Real-time motion tracking using 3D ultrasound

Three-dimensional (3D) ultrasound is ideally suited to monitor internal organ motion since it offers real-time volumetric imaging without exposing the patient to radiation. We extend a two dimensional (2D) region-tracking algorithm, which was originally used in computer vision, to monitor internal organ motion in 3D. A volume of interest is first selected in an ultrasound volume as a reference. The sum of squared differences is used as the similarity measure to register the reference to each successive volume frame. A transformation model is used to describe the motion and geometric deformation of the reference. The Gauss-Newton method is used to solve the optimization problem. In order to improve the algorithm's efficiency, the Jacobian matrix is decomposed as a product of a time-varying matrix and a constant matrix. The constant matrix is pre-computed to reduce the load of online computation. The algorithm was tested on targets under respiratory motion and cardiac motion. The experimental results show that the transformation model of the algorithm can approximate the geometric distortion of the reference template. With a properly selected reference with rich texture information, the algorithm is sufficiently accurate and robust to follow target motion, and fast enough to be used in real time.