A system for imaging a target structure of a patient's heart, the system comprising: means adapted to receive a pre-acquired image; a catheter comprising a position sensor and an ultrasonic imaging sensor, the position sensor adapted to transmit electrical signals indicative of positional information of a portion of the catheter in the patient's body, and the ultrasonic imaging sensor adapted to transmit ultrasonic energy at the target structure of the patient's heart, receive ultrasonic echoes reflected from the target structure of the patient's heart and transmit signals relating to the ultrasonic echoes reflected from the target structure of the patient's heart; a positioning processor operatively connected to the catheter and adapted to determine 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 adapted to generate multiple two-dimensional ultrasonic images based on the signals transmitted by the ultrasonic sensor and determine positional information for any pixel of the ultrasonic images, and characterised in that the image processor is further adapted to: assign 3D coordinates to contours of interest (94, 96) manually tagged in one or more of said two-dimensional ultrasonic images after said one or more two-dimensional ultrasonic images are generated, thereby to identify features of the heart; automatically identify and reconstruct contours (84, 86, 88) corresponding to said contours of interest in at least some of any remaining untagged two-dimensional ultrasonic images; reconstruct a three-dimensional model of the target structure of the patient's heart from the two-dimensional ultrasonic images, wherein said contours are used to segment three-dimensional structures in the three-dimensional model; and register the pre-acquired image with at least one of the two-dimensional ultrasonic images, and with the three-dimensional model; the system further comprising: a display adapted to display the registered pre-acquired image together with the three-dimensional model and/or with at least one of the two-dimensional ultrasonic images. means adapted to receive a pre-acquired image; a catheter comprising a position sensor and an ultrasonic imaging sensor, the position sensor adapted to transmit electrical signals indicative of positional information of a portion of the catheter in the patient's body, and the ultrasonic imaging sensor adapted to transmit ultrasonic energy at the target structure of the patient's heart, receive ultrasonic echoes reflected from the target structure of the patient's heart and transmit signals relating to the ultrasonic echoes reflected from the target structure of the patient's heart; a positioning processor operatively connected to the catheter and adapted to determine 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 adapted to generate multiple two-dimensional ultrasonic images based on the signals transmitted by the ultrasonic sensor and determine positional information for any pixel of the ultrasonic images, and characterised in that the image processor is further adapted to: assign 3D coordinates to contours of interest (94, 96) manually tagged in one or more of said two-dimensional ultrasonic images after said one or more two-dimensional ultrasonic images are generated, thereby to identify features of the heart; automatically identify and reconstruct contours (84, 86, 88) corresponding to said contours of interest in at least some of any remaining untagged two-dimensional ultrasonic images; reconstruct a three-dimensional model of the target structure of the patient's heart from the two-dimensional ultrasonic images, wherein said contours are used to segment three-dimensional structures in the three-dimensional model; and register the pre-acquired image with at least one of the two-dimensional ultrasonic images, and with the three-dimensional model; the system further comprising: a display adapted to display the registered pre-acquired image together with the three-dimensional model and/or with at least one of the two-dimensional ultrasonic images. a display adapted to display the registered pre-acquired image together with the three-dimensional model and/or with at least one of the two-dimensional ultrasonic images.