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Tumor detection on chest images is improved when image sites that receive prolonged visual attention are identified by eye-position recording and physically circled on the image. Missed lesions on mammograms can also be identified by eye- position recording but there is no data about whether site identification and subsequent re-evaluation improves performance. The goal of this work it to determine if a circle cue improves the detectability of a mass on a mammogram independently of indicating high yield sites. Simulated masses were added to the exact center of one of a pair of 7.7 multiplied by 7.7 cm patches of either Gaussian noise or mammogram parenchyma displayed on a 2000 by 2000 pixel Tektronix monitor. The mammogram parenchyma patches were selected from a set of 10 digitized mammograms by first randomly selecting a mammogram and then randomly selecting a point within the mammogram to locate the center of the patch. On about half of the trials a 6.4 cm diameter circle was drawn about the center of both patches. The mass was always centered in the patch and in the circle. The subject had to indicate which side contained a mass on each trial. Five levels of mass contrast were used. In this 2 AFC task, each of three subjects saw each contrast level 200 times in the circle and no-circle condition and with a Gaussian noise and mammogram background for a total of 4000 trials per subject. The index of detectability, d', was calculated and the results were analyzed using an analysis of variance with pooling of the d' values over subjects. As would be expected the d' for the detection of the masses increase linearly with SNR under all conditions. The d' for detection increased for all subjects and at all signal levels when the mass was physically circled on the image. The increase was not statistically significant for the noise background (p equals. 19) but was statistically significant for the mammographic background (p equals .001). Regression analysis showed an increase in d' of .41 for the Gaussian noise background and .54 for the mammogram. Cues like arrows, circles, and luminance pedestals are thought to act by reducing uncertainty about the exact location of a target. The experimental situation used here where the tumor was always in the exact center of a square suggests that an additional perceptual mechanism is operating. The circle may limit the area over which the retina has to integrate image noise or it may improve retinal contrast sensitivity by providing a fiducial marker to help stabilize the fine eye movements.
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