A method and system is presented in image-guided radiosurgery for determining the measure of similarity of two digital images, for example a 2D x-ray image and a 2D DRR synthesized from 3D scan data. A two-dimensional array of pixel values of a difference image is formed by subtracting each pixel value of the second image from the corresponding pixel value of the first image. The pattern intensity function is constructed by taking the summation of asymptotic functions of the gradients of the difference image. The neighborhood R is defined so as to allow the gradients of the difference image to be considered in at least four directions.