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PhD course on Nuclear Microelectronics

Towards M illimeter Resolution In Time-of-Flight PET. PhD course on Nuclear Microelectronics. Luca Miari 785753. Alessandro Ruggeri 802434. Outline. Positron Emission Tomography Image Reconstruction SNR Enhancement in TOF PET Scintillators & Detectors Evolution

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PhD course on Nuclear Microelectronics

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  1. Towards Millimeter Resolution In Time-of-Flight PET PhD course onNuclear Microelectronics Luca Miari 785753 Alessandro Ruggeri 802434

  2. Outline Positron Emission Tomography Image Reconstruction SNR Enhancement in TOF PET Scintillators & Detectors Evolution Discrete vs Integrated Acquisition Chain State-of-the-art timing TAC

  3. Positron Emission Tomography • Patients are injected with radioactive drug • Radionuclide decays emitting a positron • Positron annihilates with an electron of the tissue • Back-to-back gamma ray@ 511 keV is emitted • Two detectors register a hit • Positron lies on line defined by detector pair.

  4. Image Reconstruction • It contains information about the positronemission rate along the line of response • The projection of a 2D image on a 1D line is called Radon Transform • By rotating the projection line of an angle θ I get the sinogram of the 2D image • Sinogram is 2π periodic

  5. Image Reconstruction • The back-projection algorithm allows the reconstruction of the image original image (G) • Star artifacts arises when a limited number of projection angles are used(E,F) • Blurring (G) arises from the the finite sample- width • By HP-filtering the sinogram the resulting image becomes sharper

  6. Image Reconstruction • The back-projection algorithm allows the reconstruction of the image original image (G) • Star artifacts arises when a limited number of projection angles are used(E,F) • Blurring (G) arises from the the finite sample- width • By HP-filtering the sinogram the resulting image becomes sharper

  7. Image Reconstruction • Arc correctionresamplingisnecessarybefore back-projectingbecause of the circulardistribution of the detectors • False coincidence events decrease SNR

  8. Signal-to-Noise Ratio • T is the True coincidence event rate(indep.) • S is the Scattered event rate() • R is the Random event rate () • n is the number of volume elements influencing the noise ( M. E. Casey, “Improving PET With HD • PET + Time of Flight”, Siemens AG, 2008 W. W. Moses, “Time of Flight in PET Revisited”, IEEE Transactions on Nuclear Science, 2003

  9. Signal-to-Noise Ratio When nbecome smaller: • TOF SNR Gain: M. E. Casey, “Improving PET With HD • PET + Time of Flight”, Siemens AG, 2008 M. Conti, “State of the art and challenges of time-of-flight PET”, Physica Medica, 2009

  10. TOF: Benefits & Requirements • BENEFITS • Temporal resolution of 50 ps allows sub-centimeter spatial resolution • Current resolution of 500 ps () do not improve spatial resolution but reduces statistical noise: • Usually : reduces computational time and numbers of iteration • REQUIREMENTS FOR MILLIMETER RESOLUTION • Fast scintillators (with ns decay constant) • Low Jitter detectors • Timing Electronics M. Conti, “Focus on time-of-flight PET: the benefits of improved time resolution”, Eur J Nucl Med Mol Imaging, 2011 W. W. Moses, “Time of Flight in PET Revisited”, IEEE Transactions on Nuclear Science, 2003

  11. Scintillators: 1980 • First TOF faced scintillators limitations: • High Z scintillator had high output yield, high energy resolution, could be made smaller but had slow decay constant • Scintillator with fast decay constant (like CsF) had opposite performance and so TOF studies were abandoned M. Ter-Pogossian, D. Ficke, M. Yamamoto, J. Hood, “Super PETT 1: A Positron Emission Tomography UtilizingPhoton Time-of-Flight Information“, IEEE Transactions on MedicalImaging, 1982

  12. Scintillators: 2000 • 1980: CsF and BaF2 • Very Fast Decay Constant • Very Low Light Yield • Low Energy Resolution • 2000: LaCl3 and LaBr3 • Fast DecayConstant • Very High Light Yield • High Energy Resolution • Measurements @ 664 keV shows • Energy Resolution of 4.6 % FWHM • Timing Resolution of 350 ps FWHM New scintillators renewed scientific interest in TOF PET S. Surti, J.S. Karp, G. Muehllehner, “Investigation of Lanthanum Scintillators for 3-D PET“, IEEE Transactions on Nuclear Science, 2003

  13. Scintillators: 2010 • Recent studies broke the 100 ps barrier • 3x3x5mm LaBr3:Ce(5%) scintillator • Hamamatsu SiPM • Digital Signal Processing • Promising new scintillators • LaBr3:Ce(30%) provides better timing resolution • New materials are under investigation: • CeBr3 • LuI3:Ce • Necessity of optimization for commercial system D.R. Schaart et al., “LaBr3:Ce and SiPMs for time-of-flight PET: achieving 100 ps coincidence resolving time“, PhysMedBiol, 2010

  14. Detectors “Fine spatial resolution of today’s PET scanners requires up to 104detectors” PMT • SiPM has slower 10-90% rise time than PMT (9 vs 3ns) • SiPM has higher PDE than PMT (45% vs 16% @380nm) • Higher PDE brings more primary charges which reduce statistical fluctuations on timing resolution even with a slower dv/dt • SiPM can be integrated in CMOS technology SiPM D.R. Schaart et al., “LaBr3:Ce and SiPMs for time-of-flight PET: achieving 100 ps coincidence resolving time“, Phys Med Biol, 2010

