There is provided an ultrasound system, which includes: an ultrasound diagnostic unit having a probe for transmitting an ultrasound beam to a target object and receiving ultrasound echo signals reflected from the target object to form ultrasound images a position tracking unit for providing position information of the probe and ultrasound beam direction information an external medical image signal providing unit for providing external medical image signals acquired from an external medical imaging device to form at least one external medical image a user input unit for inputting position information of a lesion in the external medical image from a user and an image processing unit for forming a fusion image of the ultrasound image and the external image based on the position information of the probe, the ultrasound beam direction information and the position information of the lesion in the external image.
