Abstract. We present a method of 3D/2D image registration. The algorithm is based on the property of near projective invariance in tubular objects. The skeletons of tubular anatomical structures (e.g., intracerebral blood vessels) are used as registration primitives. Experiments with Magnetic Resonance Angiogram (MRA) patient studies and both simulated and actual X-ray angiograms suggest that the algorithm is very accurate and robust. The algorithm requires only a small number of primitives. In addition, the algorithm is relatively insensitive to the choice of tubular structures used. Experimental results justifying these claims are included.