1. A method of fusing a real-time ultrasound image with a pre-acquired image of another modality, the method comprising: transforming a coordinate system of a real-time ultrasound (US) image to a coordinate system of a US sensor; transforming the coordinate system of the US sensor to a coordinate system of a tracking system, operative to track a position of the US sensor; and transforming the coordinate system of the tracking system to a coordinate system of the pre-acquired image.