Simulation of CT-images:from X-ray spectrum to CT-numbers

Simulation of CT-images:from X-ray spectrum to CT-numbers

S. Qamhiyeh1), A.Wysocka-Rabin2), M. Ellerbrock1), C. Penssel3), M. Kachelriess3) and O. Jaekel1)

1) German Cancer Research Center (DKFZ), Medical Physics, Heidelberg, Germany2)Andrzej Soltan Institute for Nuclear Studies, Department of Accelerator Physics, Swierk, Poland3) Friedrich-Alexander-University, Institute of Medical Physics, Erlangen-Nuremberg, Germany

MCNEG / 2007NPL, Teddington 28.03.2007 wysocka@ipj.gov.pl

a.wysocka@dkfz.de

Ion therapy vs. Photon radiotherapy

Bragg-peaks for different Carbon ion energies

Introduction (CT-calibration)Introduction (CT-calibration)

)1(1000numberCT −⋅=−wµµ

Rrel = ∆Rangewater / dmedium

Uncertainty of calibration curve 2% -3% ;

Range uncertainty 2-3 mm at 10 cm

µ(E,Z) = σ(E,Z) • ρe ( )( )⎪⎩

⎪⎨⎧

≥×±+<×±+= −

−

01012.035.4101008.041.91

4

4

HUHURrel

CT-numbers (HU)

Rel

ativ

e ra

nge

in w

ater

(Rre

l)

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

-1000 -500 0 500 1000 1500 2000

CT-numbers (HU)

Rel

. Ran

ge in

wat

er

TissuePlexiglasSubstitutesCalibration-1Calibration-2

Uncertainties of CT-numbers can be up to 300 HU for bone-like materials. This can lead to uncertainties in the range of ions (position of Bragg-peak)

Changes of the CT-numbers of bone-like materials affect the calibration curve and cause up to 6 mm change in the range of Carbon ions. Up to 80% of the dose is allocated to the critical organs.

What influences CT-numbers ?What influences CT-numbers ?Design parameters of the CT-scanner (filters, collimators, generation)

Voltage of the X-ray tube

Size of Field of view (FOV)

Reconstruction algorithms

Parameters of the scanned object (material of phantom, size, geometry, homogenity, position in the FOV, movement)

Artefacts, especially metal artefacts

The dependance of CT-numbers on the above parameters is only known emprically.

Aim ...Aim ...

Develop a method to calculate CT-numbers under different conditions.

Analyze the effects of measurement conditions on calculated CT-numbers.

Estimate range uncertainties due to different measurement conditions of CT-numbers.

Cylindrical phantom from PMMA (16 cm diameter) and Gammex-

RMI substitutes (Middletown, WI).

Siemens Emotion CT equipped with a DURA352 X-ray tube.

CT-numbers calculation chainCT-numbers calculation chain

Pulse-Height spectrum

Phase space files

PROJECTION(Raw-data

Format)Image (HU)

Photon transport simulation throughthe filters,collimators and phantom

in CT scanner using EGSnrc/BEAMnrc

Reconstruction

Measurements of CT spectrum

using SpectroX

X-ray-tube

simulation

Responsefunction

of CT detectorsMeasurements of

CT-numbers

Scheme of CT-scanner and phantom Scheme of CT-scanner and phantom

1) CT1-part was used to simulate X-ray source, filters and collimators. 1010

photons with a given energy spectrum. 1 week of CPU time.

2) CT2-part was used to simulate phantom and inserts. 5x108 photons. Input is PHSP from CT1. 1-3 hrs of CPU time.

CT-numbers calculation chainCT-numbers calculation chain

Pulse-Height spectrum

Phase space files

PROJECTION(Raw-data

Format)Image (HU)

Photon transport simulation throughthe filters,collimators and phantom

in CT scanner using EGSnrc/BEAMnrc

Reconstruction

Measurements of CT spectrum

using SpectroX

X-ray-tube

simulation

Responsefunction

of CT detectorsMeasurements of

CT-numbers

Projections and images

HU (Normalization ?)

Single projection Matrix – image

1

10

100

1000

1 65 129 193 257 321 385 449 513 577 641

Detector channels (672)

Flue

nce

per d

etec

tor c

hann

el

Fluence after phantom with Cortical bone insert

Initial fluence

Input CT-spectra calculated for the Emotion CT-scanner with DURA352 X-ray tube for 80, 110 and 130 kVInput CT-spectra calculated for the Emotion CT-scanner with DURA352 X-ray tube for 80, 110 and 130 kV

0.E+00

2.E-04

4.E-04

6.E-04

8.E-04

1.E-03

0 20 40 60 80 100 120 140Energy (KeV)

Cou

nts

(arb

itrar

y un

its)

80kV

110kV

130kV

0.E+00

1.E-02

2.E-02

3.E-02

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14

energy (MeV)

fluen

ce/M

eV/in

ciden

t par

ticle

(cm

- 2 M

eV- 1

)

