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Physics & Instrumentation inPositron Emission TomographyPhysics & Instrumentation in
Positron Emission Tomography
Paul Vaska, Ph.D.
Center for Translational NeuroscienceBrookhaven National Laboratory
July 21, 2006
P matrix = 0.6mm, = 0.1
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Non-invasive Medical Imaging Techniques
Non-invasive Medical Imaging Techniques
Anatomical• X-ray• CAT• MRI• Ultrasound
Functional• “nuclear medicine” - SPECT, PET• Optical fluorescence, …
CAT
X-Ray
MRI
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Positron Emission TomographyPositron 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
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Technical Challenges in PET ImagingTechnical 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
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PET Imaging OverviewPET Imaging Overview
- Synthesize radiotracer- Inject radiotracer- Measure gamma-ray
emissions from isotope (~20-60 min)
- Reconstruct images of radiotracer distribution (nCi/cc)
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+
+ +
NucleusNucleus
NeutronsNeutrons
ProtonsProtons
ElectronsElectrons
Positron (Positron (++) Decay) Decay
18F-FDG
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++ Decay Decay
+
+
+
+
+
+
+
+
+
Neutron-deficient isotopes can decay by emitting Neutron-deficient isotopes can decay by emitting positronspositrons
+
anti-neutrinoanti-neutrino
positronpositron Net effect: one Net effect: one proton proton replaced by replaced by
• neutronneutron• anti-neutrinoanti-neutrino• positronpositron
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Positron annihilationPositron 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-
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Raw Data & Image ReconstructionRaw Data & Image Reconstruction
0 projection
0
180
90
90 p
roje
ctio
n
image reconstruction
“sinogram”
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Important Detector PropertiesImportant Detector Properties
- Spatial resolution- Directly controls spatial resolution
in reconstructed image- Currently ~ 1 - 5 mm- Depth-of-interaction?
- Reduces “parallax”
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Important Detector PropertiesImportant Detector Properties
- Detection efficiency (aka sensitivity, stopping power)- Reduces noise from counting statistics- Currently > ~ 30% (singles)
55M Events1M Events
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Important Detector PropertiesImportant Detector PropertiesImportant 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
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Important Detector PropertiesImportant Detector PropertiesImportant 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
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ScintillationCrystal
PMTPre-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 DetectorPrototypical PET Detector
Gamma Ray
Optical reflector
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Scintillator NaI(Tl) BGO GSO LSO LuAP LPS LaBr (ns) 230 300 60 40 18 30 35
(cm-1) 0.35 0.95 0.70 0.86 0.95 0.70 0.47E/E (%) 6.6 10.2 8.5 10.0 ~15 ~10 2.9
Rel. light output (%) 100 15 25 70 30 73 150175
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New Developments: DetectorsNew Developments: Detectors
• Scintillators• No perfect choice - tradeoffs
• Also practical qualities• Rugged?• Hygroscopic?• Cost?
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New Developments: DetectorsNew Developments: Detectors
• Photosensors• Photomultiplier tubes• Avalanche photodiodes
• Arrays, position-sensitive• Compact but noisier
• Silicon photomultipliers• Very new• Best of both?
APD array
PMT
SiPM
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New Developments: DetectorsNew Developments: Detectors
• Solid-state detectors• Direct conversion,
no photodetector • Great dE/E &
spatial resolution• Poorer timing &
stopping power• CZT Z2
Z1
Sa2
Sa1
Sc
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New Developments: DetectorsNew Developments: Detectors
• Pb converters & ionization
HIDAC Pb-walled straws (50 cm long)
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New Developments: DetectorsNew Developments: Detectors
• 3D gamma-ray event positioning• Depth of interaction• Reduces parallax problem
LSOslab
APDslab
s
vs.
LSO slab
crystal holder
APD
decoupling capacitor
HV filter capacitor
Current-limiting resistor
signal output connector
SHV connector
unused APD slot
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New Developments: DetectorsNew Developments: Detectors
• Time of flight using LaBr3n
o T
OF
300 p
s T
OF
1 Mcts 5 Mcts 10 Mcts
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New DevelopmentsNew 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
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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
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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
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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 m
m a
xial F
OV
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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
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Imaging Conditions
Anesthetized 250-350 g rats
Limited DAQ livetime >> long scans for statistics
Artifacts
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