Combined Analog/Digital Approach to Performance...
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Combined Analog/Digital Approach to Performance Optimization for the LAPET Whole-Body TOF PET Scanner W.J. Ashmanskas, Z.S. Davidson, B.C. LeGeyt, F.M. Newcomer, J.V. Panetta, 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 • Obtaining optimal timing and energy resolution at clinical count rates requires careful handling of channel-to-channel variations and of pile-up effects. • To address these issues, we have designed and built (in prototype quantities) replacements for all facets of scanner electronics: high voltage, analog shaping, trigger, digitization, and readout. • Bench tests of new electronics show encouraging results with partial detector module and prototype electronics. After testing on 2/24 of LaPET scanner, we will re-instrument full scanner (432 PMTs, 38880 crystals) with production-run electronics. Abstract LAPET is a LaBr 3 -based whole-body time-of-flight PET scanner. We previously reported coincidence timing resolution 315-330 ps (fwhm) in benchtop measurements and 375 ps in full-system measurements. We are currently testing prototype units for a complete redesign of LaPET’s electronics, aimed at further improving full-system timing performance and at preserving that performance at high count rates. We report on four facets of the new design. First, PMT-by-PMT high-voltage control at two points per dynode chain permits both gains and timing offsets to be equalized across the scanner. Second, analog pulse shaping reduces the duration of each PMT pulse (t ~ 20 ns) from 75 ns to 35 ns, reducing pileup effects. Third, custom circuit boards use the DRS4 waveform- sampling ASIC to provide oscilloscope-quality readout of each PMT signal, enabling digital processing of PMT waveforms. Finally, an FPGA-based trigger provides the coarse energy and timing measurements used to detect coincident pairs. Tests are underway of prototype High Voltage Control boards, Shaper/ Analog Mezzanine cards, and the DRS4-based Module Readout Board; the Master Coincidence Unit design is in progress. 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 obey the same 24-fold symmetry, sharing edge PMT data between neighboring sectors. High Voltage Control (HVC) board The first facet of our electronics redesign enables PMT-by-PMT high-voltage control at two points per dynode chain, permitting both gains and timing offsets to be equalized across the scanner, eliminating a significant contribution to timing resolution. The new HVC board is a digitally programmable power supply capable of independently controlling the dynode and photocathode biasing voltages for each of 18 PMTs. The full scanner requires 24 HVC boards. To date, 2/24 of LaPET have been powered continuously by HVC boards for several months. (See figure below.) Benchtop HVC tests have established that at least a factor of 3 in gain and at least 1 ns in transit-time offset can be corrected. Tests are in progress to develop an iterative calibration procedure, which can in turn be implemented in situ on the full LaPET scanner. High Voltage Control boards have stably powered 2/24 of LaPET (as shown in photo) for several months. HVC controls and programmably adjusts PMT-by- PMT gain and timing in benchtop tests. Waveform sampling at 2 GSPS enables flexible definition of leading-edge timing and of integrated light collected by PMT, from a single data stream. Digital baseline restoration and shape-based pile-up detection help to preserve performance at high count rates. Module Readout Board provides DRS4-based readout for LaPET. Module Readout Board (MRB): readout and trigger 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. Block diagram of Module Readout Board (MRB) Benchtop tests of prototype Module Readout Board. Each MRB hosts three SAM cards (two are shown in photo). PMT pulses recorded by prototype MRB. Waveforms have had baselines subtracted and leading-edge times aligned in software. Energy spectrum from positron decay (from Na22 source), triggered and recorded by prototype MRB. Shaper/Analog Mezzanine (SAM) card: analog shaping Shaper/Analog Mezzanine card input and filtered outputs. The “readout shape,” sent to DRS4 for 2GSPS sampling, preserves fast leading-edge timing while canceling slow tail of PMT pulse, to reduce pile-up effects. The “trigger shape” makes rise and fall times roughly equal, so that 100 MSPS trigger ADCs and FPGA logic can determine coarse energy and timing (via centroid algorithm) for coincidence detection, to select DRS4 chips for readout. Shaper/Analog Mezzanine cards have been assembled, tested, and used to record data both with MRB prototype and with commercial DRS4-based DAQ. Single-crystal energy resolution (blue) and response (red) vs. integration time, for pulses recorded after SAM “readout” shaping. Goal is to optimize tradeoff between energy resolution (long integration) and high-count-rate pile-up minimization (short integration). Partial flood map obtained on test bench using HVC board, SAM cards, and commercial DRS4-based digitizer. Status of system implementation • HVC board succeeds at programmably adjusting PMT-by-PMT gain and timing • SAM card provides analog shaping for high-rate readout (by suppressing exponential pulse tail) and for trigger processing; each MRB hosts 3 SAM cards • Benchtop tests using a partial LaPET detector module have demonstrated the methods of upgraded electronics. Tests thus far use real HVC, real SAM, and commercial DRS4-based digitization. • MRB is a custom DRS4-based board designed to match LaPET’s needs for detector geometry, high-rate trigger scheme, and high-rate event readout. • MRB prototypes exist and have begun to reproduce results we have obtained earlier using commercial DRS4-based boards. Work is underway to read out 2/24 of actual LaPET scanner with MRB prototypes. Then extend MRB readout to full scanner after fabrication of complete set of boards. We gratefully acknowledge support for this work by NIH grant R01-CA113941. The DRS4 chip, from Paul Scherrer Institut, is documented at drs.web.psi.ch .
Transcript of Combined Analog/Digital Approach to Performance...
