The Physics of Positron Emission Tomography (PET)

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The Physics of Positron Emission Tomography (PET)

  1. 1. 1 Physics & Instrumentation in Positron Emission Tomography Paul Vaska, Ph.D. Center for Translational Neuroscience Brookhaven National Laboratory July 21, 2006
  2. 2. 2 Non-invasive Medical Imaging Techniques Anatomical • X-ray • CAT • MRI • Ultrasound Functional • “nuclear medicine” - SPECT, PET CAT X-Ray MRI
  3. 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. 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. 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. 6 + + + NucleusNucleus NeutronsNeutrons ProtonsProtons ElectronsElectrons Positron (Positron (ββ++ ) Decay) Decay 18 F-FDG
  7. 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. 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. 9 Raw Data & Image Reconstruction 0° projection 0° 180° 90° 90°projection image reconstruction “sinogram”
  10. 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. 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. 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. 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. 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. 15 New Developments • Detectors • Multimodality imaging • Specialized applications
  16. 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. 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. 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. 19 New Developments: Detectors • Pb converters & ionization HIDAC Pb-walled straws (50 cm long)
  20. 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. 21 New Developments: Detectors • Time of flight using LaBr3 noTOF300psTOF 1 Mcts 5 Mcts 10 Mcts
  22. 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. 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. 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. 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. 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. 27 Imaging Conditions Anesthetized 250-350 g rats Limited DAQ livetime >> long scans for statistics Artifacts
  28. 28. 28 F-18 Fluoride Bone Scan 1.3 mCi fluoride RatCAP microPET R4
  29. 29. 29 C-11 Raclopride 1.8 mCi raclopride In the RatCAP
  30. 30. 30 C-11 Methamphetamine Time-activity curve for striatum
  31. 31. 31 Thanks! DOE OBER funding

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