Sensitivity Study of the RPC–PET whole-body scanner

1. Sensitivity Study of the RPC–PET whole-body scanner M. Couceiro1,2, A. Blanco1, Nuno C. Ferreira3, R. Ferreira Marques1,4, P. Fonte1,2, L. Lopes1 1 LIP, Laboratório de Instrumentação e Física Experimental de Partículas, Coimbra, Portugal 2 ISEC, Instituto Superior de Engenharia de Coimbra, Coimbra, Portugal 3 IBILI-FMUC, Instituto Biomédico de Investigação da Luz e Imagem, Faculdade de Medicina, Coimbra, Portugal 4 Departamento de Física, Universidade de Coimbra, Coimbra, Portugal

2. Summary • Resistive Plate Chambers (RPCs) • Simulation Results of Sensitivity for Human Full-Body Axial Field Of View PET Systems • Conclusions

3. HV Time signal XY readout plane RC passive network Y-strips RC passive network X-strips RPCs for PET Sensitive Area (precise small Gas Gap ~300 µm) Resistive Cathode E e- Resistive Anode At least one resistive electrode Photon • For 511 keV photons, and commonly used materials • ~300 ps FWHM for photon pairs • < 0.4% efficiency per gap for singles • No energy resolution

4. Stacked RPCs e- e- .......... e- Improving Detection Efficiency • Efficiency depends both on • Photon interaction probability in the converter plate • Electron extraction probability from the converter plate Converter plates with high interaction probability for 511 keV photons Optimize plate thickness for optimum electron extraction

5. N Plates Lead Glass Lead-Glass N Plates Optimum Thickness (µm) Detection Efficiency: 511 keV Photons (GEANT4 Simulation) Photon (511 keV) Detection Probability Plate Thickness (m) Bateman et al. 5 plates of 0.4 mm glass: Our measurements: 0.8% GEANT4 Simulation: 1.0% Optimum efficiency is balanced by beam absorption (thicker plates) and extraction probability (thinner plates) Optimum thickness depends on the number of plates and on the material.

6. N Plates Lead Glass Lead-Glass Photon Detection Probability Incident Photon Energy (keV) Detection Efficiency: Energy Dependence (GEANT4 Simulation) @ optimum thickness @ optimum thickness @ optimum thickness Strong ENERGY SENSITIVY scattered photonsstatistically rejected

7. Full-BodyHuman TOF-PET Comparison With the Standard PET Technology • Disadvantages • Much smaller detection efficiency: 20% to 50%. • No energy resolution, although energy sensitivity. • Advantages • Increased position accuracy • Sub-millimetric spatial resolution • Full 3D detection • Increased system sensitivity • Inexpensive • Large areas possible • large solid angle coverage • Excellent timing resolution (300 ps FWHM for 511 keV photon pair) • TOF-PET • Optimum randoms rejection Possible specialized PETapplications

8. Human Full Body FOV RPC–PET: Sensitivity Advantage • Main Goals • Study the sensitivity gain for large Axial Field Of View human PET systems (> 200 cm) • Crystal based • RPC based • Comparison with commercial scanners (e.g. GE Advance) • Methods • Simple analytical model for the sensitivity to true coincidences • GEANT4 simulations of photon transport through sensitivity phantoms (NEMA NU2 1994 and an extended version) • Photon interaction within detectors not simulated • Post processing of detection and coincidences (detection efficiency, packing fraction, energy blurring, energy window, coincidence window, etc.): • Scintillator based detectors • RPC based detectors • 3D true sensitivity computation followed the NEMA NU2 1994 procedure

9. L z Tomograph x1 d   R  Water Phantom d d x2 Z Human Full Body PET Sensitivity: Simplified Analytical Model Emitting Point

10. 15 cm AFOV by 92.7 cm Ø NEMA NU 2 1994 sensitivity phantom (b) 15 cm AFOV by 92.7 cm Ø with an extended phantom (c) 240 cm AFOV by 92.7 cm Ø with an extended phantom (a) (b) (c) Human Full Body PET Sensitivity: Simulation Setups

11. Human Full Body PET Sensitivity: Validation for Crystal Based Systems • Data processing • Photons assigned to a Module/Block/Crystal according to the GE Advance segmentation • Gaussian energy blurring with 20% FWHM at 511 keV • 300 – 650 keV energy window • Detection efficiency adjusted to obtain reasonable agreement with published data.

12. Human Full Body PET Sensitivity: Results for Crystal Based Systems Normalized to the GE Advance sensitivity ring difference of 11(axial acceptance angle ~5.7 deg) + Badawi et al. normalized ~100-fold sensitivity advantage for standard PET without TOF ? ~300-fold sensitivity advantage for LSO based PET with TOF ?

13. Human Full Body AFOV PET • Full Body AFOV Crystal Based PET Scanners • Maintaining crystal thicknessfor full body AFOV PET scanners • unaffordable • Keeping overall crystal volume, reducing crystal thickness • reduce detection efficiency, and sensitivity • RPC TOF-PET Scanners?

14. Photon Detection Efficiency Incident Photon Energy (keV) Human Full Body RPC TOF–PET Sensitivity: Selected Efficiencies Efficiencies of 60 and 120 stacked RPCs based on 0.4 mm glass plates 19.4% 11.0%

15. 3D True Sensitivity (kcps/(µCi/cc)) Axial Field of View (cm) Human Full Body RPC TOF–PET Sensitivity: Simulation Results ~20-fold sensitivity increase With TOF information t = 300 ps FWHM GE Advance (max. ~1240 kcps/(µCi/cc))

16. Scatter • Important source of image noise; • Typically rejected by energy discrimination. Scatter Fraction in Full Acceptance Mode Axial Field of View (cm) Human Full Body RPC TOF–PET Scatter Fraction: Simulation Results RPC Energy Sensitivity Efficiency Incident Photon Energy (keV) Moderate excess of scatter over most standard PET systems

17. Conclusions • Full-body AFOV sensitivity • ~20 fold sensitivity gains for 240 cm AFOV RPC TOF-PET may be attainable • Scatter is partially rejected by detector energy sensitivity • RPC application to PET seems possible in • Full-body human PET, offering larger throughput - hopefully without extra cost • Comprehensive study of a full system in progress • A first detector has been assembled and Luís Lopes in his talk will show the detector details, and present some preliminary results obtained with it