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 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 determine first and second objective functions associated with first and second regions of the target object, respectively, based on intensity and gradient information upon portions corresponding to the first and second regions in each of the 3-dimensional ultrasound and CT images, 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.