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Position-Sensing Photon Position-Sensing Photon Detectors (for Positron Detectors (for Positron Emission Tomography)Emission Tomography)
Simon R. Cherry
Department of Biomedical Engineering
Center for Molecular and Genomic Imaging
University of California, Davis
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In Vivo Biomedical Imaging Technologies
Anatomic Physiologic Metabolic Molecular
PET/SPECT
x-ray CT
MRI
ultrasound
optical imaging
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Positron Emission Tomography
PET quantitatively and non-destructively measures the 3-D distribution of radiolabeled biomolecules in vivo
Primary tasks for imaging physics/engineering:–detect as many events as possible (sensitivity)–put events in the right place (spatial resolution)–make corrections and reconstruct quantitative
images (Bq/cc)
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+ Decay and Annihilation
QuickTime™ and aSorenson Video 3 decompressorare needed to see this picture.
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Positron Emission Tomography• Inject radiotracer• Detect (scintillation detectors) two
annihilation photons in coincidence• Defines line along which
annihilation lies• Collect ~107-108 events• Use reconstruction algorithms to
compute image of radiotracer distribution using all the different angular views
• Analyze data– Lesion detection
– Quantify radiotracer distribution
– Tracer kinetics
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PET Radionuclides and Labeled ProbesSmall molecules
• enzyme substrates, ligands, drugs…
Peptides• receptor targeted…
Antibodies• fragments, minibodies, diabodies
Reporter Genes• enzyme-based, receptor-based
Cells• T-cells, stem cells…
Particles• Liposomes, lipospheres, nanoparticles…
T1/2
11C 20.4 mins18F 110 mins64Cu 12.6 hrs68Ga 68 mins89Zr 3.3 days124I 4.2 days
PET Radionuclides
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PET Imaging in Cancer18F-Fluorodeoxyglucose
Images courtesy of GE Medical Systems
[18F]-fluoro-2-deoxy-D-glucose
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Preclinical In Vivo Imaging Bridging the Divide
GENOMICS/PROTEOMICSCOMBINATORIAL CHEM
IN VITRO, EX VIVO IN VIVO
In Vivo
Imaging
MEDICAL DIAGNOSTICS AND THERAPEUTICS
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Small Animal Imaging
Requires both:High spatial resolution - mouse organs ~1000-fold smaller volume than humanHigh sensitivity - number of targets also smaller, radiation dosimetry can be limiting
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Spatial Resolution and Sensitivityspatial resolution determined by detector width
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Spatial Resolution and Sensitivitysmaller detectors yield better resolution and better sampling
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Spatial Resolution and SensitivityThicker detectors improve sensitivity, but spatial resolution degrades due to parallax effects
€
sensitivity∝ (1−e−μd )2
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Spatial Resolution and SensitivityThicker detectors improve sensitivity, but spatial resolution degrades due to parallax effects
€
sensitivity∝ (e−μd )2
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Requirements
• High Sensitivity– High efficiency (thick) detectors– High solid angle coverage
• Small Detector Ring Diameter • Long axial extent
• High Spatial Resolution– Very small cross-section detector elements– Depth-Encoding Detectors
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Approaches
• Scintillation Detector– Photomultiplier tube (PMT)– Avalanche photodiode (APD)– Silicon photomultiplier (SiPM)
• High Density Semiconductors– CdTe or CZT– Ge– TlBr
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PET Scintillators
Scintillator 90% efficiency (cm)
Light output
(photons/MeV)
Decay time
(nsecs)
BGO 2.4 7,000 300
BaF2 5.1 2,000 0.8
CsF 5.4 1,900 4
LSO, LYSO 2.6 25,000 42
LaBr3 4.9 60,000 27
LuI3 4.1 100,000 30
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Factors Determining Spatial Resolution
Crystal width
Positron range
Scanner geometry
Non-colinearity
Depth of interaction
Detector scatter
How narrow should the crystals be?
What is the best resolution achievable
with PET?
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Positron Range
511 keV
positron range
511 keV
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Positron Range
fluorine-18(0.64 MeV) carbon-11
(0.97 MeV)
nitrogen-13(1.19 MeV)
oxygen-15(1.72 MeV)1 mm
Exponential distribution, range depends on energy of emitted positrons
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Non-Colinearity
511 keV
511 keV180 ± 0.25°
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Non-colinearity
• Annihilation photons emitted with angle of 180° ± 0.25°
• Non-colinearity blurring given by 0.022 x detector separation at center of scanner
D= 80 cm
R180° = 1.8 mm
D= 12 cm
R180° = 0.26 mm
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Spatial Resolution in PET
Detector Interactions2 cm LSO (log scale)
Small Animal Scanner (D = 8 cm)
Human Whole-Body Scanner (D = 80 cm)
Positron Range18F
1 mm
Total
R~2.0 mm
R~0.4 mm
Non-Colinearity
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Effect of Detector Size in Small Animal PET
“best” achievable resolution
detector size 1.5 mm1.0 mm0.5 mm0.25 mm
1 mm
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Resolution versus Detector Size18F tracer, 8 cm diameter scanner
FWTM
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Manufacturing of LSO ArraysStart off with a solid LSO crystal1
Slice the crystal to a desirable thickness (i.e. 0.5 mm) and glue reflector between the slices of scintillator
2
Glue the slices of scintillator together with the reflector between the scintillator slices.
