Synergies Between Calorimetry and PET

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Synergies Between Calorimetry and PET
William W. Moses Lawrence Berkeley National
Laboratory March 26, 2002

• Outline
• Fundamentals of PET
• Comparison of Calorimetry PET
• Areas of Common Interest
• Conclusions

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Step 1 Inject Patient with Radioactive Drug

• Drug is labeled with positron(?) 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(lt1 rem).

Drug Distributes in Body
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Ideal Tracer Isotope

• Interesting Biochemistry
• Easily incorporated into biologically active
  drugs.
• 1 Hour Half-Life
• Maximum study duration is 2 hours.
• Gives enough time to do the chemistry.
• Easily Produced
• Short half life ? local production.

18F 2 hour half-life 15O, 11C, 13N 2, 20,
10 minute half-lives
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Step 2 Detect Radioactive Decays
Ring of Photon Detectors

• Radionuclide decays, emitting ?.
• ? 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 chord or a line of response or a LOR).

Detect Pairs of Back-to-Back 511 keV Photons
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Multi-Layer PET Cameras
Scintillator
Tungsten Septum
Lead Shield

• 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 Image
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Step 3 Reconstruct with Computed Tomography
2-Dimensional Object
By measuring all 1-dimensional projections of
a 2-dimensional object, you can reconstruct the
object
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Why Do Computed Tomography?
Planar X-Ray
Computed Tomography
Separates Objects on Different Planes
Images courtesy of Robert McGee, Ford Motor
Company
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Attenuation Correction
? Source

• Use external ? 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

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Time-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
  BaF2 and CsF.
• These scintillators force other tradeoffs that
  reduce performance.

c 1 foot/ns
Not Compelling with Present Technology...
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NMR PET Images of Epilepsy
NMR
PET

• NMR Sees Structure with 0.5 mm Resolution
• PET Sees Metabolism with 5.0 mm Resolution

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PET Images of Cancer
Brain
Heart
Bladder
Treated Tumor Growing Again on Periphery
Metastases Shown with Red Arrows
Normal Uptake in Other Organs Shown in Blue
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PET Camera Design

• Typical Parameters
• Detector Module Design

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PET Cameras

• Patient port 60 cm diameter.
• 24 to 48 layers, covering 15 cm axially.
• 45 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 Element
BGO Scintillator Crystal (Converts ? into Light)
Photomultiplier Tube (Converts Light to
Electricity)
10 30 mm high (determines axial spatial
resolution)
30 mm deep (3 attenuation lengths)
3 10 mm wide (determines in-plane spatial
resolution)
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Modern PET Detector Module
4 PMTs (25 mm square)

• 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).

50 mm
BGO Scintillator Crystal Block (sawed into 8x8
array, each crystal 6 mm square)
50 mm
30 mm
Good Performance, Inexpensive, Easy to Pack
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Crystal Identification with Anger Logic
Profile through Row 2
Uniformly illuminate block. For each event,
computeX-Ratio and Y-Ratio,then plot 2-D
position. Individual crystals show up as dark
regions. Profile shows overlap (i.e.
identification not perfect).
Y-Ratio
X-Ratio
Can Decode Up To 64 Crystals with BGO
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Fundamental Limits of Spatial Resolution

• Dominant Factor is Crystal Width
• Limit for 80 cm Ring w/ Block Detectors is 3.6 mm

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Radial 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.

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PET Front End Electronics
Custom ASIC
Off the Shelf
RAM
SinglesEvent Word
Energy
ADC
PMT A
X
ADC
PMT B
Analog ASIC
FPGA
Y
PMT C
ADC

• Position
• Time

PMT D
Time
TDC

• 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)
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PET Readout Electronics
Off the Shelf
From Each Camera Sector
CoincidenceEvent Word
Fiber Optic Interface
. . .
FPGAs

• Locationof Chord

• 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)
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Similarities and DifferencesBetween Calorimetry
PET

• Similarities
• The PET World Picture...

