Monte Carlo model and spectral measurements of a computed tomography x-ray tube Magdalena Bazalova Frank Verhaegen

Overview MotivationMotivation Diagnostic x-ray beam productionDiagnostic x-ray beam production Monte Carlo (MC) simulationsMonte Carlo (MC) simulations Spectral measurementSpectral measurement –in computed tomography (CT) geometry –experimental setup and Compton scatter –detector Corrections of measured spectraCorrections of measured spectra Results: Comparison of MC simulated spectra with measured spectraResults: Comparison of MC simulated spectra with measured spectra ConclusionsConclusions

Motivation It is very difficult to assess patient dose from CT imaging and mostly approximative methods are used.It is very difficult to assess patient dose from CT imaging and mostly approximative methods are used. A Monte Carlo model of the x-ray tube of a CT scanner in combination with patient CT images will allow for an accurate estimate of CT dose.A Monte Carlo model of the x-ray tube of a CT scanner in combination with patient CT images will allow for an accurate estimate of CT dose. However, the MC model has to be accurate in order to calculate the dose accurately.However, the MC model has to be accurate in order to calculate the dose accurately. To developed and validate (using spectral measurements) a MC model of a Philips PQ5000 CT simulator x-ray tube.To developed and validate (using spectral measurements) a MC model of a Philips PQ5000 CT simulator x-ray tube.

X-ray beam production tungsten target

Monte Carlo model of the PQ5000 x-ray tube using the BEAM code and manufacturer’s specifications of the x-ray tubeusing the BEAM code and manufacturer’s specifications of the x-ray tube anode material: W anode material: W anode angle: 7° anode angle: 7° tube voltage: kVp tube voltage: kVp additional filter: 9 mm Al additional filter: 9 mm Al

Spectral measurements of diagnostic x-rays Not trivial due to the high photon flux causing pile-up in the detector of choice.Not trivial due to the high photon flux causing pile-up in the detector of choice. Can be solved by using very small collimators and/or increasing the x-ray source to detector distance; the alignment is then detrimental.Can be solved by using very small collimators and/or increasing the x-ray source to detector distance; the alignment is then detrimental. Not applicable for CT geometry and our detector, therefore, the x-ray flux has to be reduced in another way.Not applicable for CT geometry and our detector, therefore, the x-ray flux has to be reduced in another way.

Problems of spectral measurements in CT geometry a few meters 80 cm Attenuation? Scatter!

Experimental setup using Compton scatter

Compton scatter- scatterer material interaction of a photon of energy hν with a loosely bound orbital electron of an absorberinteraction of a photon of energy hν with a loosely bound orbital electron of an absorber

Scatterer material Solid line: Z=6 Dashed line: Z=50

Measurements of x-ray spectra: CdTe detector 3 ×3×1 mm 3 CdTe crystal (Z Cd =48, Z Te =52)3 ×3×1 mm 3 CdTe crystal (Z Cd =48, Z Te =52) Pt and In contactsPt and In contacts

Spectrum corrections-detector response 2 mm 3 mm 1 mm

Spectrum corrections-Compton effect

Spectrum corrections-peak broadening

Spectrum corrections - final

Comparison of MC simulations and spectral measurements filtering kVp

Half value layer comparison

Conclusions An accurate Monte Carlo model of the PQ5000 CT simulator x-ray was developed.An accurate Monte Carlo model of the PQ5000 CT simulator x-ray was developed. The model was validated with spectral measurements using a CdTe detector in a Compton scattering setup.The model was validated with spectral measurements using a CdTe detector in a Compton scattering setup. A very good agreement between the MC simulated and measured spectra for the extreme tube voltages with and without an additional 9 mm thick aluminum filter was found.A very good agreement between the MC simulated and measured spectra for the extreme tube voltages with and without an additional 9 mm thick aluminum filter was found. The MC model can be used for CT dose calculation.The MC model can be used for CT dose calculation.

Acknowledgements 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 studentsMedical Physics Unit staff and students NSERCNSERC