1. A computer implemented method for registering pre-operative images of an object with an intra-operative image of the object during an operative procedure, said method performed by the computer comprising the steps of: providing a knowledge base extracted from pre-operative 3D images of an anatomical region of a patient, said knowledge base including 2D digitally reconstructed radiographs (DRRs) computed from said pre-operative 3D images for a plurality of poses and a set of signatures indicative of each said DRR; acquiring a 2D intra-operative image of the anatomical region of the patient; extracting a signature from the intra-operative image; comparing the intra-operative signature to the stored pre-operative signatures; retrieving a pre-operative image having a best signature match to the intra-operative signature; and registering the retrieved best matching pre-operative image with the intra-operative image.