Embodiments for providing a 2-dimensional (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 comprises a CT imaging unit, an ultrasound image forming unit, a storing unit, a processor and a display unit. The processor 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 are obtained through the rigid-body transform.