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Motivation Reflector Optimization LSO Optimization PMT Optimization. Optimization of LSO for Time-of-Flight PET. W. W. Moses 1 , M. Janecek 1 , M. A. Spurrier 2 , P. Szupryczynski 2,3 , W.-S. Choong 1 , C. L. Melcher 2 , and M. Andreaco 3

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optimization of lso for time of flight pet
Motivation

Reflector Optimization

LSO Optimization

PMT Optimization

Optimization of LSOfor Time-of-Flight PET

W. W. Moses1, M. Janecek1, M. A. Spurrier2, P. Szupryczynski2,3 ,W.-S. Choong1, C. L. Melcher2, and M. Andreaco3

1Lawrence Berkeley National Laboratory2University of Tennessee, Knoxville3Siemens Medical Solutions

October 21, 2008

Outline:

This work was supported by the NIH (NIBIB grant No. R01-EB006085).

time of flight in pet

500 ps timing resolution

 7.5 cm localization

D

Time-of-Flight in PET

c = 30 cm/ns

  • Can localize source along line of flight.
  • Time of flight information reduces noise in images.
  • Variance reduction given by 2D/ct.
  • 500 ps timing resolution5x reduction in variance!
  • Time of Flight Provides a Huge Performance Increase!
  • Largest Improvement in Large Patients
commercial tof pet w lso
Commercial TOF PET w/ LSO

~550 ps Coincidence Timing Achieved

our goal demonstration tof pet camera
Our Goal:“Demonstration” TOF PET Camera
  • With better timing resolution (t), huge gains predicted(23x variance reduction for 100 ps timing)
  • Measure image improvement vs. timing resolution
  • Use LSO scintillator
    • Don’t change other factors that influence SNR(efficiency, scatter fraction, etc.)

Achieve the Best Timing Possible w/ LSO

what limits timing resolution
What Limits Timing Resolution?

Non-TOF Block Detector Module

Baseline 160 ps

Crystal

Geometry 326 ps

Light Sharing 454 ps

PMT 422 ps

PMT Array 274 ps

  • Many Factors
  • “Optical Geometry” Particularly Important
timing values will be for a single detector
Timing Values will be for a Single Detector*

Windowed Timing

Photopeakevents

LSO

1 cm3BaF2

TriggerDetector

H5321

Ge-68

R9800

TestDetector

CFD

Start

TAC

Stop

CFD

Canberra 454

F=0.2, D=0.6 ns

Canberra 454

F=0.2, D=0.6 ns

Ortec 566

Timing

ShapingAmplifier

ADC

Pulse Height

NI-7833R

+/- fwhm

Photopeak events

*Unless explicitly mentioned otherwise

  • Only Accept Events in Photopeak Window
  • Subtract (in Quadrature) 150 ps Trigger Contribution
proposed side coupled design
Proposed Side-Coupled Design

PMT

384 ps

(543 ps coinc.)

Conventional Geometry

(End-Coupled Crystal)

ScintillatorCrystal

218 ps

Proposed Geometry(Side-Coupled Crystal)

PMT

Shorter Optical Path Length & Fewer Reflections

detector module design

Hole in ReflectorOn Top Face of Crystals

Two LSO Crystals

(each 6.15 x 6.15 x 25 mm3)

Detector Module Design

Reflector

(on all five faces of each crystal, including the face between the two crystals)

PMT

(HamamatsuR-9800)

Optical Glue

(between lower crystal faces and PMT)

Two Side-Coupled Scintillator Crystals per PMT

detector ring geometry

Crystal ofInteraction

Detector Ring Geometry
  • Top face of each crystal (with hole in reflector) is coupled via a small (<1 mm) air gap to the edge of one opposing PMT.
  • Light seen by the opposing PMT is used to decode the crystal of interaction.

Exploded View

Crystals Decoded by Opposing PMT

camera geometry
Camera Geometry

Section of Detector Ring

Lead Shielding

Modules

  • Detector ring is 825 mm diameter, 6.15 mm axial
  • 192 detector modules, 384 LSO scintillator crystals
  • Adjustable gap (6 – 150 mm) between lead shields allows “scatter-free” and “3-D” shielding geometries

“Real” Single-Ring PET Camera for Humans & Phantoms

optimization surface finish reflector
Optimization: Surface Finish & Reflector

Requirements

  • Best Possible Timing Resolution
  • Good Light Collection Efficiency
  • Good Energy Resolution
  • Manufacturable:
    • Easy
    • Reliable & Reproducible
    • Rugged
    • Well-Controlled Light from Top Surface
  • Starting Point: Saw Cut LSO w/ Teflon Tape

Both Performance & Manufacturing Important

surface reflector optimization method
Surface & Reflector Optimization Method
  • 6.15 x 6.15 x 25 mm3
  • Reflector on 5 Sides
  • Optical Grease
  • No Hole on Top
  • Measure Timing of “Raw” Crystal(saw cut finish, Teflon tape reflector)
  • Apply Surface Treatment
  • Apply Reflector
  • Re-Measure Timing
  • Compute Percent Change
  • Repeat for 5 Crystals & Average Results
  • Do for All Surface / Reflector Combinations(>100 crystals, each measured twice)