3
Rotate the block 90°4
Again, slice the crystal to a desirable thickness (i.e. 0.5 mm) and glue reflector between the slices of scintillator
5
Glue the slices of scintillator together with the reflector between the scintillator slices.
6in collaboration with Agile Engineering
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LSO Arrays
• 56 x 56 array• 0.22 x 0.22 x 20 mm elements• 90:1 aspect ratio• Vikuiti ESR specular reflector
in collaboration with Agile Engineering
• Down to ~220 micron pixels• Up to 20 mm thick• Diffuse or specular reflectors
– Vikuiti ESR (3M) (65 µm)– Lumirror E20 (Toray) (50 µm)
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Photomultiplier Tubes
• PMTs (arrays of individual PMTs, MC-PMTs or PS-PMTs) struggle to resolve LSO arrays with element sizes much less than 0.5 mm.
0.5 x 0.5 x 10 mm elements Hamamatsu M64 MC-PMT
Flood Histogram
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Limitations• Resolution limited by
– brightness of scintillator– light transport to PMT– quantum efficiency of PMT (~20%)– Size of anode structures (~ 2-4 mm)
• Solution - solid state detectors?– Finer feature sizes– Higher quantum efficiency– Considerations
• Gain, noise, timing, area, # of channels…
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Depth-Encoding Detectors
“phoswich” design
photo-detector
photo-detector photodetector
photodetector
“offset” design “dual-ended” design
discrete depth informationcontinuous
depth information
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Avalanche Photodiodes• compact Si devices• electric field high enough for
multiplication of e-h pairs• magnetic field insensitive• range of sizes• monolithic arrays available
– but need lots of electronics
• compared with PMTs– lower gain– sensitive to temperature and bias
voltage
p+ drift region
avalanche regionn+ Si substrate +V
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Position-Sensitive Avalanche Photodiodes
Si avalanche photodiode with position-
sensitive resistive anode - only four
readouts from entire surface
Active area: 8x8 to 28x28 mm
Gain ~ 1000 at 1750 V
Noise = 200 e– (FWHM)
Q. E. ≥ 60 % (400-700 nm)
Rise time ~ 1 ns
Capacitance 0.7 pf/mm2
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Depth-Encoding PET Detectors
13 x 13 LSO array(0.5 x 0.5 x 20 mm3
elements)
PSAPD 1 (8 x 8 mm2)
PSAPD 2(8 x 8 mm2)
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Flood Histograms and Energy Resolution
room temperature 10°C 0°C –10°C
Crystal Size Reflector Energy Resolution (%)
0.5 x 0.5 x 20 mm3 Specular (ESR) 19.8 +/- 3.0
Diffuse (Toray) 21.1 +/- 4.9
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Depth of Interaction ResolutionPSAPD
Average DOI resolution
1.9 mm +/– 0.2 mm
Energy ratio = E1/E1+E2
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Intrinsic Spatial ResolutionRectangle, all events
0
1000
2000
3000
4000
5000
6000
1 4 7 10 13 16 19 22 25 28 31 34 37
Source position
Counts
MeasurementGaussian fit
FWHM: All events: 0.674 mm Photopeak: 0.681 mm Compton: 0.662 mm
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New Small Animal PET Scanner Design with Depth-Encoding Detectors
1.6 x higher sensitivity
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Beyond APDs - SiPMs
• Silicon Photomultipliers– Higher gain– Faster response time– Low bias voltage (tens of volts)– Less dependence on bias and temperature
– Non-linearity at higher light levels– Dark noise a problem at very low light levels– Less mature technology
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SiPM Structure
“low” light level
“high” light level
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Solid State Photomultipliers (SSPMs)• CMOS process
– Lower cost for mass production
– On-chip integration with electronics possible
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Position-Sensitive SPPMs
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Pixel Level PS-SPPMs
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Summary• LSO arrays with pixel sizes as low as 0.22 mm and
as thick as 20 mm can be fabricated for high resolution PET
• Dual-ended readout of these arrays with PSAPDs provides good crystal identification and depth encoding ability
• PS-SPPMs in a similar design may provide even better performance in the future
• Such detectors provide a pathway to constructing small animal PET scanners at or close to the physical resolution and sensitivity limits.
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AcknowledgmentsInstrumentation & Preclinical Molecular ImagingSimon CherryYongfeng YangGregory MitchellYibao WuChangqing LiEmilie RoncaliSara St. JamesMelissa FreedenbergBo PengRuby GillJulien Bec
Computational Molecular ImagingJinyi QiGuobao WangNannan CaoLin FuMichel TohmeJinxiu Liao
Clinical Molecular Imaging PhysicsRamsey BadawiAbhijit ChaudhariJonathan PoonSpencer BowenFelipe Godinez
Radiation Monitoring Devices Inc.Kanai ShahRichard FarrellPurushottam DokhaleMickel McClishChristopher StapelsErik JohnsonJames Christian
Keith Vaigneur (Agile Engineering)
Recent Lab MembersCiprian Catana (MGH/Harvard)Bernd Pichler (U. Tübingen)Martin Judenhofer (U. Tübingen)Jennifer StickelHongjie Liang (Philips Medical)Huini Du (Toshiba Medical)Shrabani Sinha
Funding:National Institutes of Health R01 EB000993 R01 EB006109 R01 CA134632Department of Energy DE-FG02-08ER64677
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