1. Optimization of LSO for 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 Laboratory 2University of Tennessee, Knoxville 3Siemens Medical Solutions
October 21, 2008
Outline:
Motivation
Reflector Optimization
LSO Optimization
PMT Optimization
This work was supported by the NIH (NIBIB grant No. R01-EB006085).

2. Time-of-Flight in PET
c = 30 cm/ns
500 ps timing resolution
 7.5 cm localization
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! D
One normal variation in PET cameras is the “time-of-flight” or TOF design. By measuring the difference in arrival time at the two detectors, the positron source can be localized along the line of flight. Doing this does not improve the spatial resolution, but improves the signal to noise ratio (the mechanism will be described in more detail in the next slide) — the variance improves by a factor of 2D/ct, where D is the diameter of the radionuclide distribution, c is the speed of light, and t is the TOF resolution. Several TOF PET systems were built in the 1980’s with barium fluoride or cesium fluoride scintillators. They achieved ~500 ps timing resolution, which results in 8 cm localization. For objects the size of the human head (which was what most PET cameras imaged in the 1980’s) the net result is a tomograph with a factor of ~2 lower variance than a non-TOF BGO tomograph.
Problems arose from the use of barium fluoride as a scintillator. It is less dense than BGO, and so the spatial resolution is degraded. In addition, the wavelength of its fast emission is in the hard UV, which made it difficult to work with (i.e. expensive) because it does not penetrate glass-windowed photomultiplier tubes or any known glue (to couple the crystal to the photomultiplier tube). Finally, it was difficult to keep these cameras in tune. Thus, TOF PET largely died at the end of the 80’s. However, the advent of LSO and other new PET scintillators (that can provide excellent timing resolution without the material drawbacks of barium fluoride) makes TOF a promising direction for modern PET.
Time of Flight Provides a Huge Performance Increase!
Largest Improvement in Large Patients

3. Commercial TOF PET w/ LSO
~550 ps Coincidence Timing Achieved

4. 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

5. 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

6. Timing Values will be for a Single Detector*
LSO
Trigger Detector
1 cm3 BaF2
H5321
Ge-68
R9800
Test Detector
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
Shaping Amplifier
ADC
Pulse Height
NI-7833R
+/- fwhm
Photopeak events
Windowed Timing
Photopeak events
*Unless explicitly mentioned otherwise
Only Accept Events in Photopeak Window
Subtract (in Quadrature) 150 ps Trigger Contribution

7. Proposed Side-Coupled Design
PMT
384 ps
(543 ps coinc.)
Scintillator Crystal
Conventional Geometry
(End-Coupled Crystal)
218 ps
Proposed Geometry (Side-Coupled Crystal)
PMT
Shorter Optical Path Length & Fewer Reflections

8. Detector Module Design
Hole in Reflector On Top Face of Crystals
Two LSO Crystals
(each 6.15 x 6.15 x 25 mm3)
Reflector
(on all five faces of each crystal, including the face between the two crystals)
PMT
(Hamamatsu R-9800)
Optical Glue
(between lower crystal faces and PMT)
Two Side-Coupled Scintillator Crystals per PMT

9. 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.
Crystal of Interaction
Exploded View
Crystals Decoded by Opposing PMT

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

11. 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

12. 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 for all measurements
Measure Percentage Change in Timing

13. Surface & Reflector Results
Reflector Saw Cut Chemically Mechanically Etched Polished
Air Gap
Teflon 1.00 ± ± ± 0.09
ESR 1.01 ± ± ± 0.08
Lumirror 1.03 ± ± ± 0.12
Glued
ESR 0.99 ± ± ± 0.18
Lumirror 1.04 ± ± ± 0.22
Melinex 1.01 ± ± ± 0.20
Epoxy 1.00 ± ± ± 0.15
Average
1.00
1.02
1.01
0.99
0.98
Average
Paint ± 0.03

14. Etched Surface, White Primer Spray Paint
Finished Crystal
Etched Surface, White Primer Spray Paint

15. Pulse Height Performance
Well-Coupled PMT
Poorly-Coupled PMT
18% fwhm
Good Energy Resolution & Crystal Decoding

16. Optimization: LSO Composition
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

17. Optimization: LSO Composition
High Light Out
Short 
Normal LSO
The Good Stuff!
= Ca-doped
0.1%
0.2%
0.4%
0.3%
Ca-Doping Gives High Light Output & Short 

18. 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 for all measurements
Measure Time Resolution vs. Initial Intensity

19. Measured Results: LSO Composition
Normal LSO
Scaled by 1/sqrt(I0)
= Ca-doped
0.1%
0.2%
0.4%
0.3%
Ca-Doping Gives Good Timing Resolution
~15% Improvement Over Normal LSO

20. Optimization: Photomultiplier Tube
Blue Sensitivity Index
Peak QE
Predicted Timing Resolution  1/sqrt(QE)
Want High Quantum Efficiency Version of PMT

21. PMT Optimization Method
6.15 x 6.15 x 25 mm3
Chemically etched
Lumirror reflector
Optical Grease
Same Crystal for all 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

22. Measured Results: High QE PMTs
Normal (“28% QE”) PMTs
Scaled by 1/sqrt(Blue Index)
= “32% QE” PMTs
Increased QE Improves Timing Resolution by 7%
Expect 10% Improvement with 35% SBA PMT

23. Electronics Intrinsic Timing Resolution is 63 ps fwhm
Shaper
4 ADCs
CFD
Sum
CERN TDC
FPGA
Out…
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

24. Summary Hardware Single Coinc. TOF (ps fwhm) (ps fwhm) Gain
End-Coupled Crystal
Side-Coupled Crystal
Etched, Reflector Paint
Co-Doped LSO
32% QE PMT
35% QE “SBA” PMT
TOF PET with Significantly Better Timing is Possible
To Achieve, We Must “Think Outside the Block Detector”

25. Future TOF PET Design? Depth of Interaction & 150 ps Timing Resolution
Scintillator
Array
Thinned
SiPM Array
Depth of Interaction & 150 ps Timing Resolution
11x Reduction in Variance in Practical Geometry