R-9800

Same PMT forall measurements

Measure PercentageChange in Timing

surface reflector results

Average

1.00

1.02

1.01

0.99

1.00

1.00

0.98

Average 1.01 0.96 1.03

Surface & Reflector Results

Reflector Saw Cut Chemically Mechanically Etched Polished

Air Gap

Teflon 1.00 ± 0.17 0.94 ± 0.10 1.06 ± 0.09

ESR 1.01 ± 0.08 0.96 ± 0.16 1.08 ± 0.08

Lumirror 1.03 ± 0.13 0.96 ± 0.06 1.04 ± 0.12

Glued

ESR 0.99 ± 0.09 0.98 ± 0.03 1.01 ± 0.18

Lumirror 1.04 ± 0.10 0.97 ± 0.10 0.98 ± 0.22

Melinex 1.01 ± 0.16 0.99 ± 0.06 1.01 ± 0.20

Epoxy 1.00 ± 0.16 0.95 ± 0.09 1.00 ± 0.15

Paint 0.96 ± 0.03

finished crystal
Finished Crystal

Etched Surface, White Primer Spray Paint

pulse height performance
Pulse Height Performance

Well-Coupled PMT

Poorly-Coupled PMT

18% fwhm

Good Energy Resolution & Crystal Decoding

optimization lso composition
Optimization: LSO Composition

I0

I(t) = I0 exp(-t/)

Light Output = I0 

  • Both Scintillators Have Same Light Output (photons/MeV)
  • Red Decay Time is 2x Longer Than Blue Decay Time
  • Predicted Timing Resolution  1/sqrt(I0)
  • Want High Total Light Output & Short Decay Time
  • Possible By Co-Doping LSO With Calcium
optimization lso composition1

High Light Out

The Good Stuff!

Short 

0.1%

0.3%

0.4%

0.2%

= Ca-doped

Optimization: LSO Composition

Normal LSO

Ca-Doping Gives High Light Output & Short 

lso composition optimization method
LSO Composition Optimization Method
  • 5 x 5 x 5 mm3
  • Saw Cut Surface
  • Teflon on 5 Sides
  • Optical Grease
  • Get Samples of Normal & Co-Doped LSO
  • Measure Decay Time
  • Measure Relative Light Output
  • Compute Initial Intensity I0
  • Measure Timing Resolution
  • Plot Timing Resolution vs. I0

R-9800

Same PMT forall measurements

Measure Time Resolution vs. Initial Intensity

measured results lso composition

0.1%

0.4%

0.3%

0.2%

= Ca-doped

Measured Results: LSO Composition

Normal LSO

Scaled by1/sqrt(I0)

  • Ca-Doping Gives Good Timing Resolution
  • ~15% Improvement Over Normal LSO
optimization photomultiplier tube

Blue Sensitivity Index

Peak QE

Optimization: Photomultiplier Tube
  • Predicted Timing Resolution  1/sqrt(QE)
  • Want High Quantum Efficiency Version of PMT
pmt optimization method
PMT Optimization Method
  • 6.15 x 6.15 x 25 mm3
  • Chemically etched
  • Lumirror reflector
  • Optical Grease
  • Same Crystal forall measurements
  • Couple “Standard” LSO Crystal to PMT(“normal” LSO, etched surface finish,Lumirror reflector glued to five sides)
  • Measure Timing
  • Repeat for all PMTs
  • Measure twice for each PMT

R-9800

Measure Time Resolution vs. Blue Sensitivity

measured results high qe pmts

= “32% QE” PMTs

Measured Results: High QE PMTs

Normal (“28% QE”) PMTs

Scaled by1/sqrt(Blue Index)

  • Increased QE Improves Timing Resolution by 7%
  • Expect 10% Improvement with 35% SBA PMT
electronics

FPGA

CERNTDC

Out…

CFD

4 ADCs

Sum

Shaper

CFD

4 ADCs

Sum

Shaper

Electronics

Based on Siemens “Cardinal” electronics.

CFD triggers if any of 4 adjacent modules fire.

CERN HPTDC digitizes arrival time w/ 24 ps LSB.

Pulse height from all 8 modules read out on every trigger.

FPGA uses pulse heights to identify interaction crystal.

FPGA also does calibration, event formatting, etc.

  • Intrinsic Timing Resolution is 63 ps fwhm
  • With Detectors, Same Timing as NIM Electronics
summary
Summary

Hardware Single Coinc. TOF(ps fwhm) (ps fwhm) Gain

End-Coupled Crystal 384 544 4.3

Side-Coupled Crystal 218 309 7.6

Etched, Reflector Paint 227 321 7.3

Co-Doped LSO 182 258 9.1

32% QE PMT 155 219 10.6

35% QE “SBA” PMT 148 209 11.1

  • TOF PET with Significantly Better Timing is Possible
  • To Achieve, We Must “Think Outside the Block Detector”
future tof pet design

Scintillator

Array

Thinned

SiPM Array

Future TOF PET Design?

  • Depth of Interaction & 150 ps Timing Resolution
  • 11x Reduction in Variance in Practical Geometry