Christoph Suess PhD, Abstract Co-Author: Employee, Siemens AG

Rosemarie Banckwitz, Abstract Co-Author: Employee, Siemens AG

Michael Grasruck PhD, Presenter: Employee, Siemens AG

Andreas Schaller PhD, Abstract Co-Author: Employee, Siemens AG

The current concepts for dose specification in CT, based on CTDI100, appear practical and easy to use, but cause a systematic underestimate of the applied dose. Especially for multi-slice scanners with large z-coverage and novel volume CT scanners these effects can not be neglected. We defined and evaluated an alternative approach which eliminates the systematic errors, but still allows use of the current equipment like short CTDI phantoms and Ion chambers.

We evaluated the accuracy and systematic errors of the CTDI100 technology by measuring the dose with a 100 mm dose chamber in 16 cm and 32 cm long PMMA phantoms with diameter 16 cm and 32 cm. Data were compared to the dose measured with a 250 mm dose chamber and LiF TLD arrays in 32 cm long phantoms. Dose data were acquired for the collimations of 2x1 mm, 1x10 mm and 24x1.2 mm in a multi-slice scanner (Sensation 64, Siemens Medical Solutions) and in a prototype flat-panel volume CT scanner (Siemens Medical Solutions) for 180 mm z-coverage. The measurements were repeated for the available tube potentials 80, 100, 120 and 140 kV.

For the multi-slice CT scanner the CTDI100 significantly underestimates the applied dose for all collimations and kV-settings. The dose integrated over 250 mm as well as over 320 mm was higher by 10% to 45% depending on phantom diameter and chamber position. For the Flatpanel scanner the dose measured over 320 mm length with a collimation of 180 mm was higher by 48% compared to the standard CTDI100. The desirable 'CTDIinfinity' (CTDI∞), which integrates over the entire dose profile, can be approximated by the measured CTDI100 and conversion factors. Even when neglecting the influence of collimation and kV, the errors are still below 5%.

The dose and dose profiles measured in the 32 cm long CTDI phantoms allow to define a new metric for a 'CTDIinfinity' (CTDI∞), that accurately describes dose in multi-slice and volume CT scanners.

Routine acceptance testing and quality control dose measurements can be based on the current CTDI100 and the conversion factors.

Suess, C, Banckwitz, R, Grasruck, M, Schaller, A, A New CTDI Concept for Multi-slice and Flat-Panel CT.  Radiological Society of North America 2006 Scientific Assembly and Annual Meeting, November 26 - December 1, 2006 ,Chicago IL. http://archive.rsna.org/2006/4433876.html