29.01.2014 Nuclear Medicine Imaging MADPET High Resolution, MR-compatible PET Cooperation between Physics and Nuclear Medicine Goal: 3D Distribution of radiotracer in vivo Based on detection of gamma rays (140 keV, 511 keV) -High resolution (mm) Sibylle I. Ziegler -High sensitivity (dose, acquisition time) Department of Nuclear Medicine, Technische Universität München -Minimizing measurement of scattered radiation PET Block Detector Why Solid State Detectors in Nuclear Medicine? BGO, LSO, LYSO Block Detector: • Coincidence window: 4 - 12 ns • Energy resolution: ~ 12 - 25% Solid state light sensors Solid state detectors for direct detection Crystal identification • Compact B X= • High granularity (B+D) - (A+C) (A+B+C+D) • Scalable modules: dedicated tomographs y A C D (A+B) - (C+D) y= • Insensitive to magnetic fields: multimodal imaging (A+B+C+D) X 29.01.2014 3 1 29.01.2014 Nuklearmedizinische Klinik und Poliklinik Avalanche Photodiodes (APDs) vs. PMTs PRO: • very compact • high quantum efficiency • low bias voltage • insensitive to magnetic fields MADPET Prototype 6 Modules 4 mm avalanche photodiode array (Hamamatsu) LSO crystals (3.7 x 3.7 x 12 mm3) CON: • noise • low gain • low timing resolution - Energy resolution: 15% - Spatial resolution: 2.3 mm - sensitivity: 13 cps / µCi Eur J Nucl Med 2001 MPI Clinical PET/MR Advanced Lab Course in Physics MADPET-1 “Revival” • 3x 32 channel Sampling-ADC for data acquisition, Based on electronics for COMPASS (E18, A. Mann and I. Konorov) • Readout via USB and Linux-Computer • Data analysis with ROOT (CERN) Siemens mMR Seite 7/16 2 29.01.2014 Nuklearmedizinische Klinik und Poliklinik Sensitivity/Resolution Parallax Error Biograph mMR high sensitivity X bad spatial resolution • PET: • 448 detector blocks with X low sensitivity good spatial resolution • 8 x 8 LSO crystals and • 3 x 3 APDs each • Axial FOV: 25.8 cm Courtesy of Siemens • Spat. resolution: 4.3 mm trade-off between sensitivity and spatial resolution DOI? Depth-of-Interaction (DOI) Measurement Examples of detector schemes MADPET-II Individual readout of Two LSO layers 1.6 mm x 1.6 mm light sensor crystal crystal LSO GSO light sensor ratio of signals DOI light sensor crystal monolithic APD array light sensor signal shape DOI individual layers DOI 16 channel preamplifier 2 cm 70 mm Diameter 3 29.01.2014 Nuklearmedizinische Klinik und Poliklinik MADPET-II Prototype with 384 Channels Transaxial reconstruction of 10 line sources Silicon Photomultipliers (SiPMs) • Array of Geiger-mode APDs cell With DOI Without DOI Pros • Insensitivity to magnetic fields • High gain (> 105) • Fast (~ 1 ns rise time) TOF Cons • Limited dynamic range • Optical crosstalk • Temperature dependence Mean FWHM in central slice: (1.25±0.08) mm MADPET3 Example: Hamamatsu MPPC LYSO-SiPM Module LYSO crystals (four 1 x 1 x 20 mm3 crystals, St. Gobain), polished surface, wrapped in reflector foil PET Prototype based on SiPMs (Hamamatsu MPPCs) • Proof of principle and first experience with SiPMs • LYSO (Lutetium Yttrium Oxyorthosilicate) scintillators Interface Card • 6.63 g/cm³, Zeff=7.2, energy res. 10% @ 662 keV, 32000 ph/MeV, 40 ns decay time, 420 nm peak • • • • Individual readout of 1 x 1 x 20 mm³ crystals 64 ch, no preamps at frontend Phantoms are rotated SADC data acquisition • Features • Typical energy resolution • 20% FWHM • Coincidence time resolution • 4.8 ns FWHM • Iterative MLEM image reconstruction 4 x 1 SiPM (mppc) matrix from Hamamatsu Pulko et al. TUM 2010 4 29.01.2014 Name der Klinik Data Acquisition E18 SADC with USB readout • 32 channels per SADC • Independently digitized • 10 bit ADC • 80 MHz sampling rate MADPET3 Prototype MLEM reconstruction System matrix including detector response function Measurement: 2 line sources, 0.73 mm separation • USB 2.0 readout with Linux • Pulse maxima and timestamps • Processed with C++ and ROOT SiPM based PET SiPM for high resolution PET KETEK PM1150NT • Readout electronics less demanding • High granularity one-to-one coupling achievable • Heavily parallel readout needed KETEK GmbH München www.ketek.net • PM1150NT • 1.2 x 1.2 mm² 50 µm cells 70% FF no optical trenches • Characteristics important for PET • Peak sensitivities @ 420 nm with a PDE up to 60% • Operating range 28 – 34 V • SMD package 2.0 x 2.5 mm² • Temperature coefficient of breakdown is 18 mV/K • Gain M is very temperature stable dM/dT < 1% • Gain M = 8 · 106 at 20% Uover 5 29.01.2014 Photodetection Efficiency PDE Gain Variation with Temperature PM1150NT: PDE > 60% @ 420 nm PM3350: < 1% @ U over ,rel > 10% PM1150: < 1% @ U over ,rel > 6% • High PDE is the key for good timing and energy resolution • less need for temperature and bias control GAGG BGO LYSO • 60% PDE for blue light, 45% PDE for green light • Suitable for LYSO, GAGG and BGO • Both SiPM types • Key feature all KETEK SiPMs have in common Temperature Coefficient 22% OV 0,00 12% OV -0,50 dM/dT*1/M [%] -1,00 -1,50 -2,00 PM3350 -2,50 PM1150 -3,00 -3,50 -4,00 0,00 PDE measured by Y. Mousienko, CERN Coincidence Timing 2x PM3350 (3x3 mm², 50µm cell size) + 2x identical crystals 5,00 10,00 15,00 20,00 25,00 30,00 Relative Overvoltage [%] MADPET4 preclinical PET/MR development PET insert for a 7T small animal MRI 7T animal MR • Crystals wrapped in Teflon and coupled with silicone • Only events in the photopeak are taken into account • Coincidence Time Resolution • 210 ps for 2x2x6 mm³ LYSO • 446 ps for 2x2x6 mm³ GAGG • Requirements PET frontend • High spatial resolution < 1.5 mm • DOI (depth of interaction): dual ring concept • Individual crystal readout for high rate capability • Inner diameter ~10 cm, outer ~15 cm 6 29.01.2014 MADPET4 • Crystal length determines sensitivity • Sensitivity is essential for PET • Design study: • Inner layer 1.5 x 1.5 x 6 mm³ LYSO • Outer layer 1.5 x 1.5 x 14 mm³ LYSO MADPET4 Monte Carlo Simulations GATE simulation of Derenzo-like phantom using the 6-14 geometry • Phantom fills 87% of the FOV • 6 minutes simulation of 511 keV back-to-back emission in x-y plane • All rods clearly visible • Crystals are rotated and not blockwise • Results in better and more homogenous sensitivity in the FOV • 330 crystals per ring, 8 rings in axial direction resulting in 2640 channels MADPET Activity • Very fruitful cooperation: physics and nuclear medicine • Driven by technology • Group at the clinic is the link to application Acknowledgments Florian Schneider Ian Somlai Gizem Seker Jorge Cabello Stephan Paul Igor Konorov Dieter Renker ERC Mumi FP7 EndoTOFPETUS FP7 TRImage University of Tokyo 7
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