Toward a universal definition of speed for digitally acquired projection images

Abstract

A variety of incompatible speed metrics for digitally acquired projection images are currently provided by system vendors. As digital acquisition has become more widely adopted, the need for a universal vendor-independent speed metric is increasingly evident. This paper proposes a method for defining the speed of digital projection images that can be readily implemented on any system that acquires a projection x-ray image and produces a display-ready image. Radiographic speed for screen-film combinations is currently defined by ISO in terms of the exposure needed to produce a net density of one on the developed film. An analogous speed method is proposed for digital images. It requires that the system produce an original image (calibrated in terms of the relationship between system response and exposure for the standard set of x-ray beam qualities defined by ISO-9236-1) and a display-ready image. The speed is computed in terms of the median pixel value in the original image that corresponds to a reference pixel value in the display-ready image. The exposure response of selected digital radiography acquisition systems has been measured for the ISO beam qualities. The proposed speed metric was computed for a representative suite of digital images and correlated well with a currently available vendor-specific speed metric. Changes in patient exposure and image-processing parameters affect the speed metric in the appropriate way. In conclusion, the proposed speed metric provides a vendor-independent definition of speed for digitally acquired projection radiographs that is consistent with current speed standards for screen-film radiography and applicable to all currently available digital acquisition systems.