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PET DetectorsPET Detectors
William W. MosesLawrence Berkeley National Laboratory
March 26, 2002
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Step 1: Inject Patient with Radioactive DrugStep 1: Inject Patient with Radioactive Drug
• Drug is labeled with positron(ββββ+) emitting radionuclide.
• Drug localizes in patient according to metabolic properties of that drug.
• Trace (pico-molar) quantities of drug are sufficient.
• Radiation dose fairly small(<1 rem).
Drug Distributes in BodyDrug Distributes in BodyDrug Distributes in Body
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Ideal Tracer IsotopeIdeal Tracer Isotope
18F 2 hour half-life15O, 11C, 13N 2, 20, & 10 minute half-lives
18F 2 hour half-life15O, 11C, 13N 2, 20, & 10 minute half-lives
• Interesting BiochemistryEasily incorporated into biologically active drugs.
• 1 Hour Half-LifeMaximum study duration is 2 hours.Gives enough time to do the chemistry.
• Easily ProducedShort half life ⇒⇒⇒⇒ local production.
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Step 2: Detect Radioactive DecaysStep 2: Detect Radioactive DecaysRing of PhotonDetectors
• Radionuclide decays, emitting ββββ+.
• ββββ+ annihilates with e– from tissue, forming back-to-back 511 keV photon pair.
• 511 keV photon pairs detected via time coincidence.
• Positron lies on line defined by detector pair (known as a chordor a line of response or a LOR).
Detect Pairs of Back-to-Back 511 keV PhotonsDetect Pairs of Back-to-Back 511 keV Photons
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Multi-Layer PET CamerasMulti-Layer PET Cameras
• Can image several slices simultaneously• Can image cross-plane slices• Can remove septa to increase efficiency (“3-D PET”)
Planar Images “Stacked” to Form 3-D ImagePlanar Images “Stacked” to Form 3-D Image
Scintillator Tungsten Septum Lead Shield
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Step 3: Reconstruct with Computed TomographyStep 3: Reconstruct with Computed Tomography
2-Dimensional Object
By measuring all 1-dimensional projections of a2-dimensional object, you can reconstruct the object
By measuring all 1-dimensional projections of a2-dimensional object, you can reconstruct the object
1-Dimensional Vertical Projection
1-Dimensional Horizontal Projection
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Attenuation CorrectionAttenuation Correction
• Use external ββββ+ source to measure attenuation.
• Attenuation (for that chord) same as for internal source.
• Source orbits around patient to measure all chords.
• Measure Attenuation Coefficient for Each Chord• Obtain Quantitative Images
• Measure Attenuation Coefficient for Each Chord• Obtain Quantitative Images
ββββ+ Source
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Time-of-Flight TomographTime-of-Flight Tomograph
• Can localize source along line of flight.
• Time of flight information reduces noise in images.
• Time of flight tomographs have been built with BaF2and CsF.
• These scintillators force other tradeoffs that reduce performance.
c = 1 foot/ns500 ps timing resolution⇒⇒⇒⇒ 8 cm fwhm localization
Not Compelling with Present Technology...Not Compelling with Present Technology...
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PET Images of CancerPET Images of Cancer
Metastases Shown with Red Arrows
Brain Heart
Bladder
Normal Uptake in Other Organs Shown in Blue
Treated Tumor Growing Again on
Periphery
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PET Camera DesignPET Camera Design
• Typical Parameters• Detector Module Design
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PET CamerasPET Cameras
• Patient port ~60 cm diameter.• 24 to 48 layers, covering 15 cm axially.• 4–5 mm fwhm spatial resolution.• ~2% solid angle coverage.• $1 – $2 million dollars.
Images courtesy of GE Medical Systems and Siemens / CTI PET Systems
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Early PET Detector ElementEarly PET Detector Element
BGO Scintillator Crystal(Converts γγγγ into Light)
Photomultiplier Tube(Converts Lightto Electricity)
3 — 10 mm wide(determines in-plane
spatial resolution)
10 — 30 mm high(determines axialspatial resolution)
30 mm deep(3 attenuation
lengths)
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Modern PET Detector ModuleModern PET Detector Module
BGO Scintillator Crystal Block(sawed into 8x8 array,
each crystal 6 mm square)
4 PMTs(25 mm square)
50 mm
50 mm 30 mm
• Saw cuts direct light toward PMTs.
• Depth of cut determines light spread at PMTs.
• Crystal of interaction found with Anger logic (i.e. PMT light ratio).
