Wolfram Stiller PhD, DIPLPHYS, 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 Research Consultant, General Electric Company

MDCT beam-shaping filters modulate X-ray intensity across the fan beam, compensating for decreasing absorption due to shorter X-ray path lengths through patient body periphery compared to its center. The performance of a filter geometry dynamically adapted to elliptical specimen in dependence of projection-angle has been compared to a static beam-shaping filter geometry experimentally determined.

A theoretical dynamic beam-shaping filter model was determined for elliptical subject geometries. To this end, attenuation profiles of a homogeneous elliptical water phantom (16.0cm semimajor, 12.5cm semiminor axes) have been MC-simulated with Geant4 for projections a.p. to lateral in 15°-steps. Projection-angle dependent filter shape was then determined by inversion of the respective attenuation profile and rescaling of absorption path lengths to aluminum. To enable comparison, the static filter shape of a real CT system has been determined from primary CT X-ray spectra (120kVp) measured at different fan angles using Compton spectrometry. For both shape models detector statistics in terms of the number of surviving quanta after X-ray passage through beam-shaping filter plus homogeneous elliptical water phantom were simulated for projections a.p. to lateral every 15°.

After X-ray transmission through a set of beam-shaping filter plus homogeneous elliptical water phantom detector statistics as well as attenuation are constant throughout the complete FOV, i.e. independent of fan angle, for all projections if a dynamically adapted filter geometry is used. For elliptical objects the measured static filter produces fan-angle dependent attenuation profiles which are asymmetrical at projection angles of 15°-75°. Differences in attenuation rise from 1.8 at the border of the X-ray fan to 5 for its central rays (log. units) a.p., and from 1.2 to 6.4 lateral.

For elliptical subjects the static beam-shaping filter is systematically too thin at the fan edge. Its geometry and material composition should be reevaluated to reduce signal and thereby patients’ skin dose for lateral projections. Dynamic beam-shaping homogenizes transmission signal and could allow for dose reduction in MDCT.

In contrast to static beam-shaping filtration used in MDCT to date, dynamic filter concepts allow for signal transmission homogenization and could thus allow for radiation exposure reduction in MDCT.

Stiller, W, Veloza, S, Kauczor, H, Dynamic MDCT Beam-shaping Filtration in Dependence of Projection Angle: Performance Analysis for Elliptical Subject Geometries.  Radiological Society of North America 2011 Scientific Assembly and Annual Meeting, November 26 - December 2, 2011 ,Chicago IL. http://archive.rsna.org/2011/11016547.html