Geometrical properties of a digital beam attenuator system

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Abstract
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 x 4 blurring function on the original 64 .times. 64 test image is required for attenuator to give appropriate compensated image appearance. A proposed attenuator printing scheme prints the attenuator in a 16 .times. 16 matrix, staggering successively printed layers to achieve the required 64 .times. 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.