| Ultrasound is a safe, inexpensive, easy-to-use, and real-tune imaging tool, which is widely used for medical diagnosis and for intraoperative applications. During a surgical operation, the anatomies with soft tissue such as liver are deformed by respiration, heartbeat, or ultrasound transducer pressure. This deformation causes difficulty for the surgeon to track a tumor or an organ landmark. To compensate for the deformation, fast, robust, and automatic techniques for registration of ultrasound images are required. However, poor quality of ultrasound images and the presence of significant amount of speckle in these images makes the ultrasound registration a challenge.; This thesis proposes a novel, fast, elastic registration technique for both two-dimensional and three-dimensional ultrasound images. In this technique, a new feature vector (attribute vector) is defined for each point in the images, which is employed to find correspondence across the images. Furthermore, this vector helps select a number of key points (leading points) that are mostly located on the anatomical features of the images and can be registered robustly. The displacement of the rest of the points are derived using a fast, free-form warping technique. The proposed registration technique is cornputationally efficient since the elements of the new attribute vector are fast to be computed and no pre-segmentation or numerical optimization is required. Experiments are carried out to evaluate the proposed registration technique using ultrasound images of liver deformed both naturally and artificially. In these experiments, the effect of registration parameters on the accuracy and the speed of registration is investigated. The results show that the method is sufficiently accurate and robust even in cases where artifacts such as shadows exist in the ultrasound data. As an additional application, a new extended field-of-view algorithm is also developed, which estimates the translational displacement of the ultrasound volumes without requiring position tracking devices.; Based on the proposed elastic registration method, a temporal registration technique is also developed for registering a sequence of ultrasound images. Assuming that the images are consecutively captured in real time, this algorithm registers each image to the first image in the sequence. The speed of registration of each image is significantly improved exploiting the information from the registration of the previous image. The registration parameters that provide a good trade off between speed and accuracy are also found. |
