Department of Radiology UNIVERSITY OF PENNSYLVANIA 1 Investigation of LaBr 3 Detector Timing Resolution A.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

Department of Radiology UNIVERSITY OF PENNSYLVANIA 2 Abstract Lanthanum bromide (LaBr 3 ) 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 LaBr 3 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 LaBr 3 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 mm 3 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.

Department of Radiology UNIVERSITY OF PENNSYLVANIA 3 Properties of LaBr 3 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 mm 3 ) Low Melting Point (783 ˚C) –Easier crystal growth, reduction in material costs Scintillator  (ns)  (cm -1 )  E/E (%) At 662 keV Relative Light Output (%) NaI(Tl) BGO CsF BaF GSO LSO/LYSO LaBr Values obtained from reference [5-11]

Department of Radiology UNIVERSITY OF PENNSYLVANIA 4 Photon Transport (MonteCrystal): –Gamma-ray trajectory –Tracks gamma interactions (Compton & Photoelectric) –Defined detector materials & geometry Crystal type (LaBr 3 and LYSO) Crystal Size (varied crystal length with 4x4 mm 2 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 Model Introduction (I)

Department of Radiology UNIVERSITY OF PENNSYLVANIA 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 LaBr 3 scanner, the HM R4998 was chosen because of its extremely fast response and low TTS.

Department of Radiology UNIVERSITY OF PENNSYLVANIA 6 Model - Block Diagram Gamma ray Transport Interactions in Crystal (Compton & Photoelectric) Detector Geometry Crystal Response Crystal Surface And Reflector Properties Generation of Scintillation Photons Track Scintillation Photons PMT Transit Time Spread Convolve PMT Response Noise Anode Signal Threshold Setting DiscriminatorEvent Time PMT & Signal Model Montecrystal

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

Department of Radiology UNIVERSITY OF PENNSYLVANIA 8 Single Crystal on XP20D0 PMT SimulationMeasurement Rise time of 30% Ce LaBr 3 (~3.5 ns) is faster than 5.0% Ce LaBr 3 (~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 LaBr 3 All Crystals are 4x4x30 mm 3 Measured pulse shapes include oscilloscope response LYSO

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

Department of Radiology UNIVERSITY OF PENNSYLVANIA 10 Relative Light Output: Crystal Surface Finish Crystal cross-section is 4x4 mm 2 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) Simulated Light Collection

Department of Radiology UNIVERSITY OF PENNSYLVANIA 11 Coincidence Time Resolution: LaBr 3 : 5.0% Ce Coupled Directly to PMT Simulated Coincidence Time Resolution XP20D0 HM R4998 (Crystal cross-section is 4x4 mm 2 ) - Measured resolution with XP20D0 - Measured resolution with HM R4998 Measured Coincidence Time Resolution Two 5.0%Ce LaBr 3 (4x4x30 mm 3 ) XP20D0 HM R4998 FWHM 280 ps FWHM 240 ps Simulation

Department of Radiology UNIVERSITY OF PENNSYLVANIA 12 Coincidence Time Resolution: LaBr 3 : 30% Ce Crystal Coupled Directly to PMT (Crystal cross-section is 4x4 mm 2 ) Simulated Coincidence Time Resolution XP20D0 HM R Measured resolution on XP20D0 - Measured resolution on HM R4998 Measured Coincidence Time Resolution Two 30%Ce LaBr 3 (4x4x5 mm 3 ) XP20D0 HM R4998 FWHM 190 ps FWHM 145 ps Simulation

Department of Radiology UNIVERSITY OF PENNSYLVANIA 13 Coincidence Time Resolution: LYSO Crystal Coupled Directly to PMT (Crystal cross-section is 4x4 mm 2 ) Simulated Coincidence Time Resolution XP20D0 HM R Measured resolution on XP20D0 - Measured resolution on HM R4998 Measured Coincidence Time Resolution Two LYSO crystals (4x4x20 mm 3 ) XP20D0 HM R4998 FWHM 380 ps FWHM 310 ps Simulation

Department of Radiology UNIVERSITY OF PENNSYLVANIA 14 Detector Geometry –7 PMTs coupled to a light guide and 4x4x30 mm 3 crystal array PMT transit times varied by ~ ps Simulation indicates a significant improvement in time resolution can be achieved by utilizing a PMT with faster response Anger-logic Detector: Coincidence Time Resolution 7 XP20D0’s: Coincidence Time Resolution 7 HM R4998’s: Coincidence Time Resolution

Department of Radiology UNIVERSITY OF PENNSYLVANIA 15 Conclusions Simulated time resolution is in good agreement with the measured data points for LaBr 3 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 LaBr 3 indicate an improvement in coincidence time resolution over the 5.0% Ce LaBr 3 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 LaBr 3 and ~400 ps with LYSO

Department of Radiology UNIVERSITY OF PENNSYLVANIA 16 Acknowledgments This work was supported by NIH R33EB 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 LaBr 3 Detector Modules for TOF PET,” TNS, 51, no. 5, October [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 [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 LaBr 3 :Ce,” Nuclear Science Symposium Conference Record, 2004 IEEE Volume 2, Oct Page(s): [4] S. Surti, J. S. Karp and G. Muehllehner, " Image quality assessment of LaBr 3 -based whole-body 3D PET scanners: A Monte Carlo Evaluation," PMB, 49, , [5] S. Surti, J. S. Karp, G. Muehllehner, and P.S. Raby, ”Investigation of Lanthanum Scintillators for 3-D PET,” TNS, 50, no. 3, , [6] S. Surti, J. S. Karp and G. Muehllehner, " Evaluation of Pixelated NaI(Tl) Detectors for PET," TNS, 50, no. 1, 24-31, [7] K. Shah, J. Glodo, M. Klugerman, and et. al., "LaBr 3 :Ce scintillators for gamma ray spectroscopy," TNS, 50, no. 6, , [8] C. W. E. van Eijk, "Inorganic scintillators in medical imaging,” PMB., 47, R85-R106, [9] W. Moses and S. Derenzo, "Prospects for time-of-flight pet using LSO scintillator," TNS, 46, , [10] E. van Loef, P. D. P, C. van Eijk, K. Kramer, and H. Gudel, "High energy-resolution scintillator: Ce3+ activated LaCl 3.," Appl. Phys. Lett., 77, , [11] E. van Loef, P. D. P, C. van Eijk, K. Kramer, and H. Gudel, "High energy-resolution scintillator: Ce3+ activated LaBr 3.,”Appl. Phys. Lett., 79, , 2001.