1. A method for registering three-dimensional images with two-dimensional intra-operative images, comprising: segmenting a tubular structured organ in a three-dimensional image of the organ; projecting the three-dimensional image of the organ into two-dimensional space to provide a projected image; segmenting a medical instrument depicted in a two-dimensional image of the medical instrument; and registering the projected image to the two-dimensional image based on a best match between the projected image of the organ and a shape of the medical instrument depicted in the two-dimensional image.