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A number of digital imaging techniques in medicine require the combination of multiple images. Using these techniques, it is essential that the images be adequately aligned and registered prior to addition, subtraction, or any other combination of the images. This paper describes an alignment routine developed to register an image of a fixed object containing a global offset error, rotation error, and magnification error relative to a second image. The described routine uses sparsely sampled regional correlation in a novel way to reduce computation time and avoid the use of markers and human interaction. The result is a fast, robust, and automatic alignment algorithm, with accuracy better than about 0.2 pixel in a test with clinical computed radiography images.