Richard Van Metter PhD, Presenter: Nothing to Disclose

John Yorkston PhD, Abstract Co-Author: Nothing to Disclose

Images from commercially available CR and DR systems are characterized by a variety of incompatible speed metrics. The need for a universal vendor-independent speed metric has been recognized by the formation of AAPM Task Group 116. This paper reviews a proposed 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. It also presents the sensitivity characteristics for representative CR and DR detector technologies.

ISO-9236-1 defines radiographic speed for screen-film combinations as the detector dose needed to produce a net density of one (1) on developed films. Unlike screen film, digital detectors can acquire images over a wide range of exposures. Hence, speed is an attribute of each acquired image – rather than an attribute of the acquisition system. The speed of digitally acquired images can be defined as the detector dose used to produce a specific response in the display-ready image sent to the PACS. The standard set of ISO-9236-1 x-ray beam qualities has been used to measure and calibrate the sensitivity of CR and DR detectors.

The exposure response for screen film (GdOS), CR (BaFBrI), indirect DR (CsI), and direct DR (a-Se) systems has been measured for the four ISO beam qualities. Matched exposures of anthropomorphic phantoms have verified the ability of the proposed speed metric to provide accurate speed/dose comparisons among these technologies. The proposed speed metric is well correlated with a currently available vendor-specific speed metric. For example, in a set of 597 skull images, the proposed speed metric was linearly related to the vendor-specific speed metric with a correlation coefficient, r2 = 0.987. Changes in exposure and image processing appropriately affect the speed metric.

An image speed/dose metric has been demonstrated for digitally acquired projection radiographs that is vendor-independent, consistent with current screen-film speed standards, and is readily applied to all currently available digital acquisition systems.

R.V.,J.Y.: The authors are employees of the Eastman Kodak Company.

Van Metter, R, Yorkston, J, Proposal for a Universal Definition of Speed for Digital Radiography.  Radiological Society of North America 2005 Scientific Assembly and Annual Meeting, November 27 - December 2, 2005 ,Chicago IL. http://archive.rsna.org/2005/4410207.html