1. A system for a non-rigid image registration between ultrasound images and computerized tomography (CT) images, comprising: an ultrasound image forming unit configured to transmit/receive ultrasound signals to/from a target object having a first region and a second region to thereby output electrical receive signals, the ultrasound image forming unit being further configured to form 3-dimensional ultrasound images based on the electrical receive signals; a CT image forming unit configured to form 3-dimensional CT images of the target object; and a registration unit configured to measure a first statistical entropy based on intensity values in the first region of the 3-dimensional ultrasound images, gradient magnitudes in the first region of the CT images and an edge orientation coincidence function in the first region to determine a first objective function and to measure a second statistical entropy based on intensity values in the second region in each of the 3-dimensional ultrasound and CT images and an edge orientation coincidence function in the second region to determine a second objective function, the registration unit being further configured to perform the non-rigid image registration between the 3-dimensional ultrasound images and the 3-dimensional CT images based on the first and second objective functions.