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Investigation of LaBr 3 Detector Timing Resolution

Investigation of LaBr 3 Detector Timing Resolution. Kuhn 1 , S. Surti 1 , K.S. Shah 2 , and J.S. Karp 1 1 Department of Radiology, University of Pennsylvania, Philadelphia, PA 2 Radiation Monitoring Devices, Watertown, MA. Abstract.

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Investigation of LaBr 3 Detector Timing Resolution

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  1. Investigation of LaBr3 Detector Timing Resolution Kuhn1, S. Surti1, K.S. Shah2, and J.S. Karp1 1Department of Radiology, University of Pennsylvania, Philadelphia, PA 2Radiation Monitoring Devices, Watertown, MA

  2. Abstract Lanthanum bromide (LaBr3) scintillation detectors are currently being developed for use in time-of-flight (TOF) PET. In recent years, studies have been aimed at the parameterization of the LaBr3 scintillation properties. We have utilized the findings of these studies in the development of simulation tools to investigate and predict the performance of TOF PET detectors of realistic geometries. Here, we present a model to simulate the combined scintillator and photomultiplier tube (PMT) response to incident photons. This model allows us to study the effects of crystal response, geometry, and surface finish, PMT response, transit time spread, and noise, as well as discrimination techniques on the coincidence resolving time achievable in various detector configurations. Results from the simulations are benchmarked against several experimental measurements with two different PMTs and LaBr3 crystals of varying cerium concentration and geometry. A comparison is also made to the time resolution achievable with LYSO. Good agreement between measurement and simulation has been achieved with detectors consisting of 4x4x30 mm3 crystals suitable for use in a TOF PET scanner. Ultimately, this guides the improvement of TOF detectors by identifying the individual contribution of each detector component on the time resolution that can be achieved.

  3. Properties of LaBr3 • Fast Rise and Decay Times • Reduction in random coincidences • Excellent coincidence time resolution • Excellent Energy Resolution • Reduction in scattered events and random coincidences • Very High Light Output • Good crystal discrimination with long narrow crystals (i.e., 4x4x30 mm3) • Low Melting Point (783 ˚C) • Easier crystal growth, reduction in material costs Values obtained from reference [5-11]

  4. Model Introduction (I) • Photon Transport (MonteCrystal): • Gamma-ray trajectory • Tracks gamma interactions (Compton & Photoelectric) • Defined detector materials & geometry • Crystal type (LaBr3 and LYSO) • Crystal Size (varied crystal length with 4x4 mm2 cross-section) • Single crystal/PMT and Anger-logic detector geometries • Scintillation photons generated at each interaction point • Crystal scintillation response parameterized [3] • Path of scintillation photons traced • Modeled crystal surfaces, boundaries and reflector material

  5. Model Introduction (II) • Modeled PMT Parameters • Transit time spread (jitter) • Quantum efficiency • Response of PMT (single photoelectron) • Signal noise from dark current • Discriminator Time Pick-off • Leading edge model Two PMTs Modeled The XP20D0 represents good timing performance in a 2 inch diameter PMT and is being used in our prototype LaBr3 scanner, the HM R4998 was chosen because of its extremely fast response and low TTS.

  6. Model - Block Diagram Montecrystal Crystal Surface And Reflector Properties Detector Geometry Crystal Response Interactions in Crystal (Compton & Photoelectric) Generation of Scintillation Photons Gamma ray Transport Track Scintillation Photons PMT & SignalModel Threshold Setting PMT Transit Time Spread Noise Convolve PMT Response Event Time Discriminator Anode Signal

  7. Simulation of Pulse Shapes 5.0% Ce LaBr3 Response Photoelectrons created at PMT cathode Taken from reference [3] Response at photocathode is convolved with the measured single photo-electron PMT response Measured single photoelectron response for XP20D0 Simulated Pulse Shape 5.0% Ce LaBr3 Measured Noise Histogram XP20D0 Dark current noise (Gaussian fit to measured noise histogram) is added to the simulated PMT pulse shape

  8. Single Crystal on XP20D0 PMT Simulation Measurement LYSO • Rise time of 30% Ce LaBr3 (~3.5 ns) is faster than 5.0% Ce LaBr3 (~5 ns) • Simulated pulse shapes have slightly faster rise and decay compared to those measured due to the finite response of the oscilloscope used to record the pulses • LYSO pulses have ~20% signal amplitude compared to LaBr3 LYSO All Crystals are 4x4x30 mm3Measured pulse shapes include oscilloscope response

  9. Single Crystal on HM R4998 PMT Measurement Simulation LYSO LYSO • Response of R4998 is faster than XP20D0 • Reduced rise time of 30% Ce LaBr3 (~2 ns) and 5.0% Ce LaBr3 (~3 ns), thus improving the ability to accurately determine the start time of the pulses All Crystals are 4x4x30 mm3Measured pulse shapes include oscilloscope response

  10. Relative Light Output: Crystal Surface Finish Simulated Light Collection • Comparison of light collection for various crystal surface finishes • Large light loss for a crystal with all diffuse surfaces • Previously tested crystal samples indicate that the light output behavior is comparable to the simulation of a crystal with both specular and diffuse surfaces (i.e., 1 diffuse and 4 specular surfaces) for crystal lengths up to 30 mm (i.e., ~30% reduction in light collection compared to very small samples) Crystal cross-section is 4x4 mm2

