Geometrical properties of a digital beam attenuator system

A digital beam attenuator system has been developed to automatically generate patient‐specific compensating filters for chest radiography. An initial low‐dose test image is used to generate the attenuator, which is fabricated by overprinting multiple layers of a heavy‐metal material onto a nonattenuating substrate. The attenuator is subsequently inserted into the x‐ray beam for a final compensated radiograph. The effects of focal spot blurring and limited attenuator resolution result in the final compensated image containing only high‐spatial frequency information. The frequency response of the process is not strictly describable by a modulation transfer function, but an approximation of the frequencies remaining in the compensated image is obtained for low‐contrast conditions. It is found that a 4×4 blurring function on the original 64×64 test image is required for the attenuator to give appropriate compensated image appearance. A proposed attenuator printing scheme prints the attenuator in a 16×16 matrix, staggering successively printed layers to achieve the required 64×64 sampling with appropriate blurring. The resulting compensated image has good anatomical definition and contains a frequency response similar to that obtained by compensation techniques being investigated by Plewes and Sorenson.