1. A method of extracting irradiation field areas in an X-ray image represented by a digital signal representation, the method comprising the steps of: first, segmenting the X-ray image into regions of pixels that have similar local image characteristics by image clustering to merge regions which have similar local image characteristics; second, and after the first step, fitting line segments to boundaries of the merged regions so that the line segments correspond with candidate irradiation field boundaries and define a segmentation map; and third, and after the second step, classifying regions in the segmentation map into a first class defining an irradiation field region and a second class defining a collimated region based on at least one of local, regional, and global image characteristics; wherein a Hough transform is applied to the boundaries of the merged regions to fit the line segments corresponding with the candidate irradiation field boundaries, and a best-fit bounding box of the candidate irradiation field boundaries is determined based on a maximum line segment length in a main direction of the X-ray image and a maximum line segment length in a direction of the X-ray image perpendicular to the main direction.