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The Physics of Positron Emission Tomography (PET)

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

    • Physics & Instrumentation in Positron Emission Tomography
      • Paul Vaska, Ph.D.
      • Center for Translational Neuroscience
      • Brookhaven National Laboratory
      • July 21, 2006
    • Non-invasive Medical Imaging Techniques
      • Anatomical
        • X-ray
        • CAT
        • MRI
        • Ultrasound
      • Functional
        • “ nuclear medicine” - SPECT, PET
        • Optical fluorescence, …
      CAT X-Ray MRI
    • 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
    • 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
    • PET Imaging Overview
      • Synthesize radiotracer
      • Inject radiotracer
      • Measure gamma-ray emissions from isotope (~20-60 min)
      • Reconstruct images of radiotracer distribution (nCi/cc)
    • Positron (  + ) Decay 18 F-FDG + + + Nucleus Neutrons Protons Electrons
    •  + Decay Neutron-deficient isotopes can decay by emitting positrons anti-neutrino positron
      • Net effect: one proton replaced by
      • neutron
      • anti-neutrino
      • positron
      + + + + + + + + + +
    • 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 -
    • Raw Data & Image Reconstruction 0  projection 90  projection image reconstruction “ sinogram” 0  180  90 
    • Important Detector Properties
      • Spatial resolution
        • Directly controls spatial resolution in reconstructed image
        • Currently ~ 1 - 5 mm
        • Depth-of-interaction?
          • Reduces “parallax”
    • Important Detector Properties
      • Detection efficiency (aka sensitivity, stopping power)
        • Reduces noise from counting statistics
        • Currently > ~ 30% (singles)
      55M Events 1M Events
    • Important Detector Properties
      • Time resolution
        • Affects acceptance of random coincidences
        • Currently ~ 1 - 10 ns
        • Time-of-flight (TOF)?
          • c = ~ 1 ft/ns
          • Need << 1 ns resolution
      Random (accidental) coincidence
    • Important 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
    • Prototypical PET Detector 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 Gamma Ray Optical reflector
    • New Developments
      • Detectors
      • Multimodality imaging
      • Specialized applications
    • New Developments: Detectors
      • Scintillators
        • No perfect choice - tradeoffs
        • Also practical qualities
          • Rugged?
          • Hygroscopic?
          • Cost?
      175 25
    • 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
    • New Developments: Detectors
      • Solid-state detectors
        • Direct conversion, no photodetector
        • Great dE/E & spatial resolution
        • Poorer timing & stopping power
        • CZT
      Z 2 Z 1 S a2 S a1 S c        
    • New Developments: Detectors
      • Pb converters & ionization
      HIDAC Pb-walled straws (50 cm long)
    • New Developments: Detectors
      • 3D gamma-ray event positioning
        • Depth of interaction
        • Reduces parallax problem
      vs . LSO slab crystal holder APD decoupling capacitor HV filter capacitor Current-limiting resistor signal output connector SHV connector unused APD slot
    • New Developments: Detectors
      • Time of flight using LaBr 3
      no TOF 300 ps TOF 1 Mcts 5 Mcts 10 Mcts
    • 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
    • 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
    • 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 optical differential RatCAP
    • 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 18 mm axial FOV
    • 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  Ci/cc
      1st prototype: LLD = 150 keV average, variable
    • Imaging Conditions
      • Anesthetized 250-350 g rats
      • Limited DAQ livetime >> long scans for statistics
      • Artifacts
    • F-18 Fluoride Bone Scan
      • 1.3 mCi fluoride
      RatCAP microPET R4
    • C-11 Raclopride
      • 1.8 mCi raclopride
      In the RatCAP
    • C-11 Methamphetamine Time-activity curve for striatum
    • Thanks! DOE OBER funding