A method of fusing ultrasound images with pre-acquired images includes acquiring a first 3D ultrasound image in real-time; identifying at least one known reference pattern or object within a pre-acquired 3D image and the 3D ultrasound image; registering the real-time 3D ultrasound image with the pre-acquired 3D image by using the reference pattern or object; fusing the 3D ultrasound images of a sequence of real-time 3D ultrasound images; with the pre-acquired 3D image using data of the co-registration; and displaying fusion tomographic images, each including the corresponding real-time ultrasound image and the pre-acquired image. The registration between real-time 3D ultrasound images and the pre-acquired 3D image is performed continuously in the background by continuing to acquire the real-time 3D ultrasound images and to register them with the pre-acquired 3D images using the reference pattern or patterns or the object or objects.