A scanning system for chest radiography with regional exposure control: theoretical considerations

Conventional chest radiography is limited by the presence of scattered radiation and the small useful exposure range of radiographic film. A computer-assisted scanning system to minimize these two effects is outlined. The system uses a small beam of radiation swept over the patient's chest in a raster pattern to expose a conventional film cassette, while a slit collimator scanning between the patient and the film serves to reject scattered photons. A microcomputer measures beam attenuation by the patient with a detector placed behind the film which in turn automatically adjusts the x-ray tube output to minimize excursions in film exposure as the beam scans. A formalism which relates the patient transmission and film exposure distribution is developed and a system transfer function is given. It is shown that such a system operates as a spatial filter which attenuates film contrast for structures of spatial frequency less than the inverse scanning beam width. By manipulating the software parameters of the feedback network, it is possible to alter this filter and produce radiographs with low spatial frequency enhancement, attenuation, or contrast inversion.