronomy Department (HEP Instrumentation Group) University of Pennsylvania, Philadelphia, PA, USA • Oscilloscope-quality readout of PMT waveforms is both beneficial and feasible for whole-body TOF PET at clinical count rates • We have designed and are prototyping electronics with which we will instrument full scanner (432 PMTs, 38880 crystals) next year Abstract Waveform sampling (WFS) is an appealing technique for instruments requiring precision time and pulse-height measurements. Recent advances in switched-capacitor-array ASICs such as the Domino Ring Sampler (DRS4) have made WFS affordable for large systems. LaPET is a whole-body time-of-flight PET scanner using 38880 LaBr 3 (5% Ce) scintillator crystals of dimension 4×4×30 mm 3 , imaged by 432 Photonis XP20D0 PMTs, grouped into 24 identical detector modules. High light yield (61000 photons/MeV) and fast decay time (20 ns) make LaBr3 an excellent scintillator for TOF PET. Our group previously reported coincidence timing resolution 315-330 ps (fwhm) in benchtop measurements and 375 ps in full-system measurements using semi- custom electronics. This contribution reports on a complete redesign of the LaPET electronics, trigger, and data acquisition system. Our design uses 240 DRS4 chips to obtain oscilloscope-quality sampling of each PMT waveform at 2 GSPS. The 7 PMTs with which each crystal's scintillation light is collected map cleanly into the 8 analog inputs of a DRS4 chip, facilitating a redundant and nearly deadtime-free (at clinical rates) trigger design, in spite of the ~3 μs required for DRS4 readout. An FPGA-based trigger using analog pulse shaping and 100 MSPS sampling provides coarse energy and timing measurements used to detect coincident pairs and to select DRS4 chips for readout. Simulation studies show that oscilloscope- quality readout of each PMT signal will permit more flexible handling of detector calibrations, PMT waveform baseline offsets, and pulse pile-up effects. We thus expect the upgraded electronics to permit system-level performance that more closely approximates single-module benchtop results and to preserve that performance at clinical count rates. Our goals are both to explore the feasibility of WFS for a large scanner and to improve the overall performance of the LaPET research scanner. We present design details and initial prototyping results TOF improves SNR in PET Time-of-Flight measurement reduces statistical noise in PET by modeling the TOF uncertainty in the image reconstruction program. TOF imaging with commercial scanners has been shown for clinical studies to improve the accuracy and precision of lesion uptake measurements, as well as improve lesion detectability. Further improvements in detector technology and electronics should lead to further clinical benefit. LaPET whole-body research scanner exploits LaBr 3 (Ce) scintillation properties LaPET’s 38880 crystals are separated azimuthally into 24 detector modules and imaged by 432 PMTs (51 mm ∅). The new electronics obeys the same 24-fold symmetry, sharing edge PMT data between neighboring sectors. Benefits of WFS-based LaPET electronics • Finer control of channel-to-channel gain and timing variations should push 375 ps full-system timing closer to 315-330 ps benchtop timing • Easy to implement PMT-by-PMT timing offset in software • Digital baseline restoration preserves resolution for high-rate operation • WFS enables robust pile-up handling — important at clinical count rates • Flexible definition of time & energy measurements — which can be handy for a research scanner whose detectors may continue to evolve • Makes sense to push on electronics, since crystals + PMTs dominate cost New electronics consist of 24 Module Readout Boards (MRB). MRB/PC link uses 100/1000 Mbps ethernet for data fan-in of accepted pairs. Coincidence logic is pure digital, using Category 7 twisted-pair cable for synchronous data link between each MRB and Master Coincidence Unit. Shaper/Analog Mezzanine card (left) and Module Readout Board (right) prototypes exist and are currently being assembled and tested. Shaper/Analog Mezzanine card input and filtered outputs Status of system implementation • design vetted with benchtop data & simulation studies • analog shaping is working (several units built & tested) • MRB prototype exists & is now coming to life, in stages • MCU exists as C and Verilog models; prototype using MRBs • Ethernet / UDP readout scheme prototyped using 4 Digilent Spartan 3 evaluation boards, Netgear FS728TS switch, and Linux PC • DAQ software will write event format compatible with existing reconstruction software & will re-use calibration algorithms that measure detector quantities • software needed for event collection & electronics calibration
Transcript of Waveform-Sampling Electronics for TOF PET...
