Wolfram Stiller, Presenter: Nothing to Disclose

Stella Veloza MS, Abstract Co-Author: Nothing to Disclose

Hans-Ulrich Kauczor MD, Abstract Co-Author: Research grant, Siemens AG Research grant, Boehringer Ingelheim GmbH

Direct comparison of MC-modeled and measured spectra reveals excellent qualitative and quantitative agreement. Nonetheless, X-ray beam attenuation is stronger in simulated data, showing systematic overestimation of the filter thickness at the fan edge. Differences between simulation and measurement are predominant at high (>115keV) and very low (<25keV) energies, probably due to limited photon statistics of the simulation in these regimes. The results demonstrate that MC models of beam-shaping filters can be used in simulations of CT, e.g. for estimating attenuated spectra, evaluating new filter geometries and materials, and modeling CT X-ray exposure.

X-ray path lengths are shorter in the periphery of the patients’ bodies than at their center. Thus, beam-shaping filters are used in CT to modulate incident X-ray beam intensity across the fan beam. Resulting non-uniform flux reduces radiation exposure of the skin while homogenizing photon statistics at the detector. Accuracy of Monte Carlo (MC) modeling of CT depends in part on an accurate characterization of the beam-shaping filter. In order to construct a realistic filter model, its attenuation profile was characterized using direct measurements of X-ray spectra, and the corresponding filter model validated by MC simulation.

For a tube potential of 120kVp, primary CT X-ray spectra have been measured at different angular distances (0°–21°) from the central ray using a Compton spectrometer, and the geometrical shape of the aluminum bowtie filter was estimated. This model was implemented in a Geant4 MC simulation. Subsequently, measured and simulated spectra have been compared.

The K lines of simulated and measured spectra march well with theoretical values for tungsten anodes. Residual values for simulated vs. measured spectra were calculated: for energies between 30-100keV the residuals lie between -1.0% and 0.5%; below ~30keV the maximum difference rises up to 21%; for energies above 100keV residuals increase up to 6% at a fan angle of 21°. The X-ray beam at the FOV border is extremely hardened, and has an intensity of below 3% compared to the beam in the isocenter.

Stiller, W, Veloza, S, Kauczor, H, Characterization of the Beam-shaping Filter of an MDCT System: Spectral Measurements and Comparison with a Geant4 Monte Carlo Simulation Model.  Radiological Society of North America 2010 Scientific Assembly and Annual Meeting, November 28 - December 3, 2010 ,Chicago IL. http://archive.rsna.org/2010/9004387.html