Transcript Overview of Medical Imaging Instrumentation and Techniques

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
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
(<1 rem).
Drug Distributes in Body
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
15O, 11C, 13N
2 hour half-life
2, 20, & 10 minute half-lives
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
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
Step 3: Reconstruct with Computed Tomography
2-Dimensional
Object
1-Dimensional
Vertical Projection
1-Dimensional
Horizontal Projection
By measuring all 1-dimensional projections of a
2-dimensional object, you can reconstruct the object
Why Do Computed Tomography?
Planar X-Ray
Computed Tomography
Separates Objects on Different Planes
Images courtesy of Robert McGee, Ford Motor Company
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
Time-of-Flight Tomograph
c = 1 foot/ns
500 ps timing resolution
 8 cm fwhm localization
• 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.
Not Compelling with Present Technology...
NMR & PET Images of Epilepsy
NMR
PET
• NMR “Sees” Structure with 0.5 mm Resolution
• PET “Sees” Metabolism with 5.0 mm Resolution
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
PET Camera Design
• Typical Parameters
• Detector Module Design
PET 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
Early PET Detector Element
BGO Scintillator Crystal
(Converts  into Light)
10 — 30 mm high
(determines axial
spatial resolution)
3 — 10 mm wide
(determines in-plane
spatial resolution)
Photomultiplier Tube
(Converts Light
to Electricity)
30 mm deep
(3 attenuation
lengths)
Modern PET Detector Module
4 PMTs
• Saw cuts direct light
toward PMTs.
(25 mm square)
• 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
50 mm
30 mm
(sawed into 8x8 array,
each crystal 6 mm square)
Good Performance, Inexpensive, Easy to Pack
Crystal Identification with Anger Logic
Counts
4000
3500
3000
2500
2000
1500
1000
500
0
Profile
through
Row 2
Uniformly illuminate block.
For each event, compute
X-Ratio and Y-Ratio,
then plot 2-D position.
Y-Ratio
Individual crystals show up
as dark regions.
Profile shows overlap (i.e.
identification not perfect).
X-Ratio
Can Decode Up To 64 Crystals with BGO
Fundamental Limits of Spatial Resolution
Factor
Shape
d
FWHM
d/2
Detector Crystal Width
0 (in dividual cou pling)
2.2 mm (Anger logic)*
*empirically det ermined
from published data
Anger Logic
1.3 mm (he ad)
1.8 mm (heart)
Photon Noncollinearity
0.5 mm (18 F)
4.5 mm (82 Rb)
Positron Range
Reconstruction Algorithm
multiplicative factor
1.2 5 (in -plane)
1.0 (axial)
• Dominant Factor is Crystal Width
• Limit for 80 cm Ring w/ Block Detectors is 3.6 mm
Radial Elongation
• Penetration of 511 keV
photons into crystal
ring blurs measured
position.
Tangential • Effect variously known
Projection as Radial Elongation,
Parallax Error, or
Radial Astigmatism.
Radial
Projection
• Can be removed by
measuring depth of
interaction.
PET Front End Electronics
Custom ASIC
Off the Shelf
RAM
Energy
PMT A
PMT B
PMT C
PMT D
Analog
ASIC
X
Y
Time
ADC
ADC
ADC
TDC
“Singles”
Event Word
FPGA
• 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)
PET Readout Electronics
Off the Shelf
From Each
Camera Sector
Singles 0
Singles n
.
.
.
“Coincidence”
Event Word
FPGAs
Fiber
Optic
Interface
• Location
of 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)
Similarities and Differences
Between Calorimetry & PET
• Similarities
• The PET World Picture...
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
The PET World Picture:
Need to Image
0.000000511 TeV*
Photons
Signal Levels Are Very Low
*511 keV
No Pair Production / EM Showers
Scatter Length ≈ 10 cm
• 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 scattered
in 3-D Whole Body PET.
• Compton Scatter is Important Background
• Use Energy to Reject Scatter in Patient
Patient Radiation Dose is Limited!
• Cannot Increase Signal Source Strength
• Image Noise Is Limited by Counting Statistics
Competitive Commercial Market
CMS Calorimeter
PET Camera
• $60 Million (parts cost)
• $1 Million (parts cost)
• 72,000 Channels
• 18,400 Channels
• $833 / Channel
• $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 >10% of total parts cost
Cost is Very Important
PET Detector Requirements
Detect 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 Modules
Synergies...
• Scintillators
• Photodetectors
• Electronics
• Computation
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 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.
Very Strong Parallels...
Scintillator Properties
Density (g/cc):
Attenuation Length (cm):
Light Output (phot/MeV):
Decay Time (ns):
Emission Wavelength (nm):
Radiation Hardness (Mrad):
Dopants:
Cost per cc:
PbWO4
8.3
0.9
200
10
420
>10
Y, Nd
$1
Lu2SiO5
7.4
1.2
25,000
40
420
10
Ce
>$25
Different Tradeoffs Required
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
APD Requirements
Calorimetry
High Gain?:
Yes
High QE / Blue Sensitivity?:
Yes
Radiation Hardness?:
Yes
Nuclear Counter Effect?:
Yes
Timing Signal (low C)?:
No
High Packing Density?:
No
Sensitive to Leakage Current?: ~
PET
Yes
Yes
No
No
Yes
Yes
Yes
Different Tradeoffs Required
Electronics Requirements
Calorimetry
Low Noise Analog Amplifier?: Yes
Low Power Consumption?:
Yes
Mixed-Mode Custom ICs?:
Yes
Real-Time Data Correction?:
Yes
Highly Parallel Readout?:
Yes
High Data Rate?:
Yes
Many Similarities
PET
Yes
Yes
Yes
Yes
Yes
Yes
Electronics Requirements
Calorimetry
Radiation Damage?:
Yes
Analog Dynamic Range:
High
Self-Generated Timing Signal?: No
Asynchronous Inputs?:
No
Event Size / Complexity?:
High
Multiple Trigger Levels?:
Yes
“Good” Event Rate?:
kHz
PET
No
Low
Yes
Yes
Low
No
MHz
Different Tradeoffs Required
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?:
TB–PB
GB
Time to Finish Analysis?:
Years
Minutes
FDA Certification Required?:
No
Yes
Different Tradeoffs Required
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 are
clearly superior (not just novel)
 All requirements must be met
 Cost is very important
Difficult to Transfer Identical Technology
Need to optimize for PET tradeoffs