Embodiments for providing a 2-diemsional (D) computerized-tomography (CT) image corresponding to a 2-D ultrasound image through image registration between 3-D ultrasound and CT images are disclosed. An imaging system 100 comprises a CT imaging unit 110, an ultrasound image forming unit 120, a storing unit 130, a processor 140 and a display unit 150. The processor 140 extracts the 2-D CT image from the 3-D ultrasound-CT registered image by performing a rigid-body transform upon the 3-D ultrasound image and calculating similarities between reference images and the 2-D ultrasound images, wherein the reference images arc obtained through the rigid-body transform.
