Monte Carlo model and Monte Carlo model and spectral measurements of a spectral measurements of a computed tomography x-ray computed tomography x-ray

tube tube

Magdalena BazalovaMagdalena BazalovaFrank VerhaegenFrank Verhaegen

OverviewOverview• MotivationMotivation• Diagnostic x-ray beam productionDiagnostic x-ray beam production• Monte Carlo (MC) simulationsMonte Carlo (MC) simulations• Spectral measurementSpectral measurement

– in computed tomography (CT) geometryin computed tomography (CT) geometry– experimental setup and Compton scatterexperimental setup and Compton scatter– detectordetector

• Corrections of measured spectraCorrections of measured spectra• Results: Comparison of MC simulated spectra Results: Comparison of MC simulated spectra

with measured spectrawith measured spectra• ConclusionsConclusions

MotivationMotivation

• It is very difficult to assess patient dose It is very difficult to assess patient dose from CT imaging and mostly approximative from CT imaging and mostly approximative methods are used.methods are used.

• A Monte Carlo model of the x-ray tube of a A Monte Carlo model of the x-ray tube of a CT scanner in combination with patient CT CT scanner in combination with patient CT images will allow for an accurate estimate images will allow for an accurate estimate of CT dose.of CT dose.

• However, the MC model has to be accurate However, the MC model has to be accurate in order to calculate the dose accurately.in order to calculate the dose accurately.

• To developed and validate (using spectral To developed and validate (using spectral measurements) a MC model of a Philips measurements) a MC model of a Philips PQ5000 CT simulator x-ray tube.PQ5000 CT simulator x-ray tube.

X-ray beam productionX-ray beam production

tungsten target

Monte Carlo model of the Monte Carlo model of the PQ5000 x-ray tubePQ5000 x-ray tube

• using the BEAM code and using the BEAM code and manufacturer’s specifications of the manufacturer’s specifications of the x-ray tubex-ray tube

• anode material: Wanode material: W• anode angle: 7anode angle: 7°°• tube voltage: 100-140 kVptube voltage: 100-140 kVp• additional filter: 9 mm Aladditional filter: 9 mm Al

Spectral measurements of Spectral measurements of diagnostic x-raysdiagnostic x-rays

• Not trivial due to the high photon flux Not trivial due to the high photon flux causing pile-up in the detector of causing pile-up in the detector of choice.choice.

• Can be solved by using very small Can be solved by using very small collimators and/or increasing the x-collimators and/or increasing the x-ray source to detector distance; the ray source to detector distance; the alignment is then detrimental.alignment is then detrimental.

• Not applicable for CT geometry and Not applicable for CT geometry and our detector, therefore, the x-ray flux our detector, therefore, the x-ray flux has to be reduced in another way.has to be reduced in another way.

Problems of spectral Problems of spectral measurements in CT geometrymeasurements in CT geometry

a few meters

80 cm

• Attenuation?• Scatter!

Experimental setup using Experimental setup using Compton scatterCompton scatter

Compton scatter- scatterer Compton scatter- scatterer materialmaterial

• interaction of a photon of energy hinteraction of a photon of energy hνν with a loosely bound orbital electron with a loosely bound orbital electron of an absorberof an absorber

Scatterer materialScatterer material

Solid line: Z=6

Dashed line: Z=50

Measurements of x-ray Measurements of x-ray spectra: CdTe detectorspectra: CdTe detector

• 33×3×1 mm×3×1 mm33 CdTe crystal (Z CdTe crystal (ZCdCd=48, Z=48, ZTeTe=52)=52)

• Pt and In contactsPt and In contacts

Spectrum corrections-detector Spectrum corrections-detector responseresponse

2 mm

3 mm3 mm

1 mm

Spectrum corrections-Spectrum corrections-Compton effectCompton effect

Spectrum corrections-peak Spectrum corrections-peak broadeningbroadening

Spectrum corrections - finalSpectrum corrections - final

Comparison of MC simulations Comparison of MC simulations and spectral measurementsand spectral measurements

filte

ring

kVp

Half value layer comparisonHalf value layer comparison

ConclusionsConclusions

• An accurate Monte Carlo model of the An accurate Monte Carlo model of the PQ5000 CT simulator x-ray was developed.PQ5000 CT simulator x-ray was developed.

• The model was validated with spectral The model was validated with spectral measurements using a CdTe detector in a measurements using a CdTe detector in a Compton scattering setup.Compton scattering setup.

• A very good agreement between the MC A very good agreement between the MC simulated and measured spectra for the simulated and measured spectra for the extreme tube voltages with and without an extreme tube voltages with and without an additional 9 mm thick aluminum filter was additional 9 mm thick aluminum filter was found.found.

• The MC model can be used for CT dose The MC model can be used for CT dose calculation.calculation.

AcknowledgementsAcknowledgements

• Robin van GilsRobin van Gils• Wamied Abdel-RahmanWamied Abdel-Rahman• Derek LiuDerek Liu• Slobodan DevicSlobodan Devic• Jan SeuntjensJan Seuntjens• Joe LarkinJoe Larkin• Medical Physics Unit staff and Medical Physics Unit staff and

studentsstudents• NSERCNSERC