Mark Geurts1, Bruce Thomadsen, Ph.D.2, Reed Selwyn3
1Departments of Medical
Physics and Engineering Physics, University of
Wisconsin, Madison, WI
2Departments of Medical Physics, Human Oncology, Engineering
Physics, and Biomedical Engineering, University of Wisconsin, Madison, WI
3Department of Medical Physics, University of Wisconsin, Madison, WI
NCCAAPM 2007 Spring Meeting, Burnsville, MN
Introduction
 Most clinical settings do not have methods for validating
the activity of Yttrium-90

90Y
is commonly labeled to microspheres for selective
internal radiation therapy (SIRT)
 90Y exhibits a small internal pair production branching
ratio of 31.86±0.47 x10-6 (Selwyn et al, 2007)
 This project investigated whether a PET scanner could be
calibrated for accurate assay validation
Coincidence Spectrum of 90Y
Sensitivity (cps/mCi)
0.030
0.020
0.010
0.000
0
200
400
600
800
1000
Energy (keV)
 Spectrum was obtained using coincident NaI-HPGe
detectors (Nickles et al, 2004)
Methods
 PET scans performed on a GE Discovery LS CT/PET
scanner
 Each 90Y sample vial was assayed using a single HPGe
detector
 Each vial was then placed in a plastic
sleeve to absorb emitted electrons
& positrons
 Sinogram data was collected
with and without each
reconstruction correction
 2D and 3D scans were compared
Sinogram Correction Detail
Sinogram Correction
Purpose
Random
Corrects for random coincidences detected within the
coincidence resolving time
Well
Corrects for axial sensitivity variation within the field
of view
Geometric
Corrects for sensitivity variation caused by gaps in the
detector configuration
Decay
Corrects for isotope decay during the scan
Dead time
Corrects for electronic dead time losses
Normalization
Corrects for variations in detector pair gains
Attenuation & Scatter
Corrects for sensitivity variation caused by photon
attenuation and scatter within the scan object
2D vs 3D Acquisition
3D
2D
Sensitivity Linearity Results
 Twelve 2D PET scans were taken over a period of several
1000
2D Uncorrected Sensitivity
(cps/mCi)
Corrected Count Rate (cps)
months
 Poisson 2σ uncertainty associated with the uncorrected
sinogram count rate was less than 0.9% for each measurement
800
600
400
200
0
0
50
100
Activity (mCi)
1.2
0.8
0.4
0
0
10
20
30
PET Slice Number
Sinogram Correction Results
Type of Sinogram
Sensitivity (2σ uncert.)
Adjusted R-squared
Uncorrected
4.000 cps/mCi ± 1.754%
99.91%
Random Corrected
4.003 cps/mCi ± 1.756%
99.91%
Well Corrected
3.510 cps/mCi ± 1.174%
99.96%
Geometric Corrected
4.440 cps/mCi ± 1.700%
99.91%
Decay Corrected
4.006 cps/mCi ± 1.752%
99.91%
Dead time Corrected
4.137 cps/mCi ± 2.448%
99.82%
Normalization Corrected
3.826 cps/mCi ± 1.604%
99.92%
All Corrections Applied
8.227 cps/mCi ± 1.012%
99.97%
Discussion
 All sinogram count rates correlated strongly with 90Y activity (R2 > 98%, P <


Mean Dead Time
Correction

1.08
1.06
1.04
1.02
1.00
0
200
400
Uncorrected Count Rate (cps)
Absolute Residual (cps)

0.001)
Random and decay corrections do not significantly change the observed
sensitivity (P = 0.821 and P = 0.682)
Well and normalization corrections improve correlation (P < 0.001)
Dead time correction degrades the sensitivity correlation (P < 0.001),
suggesting that the dead time model may be inadequate
The two highest count rates contain more than 75% of the total absolute
residual when the dead time correction is applied
40
30
20
10
0
110 109 88 85 85 85 66 54 41 30 18 14
Sample Activity (mCi)
Discussion
 The count rate for the 2D scan was higher than the 3D
scan for 90Y activities greater than 100 mCi (P < 0.001)
 The increased sensitivity of the 3D modality is offset by
the increased number of third-gamma coincidences
associated with the bremsstrahlung spectrum from the
dominant beta-minus decay of 90Y
 This comparison is consistent with the results of other
research on 2D and 3D PET modalities (Schueller, 2003;
Lubberink, 1999)
Conclusions
 The provided PET scanner responds well under conditions
of high photon background with a sensitivity 2σ
uncertainty of 1.75%
 Most of the built in sinogram corrections of the GE
Discovery LS improve performance, reducing the
uncertainty to 1.01%
 Adequate results can be obtained in several minutes
 PET is a quick and accurate method for assay validation
can be created for 90Y and used to perform quality
assurance on SIRT treatments
Work in Progress
 Monte Carlo model was developed to simulate the PET
scanner response using MCNP5 (9x107 source decays)
Read in
parameters,
initialize storage
arrays
Import and
combine MCNP5
data
Advance time to
next source
emission
Combine electron
collisions into
event pulses
Apply energy
discrimination
Check block dead
time
Convolve pulse
with energy
resolution
Determine event
location within
detector
Apply module
electronic dead
time
Trigger
coincidence
window
Reject multiple
coincidences
Process & store
coincidence in
sinogram
Export results to
Excel
Apply efficiency
corrections
Bin event
separation time
Bin coincident
event energy
resolution
Simulation Results
 Simulations fit measured PET scanner sensitivity to 3.6%
 Applying new dead time model reduces sensitivity 2σ
uncertainty from 1.01% to 0.92%
 90Y activities greater than 100 mCi should use a separate dead
time model
Corrected Count Rate (cps)
Dead time correction
1.30
1.25
1.20
1.15
1.10
1.05
1.00
0
400
800
1200
Uncorrected Count Rate (cps)
1600
1200
800
400
0
0
100
Y-90 Activity (mCi)
200
Acknowledgements
 The research group would like to recognize Dr. Robert J.
Nickles and Dr. Robert Pyzalski at the University of
Wisconsin for their ideas regarding 90Y PET imaging and
expertise in positron emission tomography and
coincidence detection systems
 No funding was required for this research
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