Waveform-Sampling Electronics for TOF PET ScannerW.J. Ashmanskas, B.C. LeGeyt, F.M. Newcomer, R. Van Berg, R.I. Wiener, J.S. Karp
Radiology Department (Physics & Instrumentation Group) and Physics & Astronomy Department (HEP Instrumentation Group)
University of Pennsylvania, Philadelphia, PA, USA
• Oscilloscope-quality readout of PMT waveforms is both beneficial and feasible for whole-body TOF PET at clinical count rates
• We have designed and are prototyping electronics with which we will instrument full scanner (432 PMTs, 38880 crystals) next year
AbstractWaveform sampling (WFS) is an appealing technique for instruments
requiring precision time and pulse-height measurements. Recent advances in switched-capacitor-array ASICs such as the Domino Ring
Sampler (DRS4) have made WFS affordable for large systems. LaPET is a whole-body time-of-flight PET scanner using 38880 LaBr3 (5% Ce)
scintillator crystals of dimension 4×4×30 mm3, imaged by 432 Photonis XP20D0 PMTs, grouped into 24 identical detector modules. High light yield (61000 photons/MeV) and fast decay time (20 ns) make LaBr3 an
excellent scintillator for TOF PET. Our group previously reported coincidence timing resolution 315-330 ps (fwhm) in benchtop
measurements and 375 ps in full-system measurements using semi-custom electronics. This contribution reports on a complete redesign of the LaPET electronics, trigger, and data acquisition system. Our design
uses 240 DRS4 chips to obtain oscilloscope-quality sampling of each PMT waveform at 2 GSPS. The 7 PMTs with which each crystal's scintillation
light is collected map cleanly into the 8 analog inputs of a DRS4 chip, facilitating a redundant and nearly deadtime-free (at clinical rates) trigger design, in spite of the ~3 μs required for DRS4 readout. An FPGA-based trigger using analog pulse shaping and 100 MSPS sampling provides coarse energy and timing measurements used to detect coincident pairs and to
select DRS4 chips for readout. Simulation studies show that oscilloscope-quality readout of each PMT signal will permit more flexible handling of detector calibrations, PMT waveform baseline offsets, and pulse pile-up
effects. We thus expect the upgraded electronics to permit system-level performance that more closely approximates single-module benchtop results and to preserve that performance at clinical count rates. Our
goals are both to explore the feasibility of WFS for a large scanner and to improve the overall performance of the LaPET research scanner. We
present design details and initial prototyping results
TOF improves SNR in PETTime-of-Flight measurement reduces statistical noise in PET by modeling the TOF uncertainty in the image reconstruction program. TOF imaging with commercial scanners has been shown for clinical studies to improve
the accuracy and precision of lesion uptake measurements, as well as improve lesion detectability. Further improvements in detector
technology and electronics should lead to further clinical benefit.
LaPET whole-body research scanner exploits LaBr3(Ce)
scintillation properties
LaPET’s 38880 crystals are separated azimuthally into 24 detector modules and imaged by 432 PMTs (51 mm ∅). The new electronics obeys the same
24-fold symmetry, sharing edge PMT data between neighboring sectors.
Benefits of WFS-based LaPET electronics• Finer control of channel-to-channel gain and timing variations should
push 375 ps full-system timing closer to 315-330 ps benchtop timing
• Easy to implement PMT-by-PMT timing offset in software
• Digital baseline restoration preserves resolution for high-rate operation
• WFS enables robust pile-up handling — important at clinical count rates
• Flexible definition of time & energy measurements — which can be handy for a research scanner whose detectors may continue to evolve
• Makes sense to push on electronics, since crystals + PMTs dominate cost
New electronics consist of 24 Module Readout Boards (MRB). MRB/PC link uses 100/1000 Mbps ethernet for data fan-in of accepted pairs.
Coincidence logic is pure digital, using Category 7 twisted-pair cable for synchronous data link between each MRB and Master Coincidence Unit.
Shaper/Analog Mezzanine card (left) and Module Readout Board (right) prototypes exist and are currently being assembled and tested.
Shaper/Analog Mezzanine card input and filtered outputs
Status of system implementation• design vetted with benchtop data & simulation studies
• analog shaping is working (several units built & tested)
• MRB prototype exists & is now coming to life, in stages
• MCU exists as C and Verilog models; prototype using MRBs
• Ethernet / UDP readout scheme prototyped using 4 Digilent Spartan 3 evaluation boards, Netgear FS728TS switch, and Linux PC
• DAQ software will write event format compatible with existing reconstruction software & will re-use calibration algorithms that measure detector quantities
• software needed for event collection & electronics calibration
