Yiannis Kyriakou, Presenter: Nothing to Disclose

Paul Deak, Abstract Co-Author: Nothing to Disclose

Oliver Langner, Abstract Co-Author: Nothing to Disclose

Willi A. Kalender PhD, Abstract Co-Author: Consultant, Siemens AG Founder, VAMP GmbH Scientific Advisor, VAMP GmbH Shareholder, VAMP GmbH

To provide a method for dose assessment in flat-panel detector CT (FPD-CT) combining measurements and Monte Carlo (MC) simulations.

FPD-CT scanners provide large irradiation fields of typ. 100 mm to 250 mm in the longitudinal direction. In consequence dose assessment according to the current definition of the CTDI would demand larger ion chambers and phantoms which is not practical. We propose a method which includes a point measurement in air or in a phantom with an integrating dosimeter (e.g. TLD) to assess the dose at that point in and to combine it with MC simulations. The MC simulation tool is used to assess the dose characteristics for arbitrary objects or scan geometry. It is calibrated with the point measurement to provide absolute dose values. For the Validation measurements were performed on an Axiom Artis C-arm system (Siemens Medical Solutions, Forchheim, Germany) equipped with a flat-panel detector of 40 cm x 30 cm. Dose was assessed for various tube voltages in air and in cylindrical PMMA phantoms of 16 cm and 32 cm diameter with a varying z-extent from 15 to 60 cm. MC simulations results were compared to the values obtained with calibrated ion chambers of 100 mm and 250 mm length and to TLD dose profiles along the complete z-extent of the phantoms.

The MC simulations showed that the standard CTDI phantoms which have a z-extent of 15 cm underestimate the dose integral by up to 15-40%(depending on the phantom's diameter) whereas a z-extent of 320 mm appears to be sufficient for the given area detector. The combination of MC simulation and point measurements was in agreement with the reference TLD profile results to within 5%. As expected the measurements with ion chambers of 100 mm and 250 mm offered a limited accuracy depending on the radiation field and the phantom size.

The combination of a point measurement and a validated MC tool appears to be a flexible solution to assess dose characteristics for FPD-CT and for CT in general.

Both technical and patient-specific dose characteristics can be conveniently assessed for FPD-CT.

Kyriakou, Y, Deak, P, Langner, O, Kalender, W, A Method for Dose Assessment in Flat-Panel Detector CT Combining Monte Carlo and Measurement Methods.  Radiological Society of North America 2006 Scientific Assembly and Annual Meeting, November 26 - December 1, 2006 ,Chicago IL. http://archive.rsna.org/2006/4434272.html