Applying a proposed definition for receptor dose to digital projection images

Abstract

Digital radiographs are currently characterized in terms of a variety of incompatible, vendor-specific dose metrics, which make it difficult for users to monitor receptor dose. AAPM Task Group #116 has been formed to Standardize an Image Receptor Dose Index for Digital Radiography, prompting the publication of a proposed definition for receptor dose that can be readily implemented on any system that acquires a projection x-ray image and produces a display-ready image. 1 This paper carries that work forward by demonstrating the applicability of that proposal to a range of digital detector types and to a new set of clinical images. Digital receptor dose can be usefully defined for the four ISO beam qualities, in terms of the exposure needed to produce a specified display response in a displayed image, analogous to the ISO speed definition for screen-film 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 (represented as DICOM GSDF p-values). The receptor dose is computed in terms of the median of pixels in the original image that corresponds to near midrange pixels in the display-ready image. The exposure responses of Gd 2 O 2 S-, CsI(Tl)-, a-Se-, and BaFBrI-based digital radiography acquisition systems, as well as a commonly used Gd 2 O 2 S screen-film combination, have been measured for the four ISO beam qualities. The proposed receptor dose metric was computed for a sample of 602 clinical images for which body-part thickness technique factors (kV, mAs, and SID) were known. Analysis of this data demonstrates that the proposed receptor dose can be a useful predictor of exposure adequacy over a wide range of body parts and thickness. In conclusion, the proposed vendor-independent receptor dose metric has been shown to produce consistent results across a representative range of digital receptor technologies for a wide range of clinical images. It has also been shown to produce results that are consistent with existing standards for screen-film technology and thereby provides a measurement capable of bridging the transition to digital imaging.