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Validation of a Method for Characterizing Scanner-Specific Bowtie Filters in CT
Chris Liptak, Ashraf Morgan,Frank Dong, Andrew Primak, Xiang Li
Cleveland State UniversityCleveland Clinic
Siemens Medical Solutions USA, Inc.
Medical Physics Graduate ProgramCleveland State University
& Cleveland Clinic
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Background
2Bushberg 2011
• Compensates for the attenuation difference between center and periphery
• Provides a more uniform x-ray fluence at the detector
• Reduces peripheral dose without loss of image quality
(beam-shaping filter)bowtie filter
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Motivation
3
• Monte Carlo simulation is the most frequently used technique for patient dose assessment in CT
• In Monte Carlo simulations, accurate knowledge of a CT system’s bowtie filtration is essential
• Information about bowtie filtration is often proprietary
• Alternative: Dose measurements can be used to create a bowtie filter model
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Traditional Method: Step-and-Shoot
4Turner et al., Med. Phys. 36, no. 6 2154-2164 (2009)
θ
integrating dosimeter
probe
bowtie filter
stationary x-ray source
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Advantages:• Single acquisition• No probe repositioning
Characterization of bowtie relative attenuation (COBRA)Boone., Med. Phys. 37, 40-48 (2010)
Recent Development
http://www.radcal.com/accu-‐gold-‐detail
real-time dosimeter
5
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Purpose
To perform a validation of the COBRA method in terms of the accuracy of the simulated dose in circular and elliptical phantoms
6
www.ptw.de
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Methods
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Simulation of Real-Time Dose Signal
8
SOMATOM Definition Flash (Siemens Healthcare)
Tian et al., Radiology 270, 535-547 (2014)Li et al., Phys. Med. Biol. 59, 4525-4548 (2014)
• PENELOPE Monte Carlo program
• The program was extended to output dose as a function of time
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Simulated Experimental Setup
9
CTDI100 pencil ion chamber, capable of collecting real-time dose signals
Liptak et al., Med. Phys. 42, no. 6, 3747-3747 (2015)
θ
rotatingx-ray source
real-time dosimeter
probe
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Simulation of Real-Time Dose Signal
10
Simulation parameters:• axial scans
• 70, 80, 100, 120, 140 kVp
• 38.4 mm collimation
• 1-kHz acquisition rate
Relative bowtie attenuation profiles were extracted from simulated dose waveforms
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Relative Bowtie Attenuation
11
Relative transmitted dose as a function of the fan beam angle
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Basis Decomposition
12
2 2[ ( ) ( , )]n
i
V
VV V
F Vχ θ θ=
= −Γ∑
,( ) ( ) exp( ( ))( , )
( ) ( )
ii i E Al
i i
k E E tV
k E E
µ θθ
Ψ −Γ =
Ψ
∑
∑
experimental bowtie attenuation
theoreticalbowtie attenuation
simulated
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Basis Decomposition
12
2 2[ ( ) ( , )]n
i
V
VV V
F Vχ θ θ=
= −Γ∑
,( ) ( ) exp( ( ))( , )
( ) ( )
ii i E Al
i i
k E E tV
k E E
µ θθ
Ψ −Γ =
Ψ
∑
∑
material thickness
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Basis Decomposition
12
2 2[ ( ) ( , )]n
i
V
VV V
F Vχ θ θ=
= −Γ∑
,( ) ( ) exp( ( ))( , )
( ) ( )
ii i E Al
i i
k E E tV
k E E
µ θθ
Ψ −Γ =
Ψ
∑
∑
attenuation coefficient of aluminum
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Basis Decomposition
12
2 2[ ( ) ( , )]n
i
V
VV V
F Vχ θ θ=
= −Γ∑
,( ) ( ) exp( ( ))( , )
( ) ( )
ii i E Al
i i
k E E tV
k E E
µ θθ
Ψ −Γ =
Ψ
∑
∑
photon energy fluence(from manufacturer)
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Basis Decomposition
12
2 2[ ( ) ( , )]n
i
V
VV V
F Vχ θ θ=
= −Γ∑
,( ) ( ) exp( ( ))( , )
( ) ( )
ii i E Al
i i
k E E tV
k E E
µ θθ
Ψ −Γ =
Ψ
∑
∑
mass energy absorption coefficient of air
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Equivalent Bowtie Model
13
aluminum
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14
Equivalent Bowtie Filter
ActualBowtie Filter
Compared in terms of simulated phantom dose
Validation of Equivalent Bowtie Model
aluminumgraphite
aluminum
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Validation: Phantom Dose Simulation
15
32-cm CTDI phantomcustom-designed elliptical phantom
Simulation parameters: single axial scans at 70 and 120 kVp
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Results
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Phantom Dose at 70 kVp
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Lateral holes
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Phantom Dose at 120 kVp
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Conclusion
• First attempt to validate the COBRA method in terms of the accuracy of the simulated phantom dose
• The equivalent bowtie filter model obtained using the COBRA method compares well with the actual bowtie filter in terms of simulated dose in acrylic phantoms
19
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Thank you