Maximum Likelihood Processing of Pileup in Scintillation Cameras
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2006 Nuclear Science Symposium, Medical Imaging Conference Maximum Likelihood Processing of Pileup in Scintillation Cameras Neal Clinthorne Division of Nuclear Medicine Sam Huh Department of Biomedical Engineering University of Michigan Ann Arbor, MI USA
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Maximum Likelihood Processing of Pileup in Scintillation Cameras. Neal Clinthorne Division of Nuclear Medicine Sam Huh Department of Biomedical Engineering University of Michigan Ann Arbor, MI USA. Compton 2 nd Detector Countrate. 2 nd Detector for Compton. Pixellated LSO panel detector - PowerPoint PPT Presentation
Transcript of Maximum Likelihood Processing of Pileup in Scintillation Cameras
2006 Nuclear Science Symposium, Medical Imaging Conference
Maximum Likelihood Processing of Pileup in Scintillation Cameras
Neal ClinthorneDivision of Nuclear Medicine
Sam HuhDepartment of Biomedical Engineering
University of MichiganAnn Arbor, MI USA
2006 Nuclear Science Symposium, Medical Imaging Conference
Compton 2nd Detector Countrate
2006 Nuclear Science Symposium, Medical Imaging Conference
2nd Detector for Compton
• Pixellated LSO panel detector– Good countrate capability– Good spatial resolution– Good timing performance– Expensive!
• Pixellated LaBr3?– Same benefits– Same problems + extremely hygroscopic
• Pixellated NaI(Tl)?– Expensive– Slow decay (240 ns)
• Continuous NaI(Tl) (Anger camera)– Cheap– Even more problems with pileup due to spreading of light
Can continuous NaI(Tl) work?
2006 Nuclear Science Symposium, Medical Imaging Conference
Anger Camera Review
• Gamma-ray interacts in scintillator generating light
• Light spreads out among PMTs
• Position determined by relative outputs of PMTs
• Energy determined by summed outputs of PMTs
2006 Nuclear Science Symposium, Medical Imaging Conference
Temporal Pileup Due to Slow Decay
0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4
x 10-6
0
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1.5
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2.5x 10
10
Pu
lse
He
igh
t
Time (seconds)
Piled Up Events
2006 Nuclear Science Symposium, Medical Imaging Conference
Temporal Pileup Due to Slow Decay
0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4
x 10-6
0
0.5
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10 Piled Up Events
Time (seconds)
Pu
lse
He
igh
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2006 Nuclear Science Symposium, Medical Imaging Conference
Shorter Integration Times?
0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4
x 10-6
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10 Piled Up Events
Time (seconds)
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He
igh
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Shorter Integration Time Reduces Pileup
0 0.5 1 1.5 2 2.5 30
0.1
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Integration Time ()
Re
lativ
e U
nc
ert
ain
ty
• Shorter integration times reduce pileup
• They also decrease resolution
2006 Nuclear Science Symposium, Medical Imaging Conference
State of the Art Correction
0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4
x 10-6
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10 Li-Wong Pileup Correction
Time (seconds)
Pu
lse
He
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Pulse of Interest
Subtract Remnant
Start Integration
Stop Integration
Correct "Missing" Pulse Height
Remnant for next pulse
Wong, et al., IEEE Trans. Nucl. Sci. 45(3):1122–1127, 1998
But what about spatial correlations due to spreading of scintillation light?
2006 Nuclear Science Symposium, Medical Imaging Conference
PMT Outputs at 100K CPS
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Pu
lse
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ght
Time (seconds)
Simulation Model of Anger Camera with 9 x 75mm dia PMTs
2006 Nuclear Science Symposium, Medical Imaging Conference
PMT Outputs at 1M CPS
2 4
x 10-6
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Pu
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ght
Time (seconds)
2006 Nuclear Science Symposium, Medical Imaging Conference
PMT Outputs at 4M CPS
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ght
Time (seconds)
2006 Nuclear Science Symposium, Medical Imaging Conference
Modified Scintillation Camera
0 500 1000 1500 2000 2500 3000 3500 4000 4500 50000
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Time (ns)
Pu
lse
He
igh
t 1V
=4
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Scattering Detector
Scintillation Camera PMT
PMT
PMT
PMT
100 MHzADCs
Timing Signals
Timing Signals Coincidence
Unit
Event Trigger –> Xfers ADC data
Processing Computer
Output from one PMTCountrate ~1Mcps
2006 Nuclear Science Symposium, Medical Imaging Conference
Single Pulse Likelihood
dttEs
tEsEtf
m
T
m
mnmm
N
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Nnmn
mm
0
1 log,,|log
x
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n timeInteractio
timese-Photo
response Time
LSF
Energy
Position
mn
m
t
s
E
x
2006 Nuclear Science Symposium, Medical Imaging Conference
Multiple Pulse Likelihood
dttsE
tsEEtf
m
T
kkkmk
kkmnkmk
m
N
n
Kkk
Nnmn
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0
11 log,,|log
x
xx
Estimate energy & position for each of the k pulses
2006 Nuclear Science Symposium, Medical Imaging Conference
Maximization?
