1. An imaging system, comprising: a computerized-tomography (CT) imaging unit configured to form a 3-dimensional (D) CT image of a target object; an ultrasound image forming unit configured to form 3-D and 2-D ultrasound images of the target object; a storing unit configured to store sets of transform parameters for use in performing a rigid-body transform upon the 3-D ultrasound image; a processor configured to perform an image registration between the 3-D ultrasound and CT images, establish a 3-D coordinate system with a reference surface being set thereon in the 3-D ultrasound image, perform the rigid-body transform upon the 3-D ultrasound image by sequentially applying the sets of transformation parameters retrieved from the storing unit, calculate similarities between reference images corresponding to the reference surface and the 2-D ultrasound image, and extract a 2-D CT image corresponding to the 2-D ultrasound image from the 3-D ultrasound-CT registered image based on the calculated similarities; and a display unit configured to display the 2-D ultrasound image and the resultant 2-D CT image, wherein each of the reference images is obtained at every rigid-body transform when the sets of transformation parameters are sequentially applied thereto and the 2-D ultrasound image is related to a region of interest (ROI) of the target object.