Julia Wicklein, Presenter: Nothing to Disclose

Holger Kunze MS, Abstract Co-Author: Research Consultant, Siemens AG

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

Yiannis Kyriakou PhD, Abstract Co-Author: Employee, Siemens AG

Misalignment-Correction in C-arm-based flat-detector CT (FDCT) is a frequently discussed problem. To avoid artifacts caused by geometrical instabilities, numerous methods for misalignment correction were investigated. Most of them make use of a foregoing calibration routine, based on scanning a specific phantom. The aim of this study is to develop and evaluate an online image-content-based calibration technique without using any kind of marker or calibration phantom. With online-calibration, the restriction to pre-defined scan protocols would become obsolete.

A Gabor filter consists of a sinusoidal plane of a particular frequency and orientation, modulated by a Gaussian envelop. It is used to filter the misaligned reconstruction result to form an orientation-dependent edge description of it. A probability matrix, representing the filtered image, forms the basis for feature extraction. We use this probability matrix for creating an entropy criterion which is then used to optimize the underlying geometry parameters of the acquisition system. Phantom and patient measurements with the C-arm FDCT system Artis Zeego (Siemens AG, Healthcare Sector, Forchheim, Germany) were used to validate the algorithm for realistic applications. We validated our results with an object-independent measure for geometry comparison in C-arm systems. The back-projection mismatch (BPM) is based on a virtual object and assigns one single value to compare different geometry calibrations. Invariance towards variable calibration positions as well as independence towards different C-arm systems is warranted.

The algorithm reduced most of the actual misalignment and increased image quality drastically. Evaluations of phantom-studies showed a misalignment reduction of 75% (quantified by the BPM), started from the standard system geometry without calibration.

Online-calibration has considerable advantages over foregoing calibration routines. The proposed misalignment reduces sensibility of the FD-CT system to geometrical inaccuracy and improves image quality for C-arm FD-CT.

Online-calibration is possible with our approach and therefore, the limitation to predefined scan-protocols is obsolete.

Wicklein, J, Kunze, H, Kalender, W, Kyriakou, Y, An Online-Calibration Method for Misalignment Correction in C-arm Flat-Detector CT.  Radiological Society of North America 2011 Scientific Assembly and Annual Meeting, November 26 - December 2, 2011 ,Chicago IL. http://archive.rsna.org/2011/11034395.html