By local variation of exposure, the wide absorption-range of the thorax can be equalized. Normally, the X-ray spectrum remains constant during this process. Further improvement seems possible if the radiation quality is included in the equalization process. A system has been built to investigate the merits of such a process. An overall improvement of contrast is demonstrated.
Ronald J. Geluk,
"Equalization radiography with radiation quality modulation", Proc. SPIE 1443, Medical Imaging V: Image Physics, (1 July 1991); doi: 10.1117/12.43437; https://doi.org/10.1117/12.43437