130 kV110 kV80 kV

0.00E+00

1.00E-03

2.00E-03

3.00E-03

4.00E-03

5.00E-03

6.00E-03

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14

energy (MeV)

fluen

ce/M

eV/in

cide

nt p

artic

le (c

m- 2

MeV

- 1)

130 kV110 kV80 kV

Spectra in the center of FOV

Spectra at the edge of FOV

Input spectra

The effect of filters (Aluminium and Teflon) on the fluence and mean energy of the spectrum were investigated using 110 kV spectrum

The effect of filters (Aluminium and Teflon) on the fluence and mean energy of the spectrum were investigated using 110 kV spectrum

0.E+00

1.E-05

2.E-05

3.E-05

4.E-05

-50 -40 -30 -20 -10 0 10 20 30 40 50

distance from center of FOV (cm)

ener

gy fl

uenc

e/in

cide

nt p

artic

le(M

eV c

m-2

)

0 filters (Air)

1 filter (Al)

1 filter (Teflon)

2 filters (Al & Teflon)

0.0E+00

5.0E-03

1.0E-02

1.5E-02

2.0E-02

2.5E-02

0.02 0.04 0.06 0.08 0.1 0.12

energy (MeV)

fluen

ce/M

eV/in

ciden

t par

ticle

(c

m- 2

MeV

- 1)

0 filters (Air)1 filter (Al)1 filter (Teflon)2 filters (Al & Teflon)

0.0E+00

5.0E-03

1.0E-02

1.5E-02

2.0E-02

0.02 0.04 0.06 0.08 0.1 0.12energy (MeV)

fluen

ce/M

eV/in

ciden

t part

icle

(cm

- 2 M

eV- 1

)

0 filters (Air)1 filter (Al)1 filter (Teflon)2 filters (Al & Teflon)

55

60

65

70

-50 -40 -30 -20 -10 0 10 20 30 40 50Distance from center of FOV (cm)

Mea

n en

ergy

(keV

)

0 filters (Air)1 filter (Al)1 filter (Teflon)2 filters (Al and Teflon)

Simulated effectsSimulated effects

-1000

-500

0

500

1000

1500

2000

2500

70 80 90 100 110 120 130 140

Energy of the incident photons (kV)

Sim

ulat

ed C

T-nu

mbe

rs (H

U)

Air LN300LN450Adipose fatWaterIBCB2 50CB

-1000

-500

0

500

1000

1500

2000

10 15 20 25 30 35Phantom diameter (cm)

Sim

ulat

ed C

T-nu

mbe

rs (

HU

) LN300

LN450

Adipose fat

IB

CB2 50

CB

-1000

-500

0

500

1000

1500

2000

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

Electron density of phantom material relative to water

Sim

ulat

ed C

T-nu

mbe

rs (H

U) LN300

LN450

Adipose fat

IB

CB2 30

CB

PMMA

Water, RW3 and MuscleAir CB

Voltage of X-ray tube Material of phantom

Diameter of phantom

Comparison of simulated and measured CT-numbersComparison of simulated and measured CT-numbers

-400

-200

0

200

400

-1000 -500 0 500 1000 1500 2000 2500Measured CT-numbers (HU)

Sim

ulat

ed -

mea

sure

d C

T-nu

mbe

rs

(HU

) Gammex 80 kV Gammex 130 kV

Effect of voltage, phantom size and phantom material on Carbon rangeEffect of voltage, phantom size and phantom material on Carbon range

0.00.20.40.60.81.01.21.41.61.82.0

-1000 -500 0 500 1000 1500 2000CT-numbers (HU)

Rel

ativ

e ra

nge

(WED

/T)

110 kV, PMMA, 16 cm

120 kV, PMMA, 16 cm

100 kV, PMMA, 16 cm

110 kV, PMMA, 30 cm

110 kV, Air, 16 cm

ConclusionsConclusions

It is POSSIBLE to calculate CT-numbers from MC simulation

The presented method was used to calculate the effects of different measurement parameters on CT-numbers and Carbon range

It is necessary to improve the effeciency of the MC simulation to use more than one projection for reconstruction (time needed foreach projection is 1-3 hrs of CPU time) to decrease the uncertainties of calculted CT-numbers

The simulation can not be used to calculate CT-numbers of irregular or off-center objects

Scheme of CT-scanner and phantom Scheme of CT-scanner and phantom

1) CT1-part was used to simulate X-ray source, filters and collimators. 1010

photons with a given energy spectrum. 1 week of CPU time.

2) CT2-part was used to simulate phantom and iserts. 5x108 photons. Input is PHSP from CT1. 1-3 hrs of CPU time.

Thank you for your attentionThank you for your attention

Questions ???Questions ???

Clinical Ion Therapy facility in Heidelberg (HIT)University Heidelberg / GSI / DKFZ1000 patients / yearp, He, C, O