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Page 1: PET DetectorsPET Detectors - umich.edu

PET DetectorsPET Detectors

William W. MosesLawrence Berkeley National Laboratory

March 26, 2002

Page 2: PET DetectorsPET Detectors - umich.edu

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

Page 3: PET DetectorsPET Detectors - umich.edu

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

Page 5: PET DetectorsPET Detectors - umich.edu

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

Page 19: PET DetectorsPET Detectors - umich.edu

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