  15. Advantages of CMOS technology PET analysis requires a large number of channels that must be processed in parallel • Integration of the front-end in standard CMOS tecnhnology • Reduces size • Reduces costs • Improves system reliability • Increases system performance • Reduces power dissipation B.K. Swann et al., “A 100ps Time-Resolution CMOS Time-to-Digital Converter for PET Imaging Application“, IEEE Journal of Solid State Circuits, 2004

  16. Acquisition Chain Energy section BLH Charge preamp SA Peak stretcher ADC - Scintillator Detector Storage PC Timing section Pulse discriminator TDC Common sync

  17. Pulse Discriminators • Leading-edge discriminator: • Very simple to implement (comparator) • Suffers from variation of pulse amplitude/baseline level • Constant-fraction discriminator: • No pulse amplitude dependance • Complex circuit involving delays J.F. Genatet al, “Signal processing for picosecond resolution timing measurements”, Nucl Inst and Met in Physics Research Section A, 2009

  18. Discrete CFD: Implementation • C1: reject input pulses below a certain threshold amplitude (noise level) • C2: implement CFD difference amplifier • Down to picosecond resolution W. Becker, “Advanced Time-Correlated Single Photon Counting Techniques”, 1st ed. Springer, 2005

  19. FullyIntegrated CFD D.M. Binkleyet al , “A 10-Mc/s, 0.5-μm CMOS constant-fraction discriminator havingbuilt-in pulsetailcancellation”, IEEE Transactions on Nuclear Science, 2002

  20. Fully Integrated CFD: Example D.M. Binkleyet al , “A 10-Mc/s, 0.5-μm CMOS constant-fraction discriminator havingbuilt-in pulsetailcancellation”, IEEE Transactions on Nuclear Science, 2002

  21. Fully Integrated CFD: Example Baseline restorerremoves pulsetail → reducedpileup Current mode anddifferential circuitsminimizeharmonicdistortion and noisecoupling D.M. Binkleyet al , “A 10-Mc/s, 0.5-μm CMOS constant-fraction discriminator havingbuilt-in pulsetailcancellation”, IEEE Transactions on Nuclear Science, 2002

  22. Fully Integrated Acquisition Chain • CHAIN1 • 32 parallel channels • 2 ICON based current conveyor • 2nd order shaping filter • Energy discrimination for position information • Timing discrimination by both leading edge and zero-crossing method • CHAIN2 • Analog summing circuit for energy information • TAC for timing information • Internal baseline restorer H. Matsuda et al., “Development of ultra-fast ASIC for future PET scanners using TOF-capable MPPC detectors“, Nucl Inst&Met in Physics Research, 2013

  23. Fully Integrated Acquisition Chain • ASUM providesthe energyspectrum(to be digitallyconverted) • LEDGEcomparatoracquiresfast initialrisingedges • Zero-crossing discriminator (FAST) provides a digital time-walk free signal (DSUM) • Performance • 10.5% (FWHM) @ 662 keV • 9.8% (FWHM) @ 511 keV • 491ps (FWHM) TOF info H. Matsuda et al., “Development of ultra-fast ASIC for future PET scanners using TOF-capable MPPC detectors“, Nucl Inst&Met in Physics Research, 2013

  24. Time-to-Amplitude Converter An extremely simple operating principle with several requirements lead to a complex circuit with high performance • Time resolution down to 15.9 ps • Less than 2.5% LSB peak-to-peak DNL • 200 ns maximumdead-time M. Crotti et al., “Four Channel, 40 ps Resolution, Fully Integrated Time-to-Amplitude Converter for Time-Resolved Photon Counting“, IEEE JSSC, 2012

  25. Time-to-Amplitude Converter • Idle Phase • TG1ON (OA in buffer configuration) • Differential stage switches on M2 • Conversion Phase (Start Event) • TG1OFF • Vout increases • Wait Phase (Stop Event) • Differential stage switches on M1 • Conversion capacitor holds Vout • Reset Phase • TG1ON discharges the capacitor • M2ON resets to Idle Phase M. Crotti et al., “Four Channel, 40 ps Resolution, Fully Integrated Time-to-Amplitude Converter for Time-Resolved Photon Counting“, IEEE JSSC, 2012

  26. Conclusions Fast Analysis Improved Resolution Fast Decay High Light Yield Ruggedness Higher Efficiency Compactness Low Jitter CMOS Integration High Resolution Low Dead-Time

  27. Full digital TDC B. Markovic et al., “A High-Linearity, 17 ps Precision Time-to-Digital Converter Based on a Single-Stage Vernier Delay Loop Fine Interpolation”, IEEE Transaction on Circuits and Systems I: Regular Papers, 2013

  28. Full digital TDC Performances 10 ps bin < 20 psrms precision 160 ns range 0.9%RMS LSB DNL 4%MAX LSB DNL 0.3mm2 core area B. Markovic et al., “A High-Linearity, 17 ps Precision Time-to-Digital Converter Based on a Single-Stage Vernier Delay Loop Fine Interpolation”, IEEE Transaction on Circuits and Systems I: Regular Papers, 2013

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