Transcript Investigation of LaBr3 Detector Timing Resolution

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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, PA
2Radiation Monitoring Devices, Watertown, MA
Department of Radiology
UNIVERSITY OF PENNSYLVANIA
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.
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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
Scintillator
t (ns)
m (cm -1)
DE/E (%)
Relative Light Output (%)
At 662 keV
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
160
Values obtained from reference [5-11]
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UNIVERSITY OF PENNSYLVANIA
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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
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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.
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Model - Block Diagram
Montecrystal
Gamma ray
Transport
Detector
Geometry
Crystal
Response
Crystal Surface
And Reflector
Properties
Interactions in
Crystal (Compton
& Photoelectric)
Generation of
Scintillation
Photons
Track Scintillation
Photons
Threshold
Setting
Noise
PMT Transit
Time Spread
Discriminator
Anode Signal
Convolve PMT
Response
PMT & Signal Model
Event Time
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Simulation of Pulse Shapes
5.0% Ce LaBr3 Response
Taken from reference [3]
Photoelectrons
created at PMT
cathode
Measured single
photoelectron
response for
XP20D0
Response at
photocathode is
convolved with
the measured
single photoelectron PMT
response
Simulated Pulse Shape
5.0% Ce LaBr3
Dark current
noise (Gaussian
fit to measured
noise histogram)
is added to the
simulated PMT
pulse shape
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Measured Noise Histogram
XP20D0
UNIVERSITY OF PENNSYLVANIA
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Single Crystal on XP20D0 PMT
Simulation
Measurement
LYSO
LYSO
All Crystals are 4x4x30 mm 3
Measured pulse shapes include oscilloscope response
• 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
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UNIVERSITY OF PENNSYLVANIA
Single Crystal on HM R4998 PMT
Measurement
Simulation
LYSO
LYSO
All Crystals are 4x4x30 mm 3
Measured pulse shapes include oscilloscope response
• 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
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Relative Light Output: Crystal Surface Finish
• 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)
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Simulated Light Collection
Crystal cross-section is 4x4 mm2
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Coincidence Time Resolution:
LaBr3: 5.0% Ce Coupled Directly to PMT
Simulated Coincidence Time Resolution
Measured Coincidence
Time Resolution
Two 5.0%Ce LaBr3 (4x4x30 mm3)
XP20D0
XP20D0
FWHM
280 ps
HM R4998
HM R4998
(Crystal cross-section is 4x4 mm2)
FWHM
240 ps
- Measured resolution with XP20D0
- Measured resolution with HM R4998
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Coincidence Time Resolution:
LaBr3: 30% Ce Crystal Coupled Directly to PMT
Simulated Coincidence Time Resolution
Measured Coincidence
Time Resolution
Two 30%Ce LaBr3 (4x4x5 mm3)
XP20D0
XP20D0
FWHM
190 ps
HM R4998
HM R4998
(Crystal cross-section is 4x4 mm2)
FWHM
145 ps
- Measured resolution on XP20D0
- Measured resolution on HM R4998
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UNIVERSITY OF PENNSYLVANIA
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Coincidence Time Resolution:
LYSO Crystal Coupled Directly to PMT
Simulated Coincidence Time Resolution
Measured Coincidence
Time Resolution
Two LYSO crystals (4x4x20 mm3)
XP20D0
XP20D0
FWHM
380 ps
HM R4998
HM R4998
(Crystal cross-section is 4x4 mm2)
FWHM
310 ps
- Measured resolution on XP20D0
- Measured resolution on HM R4998
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UNIVERSITY OF PENNSYLVANIA
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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
7 XP20D0’s: Coincidence Time Resolution
7 HM R4998’s: Coincidence Time Resolution
• Simulation indicates a significant improvement in time resolution
can be achieved by utilizing a PMT with faster response
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UNIVERSITY OF PENNSYLVANIA
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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
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UNIVERSITY OF PENNSYLVANIA
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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 LaBr 3 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 LaBr 3-based whole-body 3D PET
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[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.
Department of Radiology
UNIVERSITY OF PENNSYLVANIA