A System for Image Registration in Digital Subtraction Angiography

In the diagnosis of coronary atherosclerosis radio-opaque dye is injected into the interior of the coronary arteries to make them visible in X-ray images. Because of the confusing presence of overlying or underlying soft tissue and bone and because of the small size of the coronary arteries, large dye concentrations are required to render the arteries sufficiently visible for diagnosis. Because of its increased contrast sensitivity, digital subtraction angiography (DSA) has the potential for providing diagnostic images of the coronary arteries with significantly reduced dye concentrations (Levin, 1984; Tobis et al., 1983; Riederer and Kruger, 1983). In DSA a series of images is acquired during the time period which begins before the injection of dye and continues until the arteries are opacified. These images are then combined into a final processed image in which the change in opacity of the arteries leads to enhanced arterial contrast. A particularly useful and commonly applied DSA technique, “temporal subtraction”, involves the subtraction of a “mask” image, acquired before opacification, from a “contrast” image, acquired after opacification. Ideally, temporal subtraction produces an image in which nothing appears except those arteries in which the amount of dye present has changed, but its usefulness in practice is limited by the image degradation caused by patient motion during image acquisition. The rigid motion of bones and the elastic motion of soft tissue in the field of view cause changes in X-ray opacity which are unrelated to the influx of contrast material. When the two images are subtracted these changes appear as ghost-like artifacts which obscure the arteries to be examined.