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21 March 2007 Automatic prostate localization using elastic registration of planning CT and daily 3D ultrasound images
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The prostate is known to move between daily fractions during the course of radiation therapy using external beams. This movement causes problem with 3D conformal or intensity-modulated radiation therapy, in which tight margins are used for treatment planning. To minimize the adverse effect of this motion on dose delivery, daily localization of the prostate with respect to the planning CT is necessary. Current ultrasound-based localization systems require manual alignment of ultrasound images with the planning CT. The resulting localization is subjective and has high interobserver variability. To reduce the alignment uncertainty and increase the setup efficiency, we proposed an automatic prostate alignment method using a volume subdivision-based elastic image registration algorithm. The algorithm uses normalized mutual information as the measure of image similarity between the daily 3D ultrasound images and the planning CT. The prostate contours on the CT are mapped to the ultrasound space by applying the transformation fields from image registration. The displacement of the center-of-mass of the mapped contours is calculated for automatic patient setup. For validation purposes, six experts independently and manually aligned the archived CT and 3D ultrasound images using the SonArray system and reported their readings as shifts along the three principal axes. The mean shift and standard deviation of the readings along each axis were calculated. We regarded the automatic alignment as being acceptable if the difference between the mean shift and the automatic shift is within two times the standard deviation. Three out of five patients were successfully aligned with two failures.
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Jianzhou Wu, Omkar Dandekar, Vivek Walimbe, Warren D'Souza, and Raj Shekhar "Automatic prostate localization using elastic registration of planning CT and daily 3D ultrasound images", Proc. SPIE 6509, Medical Imaging 2007: Visualization and Image-Guided Procedures, 650913 (21 March 2007);

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