Robert Bohle DIPLPHYS, Presenter: Nothing to Disclose

Daniel Kolditz, Abstract Co-Author: Employee, CT Imaging GmbH

Natalia Saltybaeva, Abstract Co-Author: Nothing to Disclose

Ferdinand Lueck DIPLPHYS, Abstract Co-Author: Employee, CT Imaging GmbH

Bernhard Schmidt PhD, Abstract Co-Author: Employee, Siemens AG

Willi A. Kalender PhD, Abstract Co-Author: Consultant, Siemens AG Consultant, Bayer AG Founder, CT Imaging GmbH Scientific Advisor, CT Imaging GmbH CEO, CT Imaging GmbH

Dose in CT can be decreased by shaped filters (SF) which reduce the x-ray intensity in the periphery. Monte Carlo (MC) dose simulations provide dose distributions from which organ doses can be derived. For precise MC dose simulations the information on SFs is needed, but is generally not provided by the manufacturers. We investigated if only the x-ray intensity as a function of the fan angle (intensity function) can be used to resolve this situation.

Air kerma at different positions was measured with a calibrated ionization chamber (UniDos, PTW, Germany) with the x-ray source positioned at 12 o’clock for different tube voltages (80,100,120,140 kV) on a SOMATOM Definition Flash scanner (Siemens AG, Germany). Positions were chosen in the central horizontal plane at z=0 in 4 cm increments. The measured doses were corrected taking into account the increasing x-ray path length for increasing fan angle. For every tube voltage the corrected x-ray intensities served as input for the modeled x-ray source of the MC dose calculation (ImpactMC, CT Imaging, Erlangen, Germany). The intensity modeling was evaluated in terms of the deviation between calculated and measured CTDI. Moreover the results were compared with CTDI values resulting from MC calculation modeling the SF based on its physical characteristics which were provided confidentially by the manufacturer. In addition the results were compared with calculated CTDI values neglecting the SF by considering a constant intensity function.

The maximal deviation of calculated CTDI values from measured CTDI values was 8% for the physical SF model, 11% for the intensity-based model and 105% for the constant intensity function. The mean absolute deviations were 4%, 9% and 62%, respectively.

The modeling of the x-ray source of the MC dose simulation is crucial for accurate calculations. Since the intensity-based model reproduced the measured CTDI with nearly the same accuracy as the physical SF model the model appears to be useful for dose calculations when no SF information is available. Since SF information generally is kept confidential it may be sufficient if manufacturers provide the intensity function for dose calculations.

When no SF information is provided the intensity-based model allows for sufficiently accurate MC dose calculations for different SFs.

Bohle, R, Kolditz, D, Saltybaeva, N, Lueck, F, Schmidt, B, Kalender, W, Comparison of CT Dose Calculations Using Either Exact Shaped Filter Information or the Intensity Function Which Results from Ionization Chamber Measurements.  Radiological Society of North America 2012 Scientific Assembly and Annual Meeting, November 25 - November 30, 2012 ,Chicago IL. http://archive.rsna.org/2012/12022890.html