VGC analysis : application of the ROC methodology to visual grading tasks
2008
To determine clinical image quality in radiography, visual grading of the reproduction of important anatomical
landmarks is often used. The rating data from the observers in a visual grading study with multiple scale steps is ordinal,
meaning that non-parametric rank-invariant statistical methods are required. However, many visual grading methods
incorrectly use parametric statistical methods. This work describes how the methodology developed in receiver operating
characteristics (ROC) analysis for characterising the difference in the observer's response to the signal and no-signal
distributions can be applied to visual grading data for characterising the difference in perceived image quality between
two systems. The method is termed visual grading characteristics (VGC) analysis. In a VGC study, the task of the
observer is to rate her confidence about the fulfilment of image quality criteria. Using ROC software, the given ratings
for the two systems are then used to determine the VGC curve, which describes the relationship between the proportions
of fulfilled image criteria for the two compared systems for all possible decision thresholds. As a single measure of the
difference in image quality between the two compared systems, the area under the VGC curve can be used.
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