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Visual grading characteristics (VGC) analysis: a non-parametric rank-invariant statistical method for image quality evaluation.

Journal article
Authors Magnus Båth
Lars Gunnar Månsson
Published in The British journal of radiology
Volume 80
Issue 951
Pages 169-76
ISSN 1748-880X
Publication year 2007
Published at Institute of Clinical Sciences
Pages 169-76
Language en
Keywords Data Interpretation, Statistical, Humans, Quality Assurance, Health Care, methods, ROC Curve, Radiography, standards, Statistics, Nonparametric
Subject categories Radiology, Radiological physics


Visual grading of the reproduction of important anatomical structures is often used to determine clinical image quality in radiography. However, many visual grading methods incorrectly use statistical methods that require data belonging to an interval scale. The rating data from the observers in a visual grading study with multiple ratings is ordinal, meaning that non-parametric rank-invariant statistical methods are required. This paper describes such a method for determining the difference in image quality between two modalities called visual grading characteristics (VGC) analysis. In a VGC study, the task of the observer is to rate his confidence about the fulfilment of image quality criteria. The rating data for the two modalities are then analysed in a manner similar to that used in receiver operating characteristics (ROC) analysis. The resulting measure of image quality is the VGC curve, which--for all possible thresholds of the observer for a fulfilled criterion--describes the relationship between the proportions of fulfilled image criteria for the two compared modalities. The area under the VGC curve is proposed as a single measure of the difference in image quality between two compared modalities. It is also described how VGC analysis can be applied to data from an absolute visual grading analysis study.

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