Department of Radiology UNIVERSITY OF PENNSYLVANIA

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Investigation of LaBr3 Detector Timing Resolution

A. Kuhn1, S. Surti1, K.S. Shah2, and J.S. Karp1

1Department of Radiology, University of Pennsylvania, Philadelphia, PA2Radiation Monitoring Devices, Watertown, MA

Department of Radiology UNIVERSITY OF PENNSYLVANIA

2Abstract

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.

Department of Radiology UNIVERSITY OF PENNSYLVANIA

3Properties 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

Scintillator (ns) (cm -1) E/E (%)At 662 keV

Relative Light Output (%)

NaI(Tl) 230 0.35 6.6 100

BGO 300 0.95 10.2 15

CsF 3 0.39 18.0 5

BaF2 2 0.45 11.4 5

GSO 60 0.70 8.5 25

LSO/LYSO 40 0.86 10.0 75

LaBr3 25 0.47 2.9 160Values obtained from reference [5-11]

Department of Radiology UNIVERSITY OF PENNSYLVANIA

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• 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

Model Introduction (I)

Department of Radiology UNIVERSITY OF PENNSYLVANIA

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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.

Department of Radiology UNIVERSITY OF PENNSYLVANIA

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Model - Block Diagram

Gamma rayTransport

Interactions inCrystal (Compton& Photoelectric)

DetectorGeometry

CrystalResponse

Crystal SurfaceAnd Reflector

Properties

Generation ofScintillation

Photons

Track ScintillationPhotons

PMT TransitTime Spread

Convolve PMTResponse

Noise

Anode Signal

ThresholdSetting

DiscriminatorEvent Time

PMT & Signal Model

Montecrystal

Department of Radiology UNIVERSITY OF PENNSYLVANIA

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Simulation of Pulse Shapes5.0% Ce LaBr3 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 LaBr3

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

8Single Crystal on XP20D0 PMTSimulation Measurement

• 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

All Crystals are 4x4x30 mm3 Measured pulse shapes include oscilloscope response

LYSOLYSO

Department of Radiology UNIVERSITY OF PENNSYLVANIA

9Single Crystal on HM R4998 PMT

Simulation Measurement

• 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 mm3 Measured pulse shapes include oscilloscope response

LYSOLYSO

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Relative Light Output: Crystal Surface Finish

Crystal cross-section is 4x4 mm2

• 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

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11Coincidence Time Resolution:LaBr3: 5.0% Ce Coupled Directly to PMT

Simulated Coincidence Time Resolution

XP20D0

HM R4998

(Crystal cross-section is 4x4 mm2)

- Measured resolution with XP20D0

- Measured resolution with HM R4998

Measured Coincidence Time Resolution

Two 5.0%Ce LaBr3 (4x4x30 mm3)

XP20D0

HM R4998

FWHM280 ps

FWHM240 ps

Simulation

Simulation

Department of Radiology UNIVERSITY OF PENNSYLVANIA

12Coincidence Time Resolution: LaBr3: 30% Ce Crystal Coupled Directly to PMT

(Crystal cross-section is 4x4 mm2)

Simulated Coincidence Time Resolution

XP20D0

HM R4998

- Measured resolution on XP20D0

- Measured resolution on HM R4998

Measured Coincidence Time Resolution

Two 30%Ce LaBr3 (4x4x5 mm3)

XP20D0

HM R4998

FWHM190 ps

FWHM145 ps

Simulation

Simulation

Department of Radiology UNIVERSITY OF PENNSYLVANIA

13Coincidence Time Resolution: LYSO Crystal Coupled Directly to PMT

(Crystal cross-section is 4x4 mm2)

Simulated Coincidence Time Resolution

XP20D0

HM R4998

- Measured resolution on XP20D0

- Measured resolution on HM R4998

Measured Coincidence Time Resolution

Two LYSO crystals (4x4x20 mm3)

XP20D0

HM R4998

FWHM380 ps

FWHM310 ps

Simulation

Simulation

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• 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

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

15Conclusions

• 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

Department of Radiology UNIVERSITY OF PENNSYLVANIA

16AcknowledgmentsThis 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.