  11. Coincidence Time Resolution:LaBr3: 5.0% Ce Coupled Directly to PMT Measured Coincidence Time Resolution Two 5.0%Ce LaBr3 (4x4x30 mm3) Simulated Coincidence Time Resolution XP20D0 XP20D0 FWHM 280 ps Simulation HM R4998 Simulation HM R4998 (Crystal cross-section is 4x4 mm2) FWHM 240 ps - Measured resolution with XP20D0 - Measured resolution with HM R4998

  12. Coincidence Time Resolution:LaBr3: 30% Ce Crystal Coupled Directly to PMT Measured Coincidence Time Resolution Two 30%Ce LaBr3 (4x4x5 mm3) Simulated Coincidence Time Resolution Simulation XP20D0 XP20D0 FWHM 190 ps Simulation HM R4998 HM R4998 (Crystal cross-section is 4x4 mm2) FWHM 145 ps - Measured resolution on XP20D0 - Measured resolution on HM R4998

  13. Coincidence Time Resolution:LYSO Crystal Coupled Directly to PMT Measured Coincidence Time Resolution Two LYSO crystals (4x4x20 mm3) Simulated Coincidence Time Resolution XP20D0 XP20D0 Simulation FWHM 380 ps HM R4998 Simulation HM R4998 (Crystal cross-section is 4x4 mm2) FWHM 310 ps - Measured resolution on XP20D0 - Measured resolution on HM R4998

  14. Anger-logic Detector:Coincidence Time Resolution • Detector Geometry • 7 PMTs coupled to a light guide and 4x4x30 mm3 crystal array • PMT transit times varied by ~ + 200 ps • Simulation indicates a significant improvement in time resolution can be achieved by utilizing a PMT with faster response 7 XP20D0’s: Coincidence Time Resolution 7 HM R4998’s: Coincidence Time Resolution

  15. Conclusions • Simulated time resolution is in good agreement with the measured data points for LaBr3 and LYSO crystals coupled directly to PMTs as well as in an Anger-logic design • The faster response and lower transit time spread of the HM R4998 PMT leads to a significant improvement in the coincidence time resolution achieved • Simulation and experimental measurements with 30% Ce LaBr3 indicate an improvement in coincidence time resolution over the 5.0% Ce LaBr3 on the HM R4998 PMT due to the faster response • Utilizing a PMT with the properties of the HM R4998 in an Anger-logic detector design can potentially yield a coincidence time resolution of ~200 ps with LaBr3 and ~400 ps with LYSO

  16. Acknowledgments This work was supported by NIH R33EB001684 and a research agreement with Saint-Gobain. We would like to thank the research members at Saint-Gobain and Radiation Monitoring Devices for their continued support. References [1] A. Kuhn, S. Surti, J. S. Karp, and et. al, ”Performance Assessment of Pixelated LaBr3 Detector Modules for TOF PET,” TNS, 51, no. 5, October 2004. [2] A. Kuhn, S. Surti, J. S. Karp, and et. al, ”Design of a Lanthanum Bromide Detector for Time-of-Flight PET,” TNS, 51, no. 5, October 2004. [3] J. Glodo, W.W. Moses, W.M. Higgins, E.V.D. van Loef, P. Wong, S.E. Derenzo, M.J. Weber, K.S. Shah, “Effects of Ce Concentration on Scintillation Properties of LaBr3:Ce,” Nuclear Science Symposium Conference Record, 2004 IEEE Volume 2,  16-22 Oct. 2004 Page(s):998 - 1001. [4] S. Surti, J. S. Karp and G. Muehllehner, " Image quality assessment of LaBr3-based whole-body 3D PET scanners: A Monte Carlo Evaluation," PMB, 49, 4593-4610, 2004. [5] S. Surti, J. S. Karp, G. Muehllehner , and P.S. Raby, ”Investigation of Lanthanum Scintillators for 3-D PET,” TNS, 50, no. 3, 348-354, 2003. [6] S. Surti, J. S. Karp and G. Muehllehner, " Evaluation of Pixelated NaI(Tl) Detectors for PET," TNS, 50, no. 1, 24-31, 2003. [7] K. Shah, J. Glodo, M. Klugerman, and et. al., "LaBr3:Ce scintillators for gamma ray spectroscopy," TNS, 50, no. 6, 2410-2413, 2003. [8] C. W. E. van Eijk, "Inorganic scintillators in medical imaging,” PMB., 47, R85-R106, 2002. [9] W. Moses and S. Derenzo, "Prospects for time-of-flight pet using LSO scintillator," TNS, 46, 474-478, 1999. [10] E. van Loef, P. D. P, C. van Eijk, K. Kramer, and H. Gudel, "High energy-resolution scintillator: Ce3+ activated LaCl3.," Appl. Phys. Lett., 77, 1467-1468, 2000. [11] E. van Loef, P. D. P, C. van Eijk, K. Kramer, and H. Gudel, "High energy-resolution scintillator: Ce3+ activated LaBr3.,”Appl. Phys. Lett., 79, 1573-1575, 2001.

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