Wolfram Stiller PhD, DIPLPHYS, Presenter: Nothing to Disclose

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

Gregor Pahn DIPLPHYS, Abstract Co-Author: Nothing to Disclose

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

MDCT beam-shaping filters modulate X-ray intensity across the fan beam, reducing patients’ skin dose while homogenizing detector statistics. A simple theoretical model for calculating beam-shaping filter geometry has been developed; and for calculated attenuation profiles Monte-Carlo (MC) simulated transmission spectra have been compared to measured X-ray spectra.

Based on the assumption that X-ray intensity after transmission through a given set of a beam-shaping filter and a cylindrical object of homogeneous composition is equal for all fan angles, a theoretical beam-shaping filter geometry has been calculated. For a set of fan angles (0°–21° relative to central ray of the fan), transmission spectra were simulated for calculated filter geometry using a Geant4 MC simulation. Simulated spectra have been compared to fan-angle dependent measurements of MDCT X-ray spectra (120kVp) acquired using a Compton spectrometer. Additionally, the initial assumption of equalized X-ray intensity after beam transmission through a set of shaping filter and homogeneous cylindrical phantom was checked by simulating resulting detector signal.

Maximum differences between measured transmission spectra and those simulated for a beam-shaping filter geometry estimated using the proposed theoretical model amount to ±2.0% for all fan angles. K-lines of simulated and measured spectra match well and are in agreement with theoretical values for tungsten anodes. Measured and simulated filter transmission and mean X-ray energy after filter passage are also in good agreement. For the modeled filter geometry the initial assumption of signal homogenization could be validated through simulation.

Comparison of measured transmission spectra to those simulated for a beam-shaping filter geometry estimated using the proposed simple theoretical model reveals excellent qualitative and quantitative agreement. Thus the proposed method can be used for calculating reliable estimates of MDCT beam-shaping filter geometry instead of having to rely on manufacturers’ data or resort to time-consuming measurements.

The simple model allows reliable estimation of MDCT beam-shaping filter geometry. Its accurate modeling is a prerequisite for reliable MC simulations of MDCT, e.g. for estimating radiation exposure.

Stiller, W, Veloza, S, Pahn, G, Kauczor, H, A Theoretical Model for Calculating MDCT Beam-shaping Filter Geometry: Comparison of Calculated Attenuation Profile and Monte Carlo Simulated Transmission Spectra to Measurements.  Radiological Society of North America 2011 Scientific Assembly and Annual Meeting, November 26 - December 2, 2011 ,Chicago IL. http://archive.rsna.org/2011/11002770.html