1. A method for registering pre-operative images of an object with an intra-operative image of the object comprising the steps of: prior to an operative procedure, generating Digitally Reconstructed Radiographs (DRRs) for the pre-operative images at various poses for each patient; extracting signatures from the DRRs; storing the signatures in a knowledge base; during the operative procedure, 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 pre-operative image with the intra-operative image.