Optimization of LSO for Time-of-Flight PET

1. 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).

2. 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

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* 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

7. 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

8. 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

9. 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

10. 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

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 forall measurements Measure PercentageChange in Timing

13. 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

14. 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 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

17. 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 

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 forall measurements Measure Time Resolution vs. Initial Intensity

19. 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

20. Blue Sensitivity Index Peak QE Optimization: Photomultiplier Tube • 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 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

22. = “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

23. 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

24. 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”

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