Synergies Between Calorimetry and PET

William W. MosesLawrence 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 BodyDrug Distributes in Body

Ideal Tracer Isotope

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.

Step 2: Detect Radioactive DecaysRing of PhotonDetectors

• 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

• 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

Scintillator Tungsten Septum Lead Shield

Step 3: Reconstruct with Computed Tomography

2-Dimensional Object

By measuring all 1-dimensional projections of a2-dimensional object, you can reconstruct the object

1-Dimensional Vertical Projection

1-Dimensional Horizontal Projection

Why Do Computed Tomography?Planar X-Ray Computed Tomography

Images courtesy of Robert McGee, Ford Motor Company

Separates Objects on Different Planes

Attenuation Correction

• 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

+ Source

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/ns500 ps timing resolution 8 cm fwhm localization

Not Compelling with Present Technology...

NMR & PET Images of Epilepsy

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

NMR PET

PET 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

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)

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)

Modern 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 Pack

Crystal Identification with Anger Logic

0500

1000150020002500300035004000

Counts

Can 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

Fundamental Limits of Spatial Resolution

• 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

Tangential Projection

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.Radial

Projection

PET 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)

PET 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)

Singles n

.

.

.

From Each Camera Sector

Similarities and DifferencesBetween Calorimetry & PET

• Similarities• The PET World Picture...

Similarities Between Calorimeters and PETPET CameraCalorimeter

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

The PET World Picture:

Signal Levels Are Very Low

*511 keV

Need to Image0.000000511 TeV*

Photons

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.

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

Scatter Length ≈ 10 cm

Patient Radiation Dose is Limited!

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

Competitive Commercial Market

Cost is Very Important

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

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

CMS Calorimeter 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

In a PET Camera:

PET 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 Modules

Synergies...

• Scintillators• Photodetectors• Electronics• Computation

Very Strong Parallels...

New Scintillators Developed Recently

Image courtesy of E. Auffray, CERN

LSOPbWO4

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.

Scintillator Properties

Different Tradeoffs Required

PbWO4Lu2SiO5

Density (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

Avalanche 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

APD Requirements

Calorimetry PETHigh Gain?: Yes YesHigh QE / Blue Sensitivity?: Yes YesRadiation Hardness?: Yes NoNuclear Counter Effect?: Yes NoTiming Signal (low C)?: No YesHigh Packing Density?: No YesSensitive to Leakage Current?: ~ Yes

Different Tradeoffs Required

Electronics Requirements

Calorimetry PETLow Noise Analog Amplifier?: Yes YesLow Power Consumption?: Yes YesMixed-Mode Custom ICs?: Yes YesReal-Time Data Correction?: Yes YesHighly Parallel Readout?: Yes YesHigh Data Rate?: Yes Yes

Many Similarities

Electronics Requirements

Calorimetry PETRadiation Damage?: Yes NoAnalog Dynamic Range: High LowSelf-Generated Timing Signal?: No YesAsynchronous Inputs?: No YesEvent Size / Complexity?: High LowMultiple Trigger Levels?: Yes No“Good” Event Rate?: kHz MHz

Different Tradeoffs Required

Computation Requirements

Calorimetry PETSignificant Computation?: Yes YesMonte Carlo Simulation?: Yes YesLarge Programming Project?: Yes YesComplexity of Analysis?: High LowData Set Size?: TB–PB GBTime to Finish Analysis?: Years

MinutesFDA Certification Required?: No Yes

Different Tradeoffs Required

Final ThoughtsMany 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 TechnologyNeed to optimize for PET tradeoffs