Dr. Dirk Ertel Profile

Dr. Dirk Ertel

at Friedrich-Alexander-Univ Erlangen-Nürnberg

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PROCEEDINGS ARTICLE | March 14, 2009

Image-based online correction of misalignment artifacts in cone-beam CT

Proc. SPIE. 7258, Medical Imaging 2009: Physics of Medical Imaging

KEYWORDS: Medicine, Detection and tracking algorithms, Sensors, Calibration, Laser scanners, Head, Image quality, Computed tomography, Reconstruction algorithms, 3D scanning

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To perform a perspective cone-beam backprojection of the Feldkamp-type (FDK) the geometry of the approximately circular scan trajectory has to be available. If the system or the scan geometry is unknown and afflicted with geometric instabilities (misalignment) reconstructing a misaligned scan can cause severe artifacts in the CT images. We propose an online and image-based iterative correction of a misaligned reconstruction geometry by using entropy minimization. Unlike current methods which use a calibration of the geometry for defined scan protocols and calibration phantoms, the proposed method is performed combining a simplex algorithm for multi-parameter optimization and a graphics card (GPU)-based FDK-reconstruction in an iterative scheme. The simplex algorithm changes the geometric parameters of source and detector with respect to the reduction of entropy. In order to reduce the size of the dimensional space required for minimization the trajectory described by a subset of trajectory points. A virtual trajectory of an approximately circular path is generated after each iteration of the algorithm. This method was validated using simulations and measurements performed on a Carm CT System equipped with a flat-panel detector (Axiom Artis, Siemens Healthcare, Forchheim, Germany). Entropy was minimal for the ideal dataset, whereas strong misalignment resulted in a higher entropy value. The use of the GPU-based reconstruction provided an online geometry correction after a total computation time of only 1-3 s using 100 to 300 iterations of the algorithm, depending on the degree of misalignment and initialization conditions.

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