A scanning equalization system for improved chest radiography

The wide variation in absorber thickness characteristic of the posteroanterior projection of the chest routinely results in film underexposure and attendant suboptimal contrast in the mediastinal, retrocardiac, and diaphragmatic regions. We propose a special scanning system to deliver a more uniform film exposure over the image by modulating the intensity of a vertically oriented narrow fan (43 x 2 cm) of radiation, swept laterally across the patient's chest. Measurements of patient attenuation obtained with a detector placed behind the film cassette are used to adjust the x-ray tube output so as to maintain a constant film exposure during the course of the scan. The scan is completed in 5 seconds and results in images of significantly improved mediastinal penetration and contrast compared with those of conventional chest radiographs.