a method of manufacturing a collimator mandrel having variable attenuation characteristics is presented . the manufacturing process includes the placement of a layer of attenuating material on a core of base material . the layer of attenuating material is relatively thin and varies in thickness circumferentially around the core . the collimator mandrel may be manufactured by placing a cast about a core of non - attenuating material , filling a void between the cast and the core with an attenuating material , allowing the material to cure , and removing the cast from the assembly .