Noise and threshold contrast characteristics of a digital fluorographic system

The noise and threshold contrast characteristics of a Picker DAS 211 digital fluorography system are described. Leeds test objects were used to measure contrast-detail-dose relationships for digital fluorography and the results compared with similar measurements using screen-film radiography and 100 mm photofluorography. Noise and spread function measurements were made and from them threshold contrast (CT) values were calculated using the theory of signal detection developed by Hay and Chesters. Comparison with the results of visual assessment of images indicated generally good agreement between theory and practice depending upon the value taken for the threshold signal-to-noise ratio. It is concluded that calculations of CT can provide a means of summarising system performance based only upon objective measurements. This would be of value in quality assurance and performance assessment but should be used in addition to, not instead of, the traditional visual methods since these are important in maintaining a link with clinical practice.