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Similarities Between Calorimeters and PET
Calorimeter
PET Camera

• Cylindrical Gamma Ray Detectors
• High Efficiency, Hermetic
• Segmented, High Density Scintillator Crystals
• High Performance Photodetectors
• High Rate, Parallel Readout Electronics

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The PET World Picture
Need to Image 0.000000511 TeV Photons
Signal Levels Are Very Low
511 keV
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No Pair Production / EM Showers

• Compton scatter in patient produces erroneous
  coincidence events.
• 15 of detected events are scattered in 2-D
  PET(i.e. if tungsten septa used).
• 50 of events are scatteredin 3-D Whole Body
  PET.

Scatter Length 10 cm

• Compton Scatter is Important Background
• Use Energy to Reject Scatter in Patient

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Patient Radiation Dose is Limited!

• Cannot Increase Signal Source Strength
• Image Noise Is Limited by Counting Statistics

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Competitive Commercial Market
CMS Calorimeter
PET Camera

• 60 Million (parts cost)
• 72,000 Channels
• 833 / Channel

• 1 Million (parts cost)
• 18,400 Channels
• 54 / Channel

In a PET Camera

• Scintillator crystals are 25 of total parts
  cost
• Photomultiplier tubes are 25 of total parts
  cost
• No other component is gt10 of total parts cost

Cost is Very Important
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PET Detector Requirements

• Detect 511 keV Photons With(in order of
  importance)
• gt85 efficiency
• lt5 mm spatial resolution
• low cost (lt100 / cm2)
• low dead time (lt1 µs cm2)
• lt5 ns fwhm timing resolution
• lt100 keV fwhm energy resolution

Based on Current PET Detector Modules
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Synergies...

• Scintillators
• Photodetectors
• Electronics
• Computation

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New Scintillators Developed Recently
PbWO4
LSO
Image courtesy of E. Auffray, CERN
Image courtesy of C. Melcher, CTI PET Systems

• Discovered in 1992.
• Approximately 10 years of RD before large scale
  production.
• Development efforts driven by end users, but
  included efforts of luminescence scientists,
  spectroscopists, defects scientists, materials
  scientists, and crystal growers.

Very Strong Parallels...
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Scintillator Properties

• PbWO4 Lu2SiO5
• Density (g/cc) 8.3 7.4
• Attenuation Length (cm) 0.9 1.2
• Light Output (phot/MeV) 200 25,000
• Decay Time (ns) 10 40
• Emission Wavelength (nm) 420 420
• Radiation Hardness (Mrad) gt10 10
• Dopants Y, Nd Ce
• Cost per cc 1 gt25

Different Tradeoffs Required
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Avalanche Photodiode Arrays
Hamamatsu Photonics
RMD, Inc.

• 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

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APD Requirements

• Calorimetry PET
• High Gain? Yes Yes
• High QE / Blue Sensitivity? Yes Yes
• Radiation Hardness? Yes No
• Nuclear Counter Effect? Yes No
• Timing Signal (low C)? No Yes
• High Packing Density? No Yes
• Sensitive to Leakage Current? Yes

Different Tradeoffs Required
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Electronics Requirements

• Calorimetry PET
• Low Noise Analog Amplifier? Yes Yes
• Low Power Consumption? Yes Yes
• Mixed-Mode Custom ICs? Yes Yes
• Real-Time Data Correction? Yes Yes
• Highly Parallel Readout? Yes Yes
• High Data Rate? Yes Yes

Many Similarities
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Electronics Requirements

• Calorimetry PET
• Radiation Damage? Yes No
• Analog Dynamic Range High Low
• Self-Generated Timing Signal? No Yes
• Asynchronous Inputs? No Yes
• Event Size / Complexity? High Low
• Multiple Trigger Levels? Yes No
• Good Event Rate? kHz MHz

Different Tradeoffs Required
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Computation Requirements

• Calorimetry PET
• Significant Computation? Yes Yes
• Monte Carlo Simulation? Yes Yes
• Large Programming Project? Yes Yes
• Complexity of Analysis? High Low
• Data Set Size? TBPB GB
• Time to Finish Analysis? Years Minutes
• FDA Certification Required? No Yes

Different Tradeoffs Required
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Final Thoughts

• Many Synergies Exist Between HEP PET
• Scintillators, detectors, electronics, computing,
  
• Tools experience are particularly valuable
• PET is a Mature, Commercial Technology
• Innovations will only be used if they areclearly
  superior (not just novel)
• All requirements must be met
• Cost is very important
• Difficult to Transfer Identical Technology
• Need to optimize for PET tradeoffs