Combined Analog/Digital Approach to Performance Optimization for the LAPET Whole-Body TOF PET Scanner
W.J. Ashmanskas, Z.S. Davidson, B.C. LeGeyt, F.M. Newcomer, J.V. Panetta, R. Van Berg, R.I. Wiener, J.S. KarpRadiology Department (Physics & Instrumentation Group) and Physics & Astronomy Department (HEP Instrumentation Group), University of Pennsylvania, Philadelphia, PA, USA
• Obtaining optimal timing and energy resolution at clinical count rates requires careful handling of channel-to-channel variations and of pile-up effects.
• To address these issues, we have designed and built (in prototype quantities) replacements for all facets of scanner electronics: high voltage, analog shaping, trigger, digitization, and readout.
• Bench tests of new electronics show encouraging results with partial detector module and prototype electronics. After testing on 2/24 of LaPET scanner, we will re-instrument full scanner (432 PMTs, 38880 crystals) with production-run electronics.
AbstractLAPET is a LaBr3-based whole-body time-of-flight PET scanner. We previously
reported coincidence timing resolution 315-330 ps (fwhm) in benchtop measurements and 375 ps in full-system measurements. We are currently testing prototype units for a complete redesign of LaPET’s electronics, aimed at further improving full-system timing performance and at preserving that performance at high count rates. We report on four facets of the new design. First, PMT-by-PMT
high-voltage control at two points per dynode chain permits both gains and timing offsets to be equalized across the scanner. Second, analog pulse shaping
reduces the duration of each PMT pulse (t ~ 20 ns) from 75 ns to 35 ns, reducing pileup effects. Third, custom circuit boards use the DRS4 waveform-
sampling ASIC to provide oscilloscope-quality readout of each PMT signal, enabling digital processing of PMT waveforms. Finally, an FPGA-based trigger
provides the coarse energy and timing measurements used to detect coincident pairs. Tests are underway of prototype High Voltage Control boards, Shaper/
Analog Mezzanine cards, and the DRS4-based Module Readout Board; the Master Coincidence Unit design is in progress.
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 obey the same 24-fold
symmetry, sharing edge PMT data between neighboring sectors.
High Voltage Control (HVC) boardThe first facet of our electronics redesign enables PMT-by-PMT high-voltage
control at two points per dynode chain, permitting both gains and timing offsets to be equalized across the scanner, eliminating a significant contribution to timing
resolution. The new HVC board is a digitally programmable power supply capable of independently controlling the dynode and photocathode biasing voltages for each of 18 PMTs. The full scanner requires 24 HVC boards. To
date, 2/24 of LaPET have been powered continuously by HVC boards for several months. (See figure below.) Benchtop HVC tests have established that at least a factor of 3 in gain and at least 1 ns in transit-time offset can be corrected. Tests are in progress to develop an iterative calibration procedure, which can in turn
be implemented in situ on the full LaPET scanner.
High Voltage Control boards have stably powered 2/24 of LaPET (as shown in photo) for several months. HVC controls and programmably adjusts PMT-by-
PMT gain and timing in benchtop tests.
Waveform sampling at 2 GSPS enables flexible definition of leading-edge timing and of integrated light collected by PMT, from a single data stream. Digital baseline restoration and shape-based pile-up detection help to preserve performance at
high count rates. Module Readout Board provides DRS4-based readout for LaPET.
Module Readout Board (MRB): readout and trigger
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.
Block diagram of Module Readout Board (MRB)
Benchtop tests of prototype Module Readout Board. Each MRB hosts three SAM cards (two are shown in photo).
PMT pulses recorded by prototype MRB. Waveforms have had baselines subtracted and leading-edge times aligned in software.
Energy spectrum from positron decay (from Na22 source), triggered and recorded by prototype MRB.
Shaper/Analog Mezzanine (SAM) card: analog shaping
Shaper/Analog Mezzanine card input and filtered outputs. The “readout shape,” sent to DRS4 for 2GSPS sampling, preserves fast leading-edge timing while
canceling slow tail of PMT pulse, to reduce pile-up effects. The “trigger shape” makes rise and fall times roughly equal, so that 100 MSPS trigger ADCs and FPGA
logic can determine coarse energy and timing (via centroid algorithm) for coincidence detection, to select DRS4 chips for readout. Shaper/Analog
Mezzanine cards have been assembled, tested, and used to record data both with MRB prototype and with commercial DRS4-based DAQ.
Single-crystal energy resolution (blue) and response (red) vs. integration time, for pulses recorded after SAM “readout” shaping. Goal is to optimize tradeoff between energy resolution (long integration) and high-count-rate pile-up
minimization (short integration).
Partial flood map obtained on test bench using HVC board, SAM cards, and commercial DRS4-based digitizer.
Status of system implementation• HVC board succeeds at programmably adjusting PMT-by-PMT gain and timing
• SAM card provides analog shaping for high-rate readout (by suppressing exponential pulse tail) and for trigger processing; each MRB hosts 3 SAM cards
• Benchtop tests using a partial LaPET detector module have demonstrated the methods of upgraded electronics. Tests thus far use real HVC, real SAM, and commercial DRS4-based digitization.
• MRB is a custom DRS4-based board designed to match LaPET’s needs for detector geometry, high-rate trigger scheme, and high-rate event readout.
• MRB prototypes exist and have begun to reproduce results we have obtained earlier using commercial DRS4-based boards. Work is underway to read out 2/24 of actual LaPET scanner with MRB prototypes. Then extend MRB readout to full scanner after fabrication of complete set of boards.
We gratefully acknowledge support for this work by NIH grant R01-CA113941.
The DRS4 chip, from Paul Scherrer Institut, is documented at drs.web.psi.ch .