Good Performance, Inexpensive, Easy to PackGood Performance, Inexpensive, Easy to Pack
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Crystal Identification with Anger LogicCrystal Identification with Anger Logic
Can Decode Up To 64 Crystals with BGOCan Decode Up To 64 Crystals with BGOX-Ratio
Y-Ratio
Uniformly illuminate block.For each event, compute
X-Ratio and Y-Ratio,then plot 2-D position.
Individual crystals show up as dark regions.
Profile shows overlap (i.e. identification not perfect).
ProfilethroughRow 2
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Fundamental Limits of Spatial ResolutionFundamental Limits of Spatial Resolution
• Dominant Factor is Crystal Width• Limit for 80 cm Ring w/ Block Detectors is 3.6 mm
• Dominant Factor is Crystal Width• Limit for 80 cm Ring w/ Block Detectors is 3.6 mm
d/2
Reconstruction Algorithm 1.25 (in-plane)�1.0 (axial)
Factor
dDetector Crystal Width
Photon Noncollinearity
180¡ ± 0.25¡
Positron Range
Shape FWHM
multiplicative factor
0.5 mm (18F)�4.5 mm (82Rb)
1.3 mm (head)�1.8 mm (heart)
0 (individual coupling)�2.2 mm (Anger logic)*� *empirically determined� from published data
Anger Logic
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Tangential Projection
Radial ElongationRadial Elongation
• Penetration of 511 keV photons into crystal ring blurs measured position.
• Effect variously known as Radial Elongation, Parallax Error, or Radial Astigmatism.
• Can be removed by measuring depth of interaction.Radial
Projection
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PET Front End ElectronicsPET Front End Electronics
AnalogASIC
Custom ASIC
PMT APMT BPMT CPMT D
Energy ADC
FPGAX ADCY ADC
Time TDC
RAM
Off the Shelf
“Singles”Event Word
• Position• Time
• Digitize Arrival Time (latch 500 MHz clock — 2 ns accuracy)• Identify Crystal of Interaction & Measure Energy• Correct Energy and Arrival Time (based on crystal)• Maximum “Singles” Event Rate is 1 MHz / Detector Module
If Energy Consistent with 511 keV,Send Out “Singles” Event Word (Position & Time)
If Energy Consistent with 511 keV,Send Out “Singles” Event Word (Position & Time)
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PET Readout ElectronicsPET Readout Electronics
FPGAsFiber Optic
Interface
Off the Shelf
“Coincidence”Event Word
• Locationof Chord
Singles 0
• Search for “Singles” in Time Coincidence (~10 ns window)• Strip Off Timing Information• Format “Coincidence” Event Word (chord location)• Maximum “Coincidence” Event Rate is 10 MHz / Camera
Search for Coincidences, Send Out“Coincidence” Event Word (Position of Chord)
Search for Coincidences, Send Out“Coincidence” Event Word (Position of Chord)
Singles n
.
.
.
From Each Camera Sector
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PET Detector RequirementsPET Detector RequirementsDetect 511 keV Photons With
(in order of importance):• >85% efficiency• <5 mm spatial resolution• “low” cost (<$100 / cm2)• “low” dead time (<1 µs cm2)• <5 ns fwhm timing resolution• <100 keV fwhm energy resolution
Based on Current PET Detector ModulesBased on Current PET Detector Modules
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Very Strong Parallels...Very Strong Parallels...
New Scintillators Developed RecentlyNew Scintillators Developed Recently
Image courtesy of E. Auffray, CERN
LSOPbWO4
Image courtesy of C. M elcher, CT I PET System s
• Discovered in ~1992.• Approximately 10 years of R&D before large scale production.• Development efforts driven by end users, but included efforts
of luminescence scientists, spectroscopists, defects scientists,materials scientists, and crystal growers.
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Scintillator PropertiesScintillator Properties
Different Tradeoffs RequiredDifferent Tradeoffs Required
PbWO4 Lu2SiO5Density (g/cc): 8.3 7.4Attenuation Length (cm): 0.9 1.2Light Output (phot/MeV): 200 25,000Decay Time (ns): 10 40Emission Wavelength (nm): 420 420Radiation Hardness (Mrad): >10 10Dopants: Y, Nd CeCost per cc: $1 >$25
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Avalanche Photodiode ArraysAvalanche Photodiode Arrays
RMD, Inc.Hamamatsu Photonics
Advantages:• High Quantum Efficiency ⇒⇒⇒⇒ Energy Resolution• Smaller Pixels ⇒⇒⇒⇒ Spatial Resolution• Individual Coupling ⇒⇒⇒⇒ Spatial Resolution
Challenges:• Dead Area Around Perimeter• Signal to Noise Ratio• Reliability and Cost