Disclosed herein is an apparatus for forming an x-ray beam. The apparatus comprises a plurality of links pivotably coupled together, in an end-to-end manner, to form a continuous loop. The plurality of links comprises two or more links configured to block a transmission of an x-ray emission. The plurality of links also comprises at least one link comprising an aperture that is configured to allow only a portion of the x-ray emission to pass through the aperture.