Non-rigid registration between CT and ultrasound images is a difficult task due to the low resolution and contrast of ultrasound images. We present a method incorporating the shape information of contours extracted from the image pairs for the registration. Firstly, the shapes are represented by the automatically detected landmarks along the contour of segmented object on CT and ultrasound images. Secondly, the landmarks are corresponded by landmark sliding and the negative mean squared distance of the corresponding landmarks is used as the shape similarity measure. Finally, the integrated similarity metric for registration combines mutual information, shape similarity and smoothness constraint. The experiment results show that registration accuracy using the integrated similarity is higher than using only mutual information with nearly same execution time. It is verified that the shape information could improve the registration results between CT and ultrasound images.
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