1. A method for registering a 2-dimensional digital subtraction angiography (DSA) image to a 3-dimensional image volume during a cardiac procedure, comprising the steps of: calculating a coarse similarity measure between a 2-dimensional digitally reconstructed radiograph (DRR) of an aorta and a cardiac DSA image, and a 2-dimensional DRR of a coronary artery and the cardiac DSA image, for a plurality of poses over a range of 2-dimensional translations; selecting one or more DRR-pose combinations for the aorta and the coronary artery with largest similarity measures as refinement candidates; calculating the similarity measure between the refinement candidate DRRs of the aorta and the DSA, and between the refinement candidate DRRs of the coronary artery and the DSA, for a plurality of poses over a range of 3-dimensional translations and in-plane rotations; selecting one or more DRR-pose combinations for the aorta and the coronary artery with largest similarity measures as final candidates; calculating the similarity measure between the final candidate DRRs of the aorta and the DSA, and between the final candidate DRRs of the coronary artery and the DSA, for a plurality of poses over a range of 3-dimensional translations and 3-dimensional rotations; and selecting a DRR-pose combination with a largest similarity measure as a final registration result.