Dose reduction in digital radiography based on the significance of marginal contrast detectability

The performance of three digital detectors was measured at two exposure index (EI) levels in terms of the effect on features at the borderline of detectability. The null hypothesis was that there would be no statistically significant difference in the CNR of marginally visible features of a baseline- (2.2 µGy) and reduced dose (1.4 µGy) images. The experiment used three digital detectors and a phantom composed of an aluminum contrast-recovery plate, with features of varying diameters and hole depths, which was placed between the detector/grid and 5-20 cm Lucite. Exposures were made using a kVp between 55 and 110 corresponding to the Lucite thickness and a mAs producing an EI of approximately 220 or 140. Images were acquired for all detectors, EI values, and all Lucite thicknesses, then scored by a team of physicists and technologists in terms of feature visibility for each feature size. Contrast-to-noise ratio (CNR) was calculated for each feature using an ROI over the feature and a local background annulus. The uncertainty in the CNR was determined by sampling the background at each feature size, finding residuals from an overall background fit, and then calculating a standard deviation in the noise for each size. The marginal feature pair for each feature size bracketed the reader score. The difference between the CNR values of corresponding marginal features in EI-paired images was significant (P < 0.05) for one detector and not significant (P > 0.05) for marginal features of the other two. Based on both reader scoring and CNR measurements of phantoms, patient doses can be lowered by 30% for those two detectors without a statistically significant difference in lesion perceptibility of the marginally visible feature, while for the other detector there was a statistically significant change in marginal feature detectability and dose reduction was not recommended.