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The Physics of Positron Emission Tomography (PET)
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The Physics of Positron Emission Tomography (PET)

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  • 1. 1 Physics & Instrumentation in Positron Emission Tomography Paul Vaska, Ph.D. Center for Translational Neuroscience Brookhaven National Laboratory July 21, 2006
  • 2. 2 Non-invasive Medical Imaging Techniques Anatomical • X-ray • CAT • MRI • Ultrasound Functional • “nuclear medicine” - SPECT, PET CAT X-Ray MRI
  • 3. 3 Positron Emission Tomography Recent mainstream acceptance - relatively expensive - cyclotron for tracer production - detectors must stop high-energy gamma-rays - low resolution (>2 mm), limited counting statistics - BUT unique functional capabilities Applications - Diagnosis of disease - cancer (WB), cardiac, … - Research - brain function - animal studies
  • 4. 4 Technical Challenges in PET Imaging Radiochemistry - better tracers Imaging Physics - better images by • Detector design – Spatial resolution – Sensitivity • Image processing – Corrections for physical effects – Image reconstruction algorithms Data Analysis & Biological Modeling - better interpretation of images
  • 5. 5 PET Imaging Overview - Synthesize radiotracer - Inject radiotracer - Measure gamma-ray emissions from isotope (~20-60 min) - Reconstruct images of radiotracer distribution (nCi/cc)
  • 6. 6 + + + NucleusNucleus NeutronsNeutrons ProtonsProtons ElectronsElectrons Positron (Positron (ββ++ ) Decay) Decay 18 F-FDG
  • 7. 7 ββ++ DecayDecay + + + + + + + + + Neutron-deficient isotopes can decay by emittingNeutron-deficient isotopes can decay by emitting positronspositrons + anti-neutrinoanti-neutrino positronpositron Net effect: oneNet effect: one protonproton replaced byreplaced by • neutronneutron • anti-neutrinoanti-neutrino • positronpositron
  • 8. 8 Positron annihilation Annihilation gives • 2x 511 keV gamma rays • 180 degrees apart • Line of response Positron range & gamma noncollinearity Scanner is just a photon counter! • Counts gamma-ray pairs vs. single gammas • Time window ~ 1 ns 511 keV 511 keV e+ e-
  • 9. 9 Raw Data & Image Reconstruction 0° projection 0° 180° 90° 90°projection image reconstruction “sinogram”
  • 10. 10 Important Detector PropertiesImportant Detector Properties - Spatial resolution - Directly controls spatial resolution in reconstructed image - Currently ~ 1 - 5 mm - Depth-of-interaction? - Reduces “parallax”
  • 11. 11 Important Detector PropertiesImportant Detector Properties - Detection efficiency (aka sensitivity, stopping power) - Reduces noise from counting statistics - Currently > ~ 30% (singles) 55M Events1M Events
  • 12. 12 Important Detector PropertiesImportant Detector Properties Random (accidental) coincidence - Time resolution - Affects acceptance of random coincidences - Currently ~ 1 - 10 ns - Time-of-flight (TOF)? - c = ~ 1 ft/ns - Need << 1 ns resolution
  • 13. 13 Important Detector PropertiesImportant Detector Properties Scatter and Attenuation 511 keV - Energy resolution - Scattered gammas change direction AND lose energy - Affects acceptance of scattered coincidences - Currently ~ 20% - Deadtime - Handle MHz count rates! 511 keV 400 keV
  • 14. 14 Scintillation Crystal PMT Pre-Amplifier + Electronics Gamma photon converts to optical photons (proportional to gamma energy, typ. 1000’s) photons are collected at the end of the crystal light is converted to an electrical signal & amplified Front-end electronics condition the signal for further processing Prototypical PET Detector Gamma Ray Optical reflector
  • 15. 15 New Developments • Detectors • Multimodality imaging • Specialized applications
  • 16. 16 Scintillator NaI(Tl) BGO GSO LSO LuAP LPS LaBr τ (ns) 230 300 60 40 18 30 35 µ (cm-1 ) 0.35 0.95 0.70 0.86 0.95 0.70 0.47 ∆E/E (%) 6.6 10.2 8.5 10.0 ~15 ~10 2.9 Rel. light output (%) 100 15 25 70 30 73 150175 25 New Developments: Detectors • Scintillators • No perfect choice - tradeoffs • Also practical qualities • Rugged? • Hygroscopic? • Cost?
  • 17. 17 New Developments: Detectors • Photosensors • Photomultiplier tubes • Avalanche photodiodes • Arrays, position-sensitive • Compact but noisier • Silicon photomultipliers • Very new • Best of both? APD array PMT SiPM
  • 18. 18 New Developments: Detectors • Solid-state detectors • Direct conversion, no photodetector • Great dE/E & spatial resolution • Poorer timing & stopping power • CZT Z2 Z1 Sa2 Sa1 Sc ⊕ ∅ ⊕ ⊕ ∅ ∅ ⊕ ∅
  • 19. 19 New Developments: Detectors • Pb converters & ionization HIDAC Pb-walled straws (50 cm long)
  • 20. 20 New Developments: Detectors • 3D gamma-ray event positioning • Depth of interaction • Reduces parallax problem LSO slab γ γ APD slab s vs. LSO slab crystal holder APD decoupling capacitor HV filter capacitor Current- limiting resistor signal output connector SHV connector unused APD slot
  • 21. 21 New Developments: Detectors • Time of flight using LaBr3 noTOF300psTOF 1 Mcts 5 Mcts 10 Mcts
  • 22. 22 New Developments • Multimodality imaging • PET/CT • PET/MRI • Specialized applications • Brain, breast, prostate • Small animal - microPET • Arterial input function • Humans - wrist scanner • Animals - microprobe • Awake rat brain - RatCAP
  • 23. 23 RatCAP: Rat Conscious Animal PET Eliminate anesthesia in preclinical neuroscience using PET in order to: • Remove confounding effects of anesthetic on neurochemistry • Enable stimulation in animal PET • Enable correlations of behavior and neuro-PET
  • 24. 24 Architecture Detector blocks x12 • LSO 2.2 x 2.2 x 5 mm in 4 x 8 array • 1:1 coupling to APD • ASIC - single all digital output Timestamp & Signal Processing Module • Programmable real-time logic (FPGA) • 1 ns bins (debugging, now 10 ns) Data acquisition • PCI card in standard PC • Up to 70 MB/s = ~10 Mcps singles • Offline software for coincidences, corrections, recon, … TSPM TDC PCI card ASIC opticaldifferential RatCAP
  • 25. 25 Architecture RatCAP TSPM LSO APD ASICs all interconnections 38 mm FOV 72 mm OD optical links to PCI high voltage 194 g data, clock, power 18mmaxialFOV
  • 26. 26 Performance Spatial resolution (FWHM @ CFOV) • FBP: 2.1 mm • MLEM: <1.5 mm Energy resolution: 23% FWHM Time resolution: 14 ns FWHM • window = 30 ns Sensitivity (point @ CFOV): 0.7% Peak Noise Equivalent Count rate: 14 kcps @ 5 µCi/cc 1st prototype: LLD = 150 keV average, variable
  • 27. 27 Imaging Conditions Anesthetized 250-350 g rats Limited DAQ livetime >> long scans for statistics Artifacts
  • 28. 28 F-18 Fluoride Bone Scan 1.3 mCi fluoride RatCAP microPET R4
  • 29. 29 C-11 Raclopride 1.8 mCi raclopride In the RatCAP
  • 30. 30 C-11 Methamphetamine Time-activity curve for striatum
  • 31. 31 Thanks! DOE OBER funding

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