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An investigation of search pattern extent in the threshold contrast detection task

Regional Medical Physics Department, Newcastle General Hospital, Newcastle-upon-Tyne NE4 6BE, UK

The threshold contrast-detail diameter test is used as a semi-quantitative measure of image quality in radiology. This observation task is called "signal known exactly/background known exactly" because the signals are usually low contrast disks in known positions, and the background is uniform except for noise fluctuations. The performance of the observer undertaking this task can to some extent be predicted from knowledge of the noise power in the image background, and adoption of the assumption that the noise is sampled through an aperture of the same area as the test feature being observed. In order to extend this approach to optimization of clinical images, the effect of the cluttered anatomical background on the detection task must be quantified. To study the effect on detection of nearby structure, a series of contrast-detail tests was carried out using a progressively restricted background area of Gaussian noise, and a range of object diameters. It was found that the observer's ability to detect low contrast objects is progressively reduced as the area of the search area is reduced, the difficulty of the task increasing rapidly as the diameter of the restricted search area falls to less than twice that of the target disk. The results suggest the presence of a search pattern that scales in proportion with the size of the test feature.