Selective Exposure Radiography Using Digitally-Formed X-Ray Beam Attenuators

Since the early 1970's, Edholm and others have shown that compensation attenuators can provide fundamental improvements in radiographic image formation by modifying patient dose-distribution, reducing scatter fractions in dark regions of the image, and easing dynamic range requirements for film and video systems. Compensating attenuators have not yet been used widely in diagnostic radiology because of the difficulties of forming masks to compensate for the anatomical variations of individual patients. We have designed a software-based system which forms a heavy-metal attenuator from a digital image of the patient. At present, the attenuators are constructed manually from a pattern generated by the computer, but several techniques are being investigated which may permit fabrication and positioning during suspension of respiration. Phantom studies demonstrate that, in nonsubtractive applications, unsharp masking by the x-ray beam attenuator enhances local contrast, while in digitally subtracted images, attenuators eliminate dark regions where iodine signals otherwise are degraded by video and quantum noise. The technique can be used to reduce patient exposure in highly transmissive areas or, at the expense of increased tube loading, to increase exposure to highly attenuating areas in order to reduce image noise. Anticipated applications of this technique include chest radiography as well as conventional and digital subtraction angiography.