1. A system for imaging a target in a patient's body, the system comprising: a pre-acquired image; a catheter comprising a position sensor and an ultrasonic imaging sensor, the position sensor transmitting electrical signals indicative of positional information of a portion of the catheter in the patient's body, and the ultrasonic imaging sensor transmitting ultrasonic energy at the target in the patient's body, receiving ultrasonic echoes reflected from the target in the patient's body and transmitting signals relating to the ultrasonic echoes reflected from the target in the patient's body; a positioning processor operatively connected to the catheter for determining positional information of the portion of the catheter based on the electrical signals transmitted by the position sensor; an image processor operatively connected to the catheter and the positioning processor, the image processor generating an ultrasonic image of the target based on the signals transmitted by the ultrasonic sensor and determining positional information for any pixel of the ultrasonic image of the target, the image processor registering the pre-acquired image with the ultrasonic image; and a display for displaying the registered pre-acquired image and ultrasonic image.