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

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

Motivation

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• 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

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

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

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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

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www.ptw.de

Methods

Simulation of Real-Time Dose Signal

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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

Simulated Experimental Setup

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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

Simulation of Real-Time Dose Signal

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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

Relative Bowtie Attenuation

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Relative  transmitted  dose  as  a  function  of  the  fan  beam  angle

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  

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

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

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)

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

Equivalent Bowtie Model

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aluminum

14

Equivalent Bowtie Filter

ActualBowtie Filter

Compared in terms of simulated phantom dose

Validation of Equivalent Bowtie Model

aluminumgraphite

aluminum

Validation: Phantom Dose Simulation

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32-­cm  CTDI  phantomcustom-­designed  elliptical  phantom

Simulation  parameters:  single  axial  scans  at  70 and  120  kVp

Results

Phantom Dose at 70 kVp

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Lateral  holes

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

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Thank you