Chair for Computer Aided Medical Procedures & Augmented Reality
Lehrstuhl für Informatikanwendungen in der Medizin & Augmented Reality

P. Dressel
Calibration of Endobronchial Ultrasound
Diploma Thesis. Technische Universität München, January 2010 (bib)

Endobronchial Ultrasound (EBUS) has become a valuable tool for guidance during bronchoscopic interventions. In this thesis a calibration method and associated phantom are developed. This calibration allows for the estimation of the spatial relationship between the optical camera of a bronchoscope and the integrated ultrasound transducer, without the need for a tracking sensor. Knowledge about this spatial relation, in conjunction with a model of the perspective projection of the camera, enables the mapping of points from the ultrasound into the camera image, and of points from the camera images to lines in the ultrasound image. A possible application for this is the compounding of freehand 3D ultrasound directly within a CT volume. For this, the position of the camera is first determined via an image based camera to CT registration, and the spatial relation is then applied to find the position of the ultrasound plane in CT coordinates. The proposed method is based on an automatic pose estimation for the camera using a dot pattern. A method based on Z-fiducial pose estimation with hollow rubber tubes is used for the ultrasound plane. After the required geometrical properties of the phantom are approximated, a precise specification is developed, a phantom is built and the resulting geometry is measured with an optical tracking system. The achievable accuracy of the proposed calibration method is evaluated. The calibration is compared to another calibration method based on the established hand-eye and singlewall techniques using an electromagnetic tracker for the latter method. The new phantom based calibration is found to be more robust, producing an average transformation with a smaller backprojection error than the established techniques. Calibrations can also be performed much faster with the phantom and do not require a tracking system, thus rendering it an interesting alternative to the established methods.
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