NPWE model observer as a validated alternative for contrast detail analysis of digital detectors in general radiography

Abstract

To propose and validate a non-prewhitening with eye filter (NPWE) model observer as an alternative means of quantifying and specifying imaging performance for general radiography detectors, in a comparative study with contrast detail analysis and detective quantum efficiency (DQE).Five different x-ray detectors were assessed, covering a range of detector technologies including powder computed radiography (CR), needle CR, and three indirect conversion flat panel digital radiography detectors (DR). For each detector, threshold contrast detail (c–d) detectability was measured using the Leeds TO20 test object. A tube voltage of 70 kV and 1 mm Cu added filtration was used and five target detector air kerma (DAK) levels were set, ranging from 0.625 µGy to 10 µGy. Three c–d images were acquired at the same DAK levels and these were scored by two observers. Presampling modulation transfer function (MTF) was measured using an edge method while contrast was measured with a 2 mm Al square of dimension 10 × 10 mm. The normalized noise power spectrum (NNPS) was calculated at the target DAK values of the c–d images. The MTF, NNPS and contrast data were then used to calculate a detectability index (d′) with the NPWE model and compared to the human observer c–d results. The standard quantitative means of evaluating detector performance i.e. DQE, was then calculated for each detector.A linear correlation was found between the logarithm of threshold contrast and the logarithm of d’ for all detectors, as DAK was increased. Furthermore, the absolute value of d′ tracked threshold contrast between the five detectors, enabling the use of detectability to quantify image quality rather than the intrinsically subjective threshold contrast scored by human observers from c–d test object images. At 2.5 µGy target DAK, d’ followed the differences in DQE between the five detectors.The NPWE detectability index can be used an alternative parameter for the quantification and specification of the image quality of general radiography x-ray detectors.