m
N
n kkkmnkmk
kkmnkmk
m
N
n
mm
tsEtsE xx loglog
“Complete data” is tkmn – with this choice, maximization is trivialDon’t know tkmn but that’s what the Expectation-Maximization (EM) algorithm is for.
dttsE
tsEEtf
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kkkmk
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Computationally difficult
2006 Nuclear Science Symposium, Medical Imaging Conference
EM Example
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x 10-7Time (seconds)
Photoelectron Emission Times for 3 NaI(Tl) Pulses
2006 Nuclear Science Symposium, Medical Imaging Conference
EM Example
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x 10-7
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Photoelectron Emission Times for 3 NaI(Tl) Pulses
Time (seconds)
EM algorithm allows photoelectrons to be associated with particular event
Of course, it may select wrong but in the end it maximizes the likelihood
2006 Nuclear Science Symposium, Medical Imaging Conference
Performance 100Kcps
-30 -20 -10 0 10 20 30-30
-20
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302 Integration
-30 -20 -10 0 10 20 30-30
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300.5 Integration
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30Li-Wong
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30ML
X Position Error (mm)
Y P
ositi
on
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or
(mm
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2006 Nuclear Science Symposium, Medical Imaging Conference
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Performance 1Mcps
X Position Error (mm)
-30 -20 -10 0 10 20 30-30
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302 Integration
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ositi
on
Err
or
(mm
)
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30ML
2006 Nuclear Science Symposium, Medical Imaging Conference
Performance 2Mcps
X Position Error (mm)
-30 -20 -10 0 10 20 30-30
-20
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302 Integration
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300.5 Integration
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30Li-Wong
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30ML
Y P
ositi
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Err
or
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2006 Nuclear Science Symposium, Medical Imaging Conference
Performance 2Mcps
-30 -20 -10 0 10 20 30-30
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-30 -20 -10 0 10 20 30-30
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30ML
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Position Error (mm)
Co
un
t
Li-Wong 2 Million Events / sec
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Position Error (mm)
Co
un
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ML 2 Million Events / sec
2006 Nuclear Science Symposium, Medical Imaging Conference
Performance 4Mcps
-30 -20 -10 0 10 20 30-30
-20
-10
0
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30Li-Wong
-30 -20 -10 0 10 20 30-30
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-10
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Position Error (mm)
Co
un
t
Li-Wong 4 Million Events / sec
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Position Error (mm)
Co
un
t
ML 4 Million Events / sec
2006 Nuclear Science Symposium, Medical Imaging Conference
Performance 6Mcps
-30 -20 -10 0 10 20 30-30
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-30 -20 -10 0 10 20 30-30
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Position Error (mm)
Co
un
t
Li-Wong 6 Million Events / sec
0 5 10 15 20 250
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100
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Position Error (mm)
ML 6 Million Events / sec
Co
un
t
2006 Nuclear Science Symposium, Medical Imaging Conference
Electronics
Coincidence trigger
-G -3 1
CFTD
∑
1 of 8 channels / board
PMT in Channel out
“Energy” outFrom other7 channels
Variable gain
Variable delay
2006 Nuclear Science Symposium, Medical Imaging Conference
Typical PMT Signals
Varying Baseline
High Noise
2006 Nuclear Science Symposium, Medical Imaging Conference
0 0.2 0.4 0.6 0.8 1 1.2
x 10-5
-0.2
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Time (seconds)
Vo
ltag
e(o
ffse
t fo
r cl
ari
ty)
PMT 55
PMT 49PMT 51
Waveform “snapshot”
Pileups
38 49 54
50 55 53
40 51 52
PMT Array
Two point sourcesSeparable due to different positions
Initial Grid Image from 2nd Detector
140 keV – countrate performance yet untested
2006 Nuclear Science Symposium, Medical Imaging Conference
Summary
• Pileup is an important issue in practical Compton camera 2nd detector in medical applications
• Present pileup rejection methods work fairly well up to moderate countrates but ignore important information in scintillation pulse
• New ML method performs better at high countrates than existing methods but computationally expensive
• May be possible to apply “band-aid” to Li-Wong method
• Next step will apply methods to recently completed camera
