The authors describe a variable compensation (VC) technique in which an x-ray equalizer and a computed radiography system are used. The VC technique allows retrospective alteration of equalized chest appearance with maintenance of improved signal-to-noise ratio in dense regions. Two imaging plates are used: one upstream of the patient to record the incident beam profile and one downstream to record the equalized image. Subtraction of a weighted version of the upstream image from the downstream image permits alteration of the appearance of the VC image, from the extremes of simulated-unequalized to highly equalized. VC image appearance was optimized with a real-time workstation. The quality of VC images obtained in 33 patients was evaluated by three chest radiologists. Mediastinal appearance was better on VC equalized images than on conventional screen-film images. The simulation of the appearance of a conventional radiograph with VC proved useful in interpretation of lung appearances on equalized radiographs.
