The objective of this work was to test the capabilities of visual-search (VS) model observers for target classification. In this paper, a localization and classification ROC study was conducted with simulated single-pinhole nuclear medicine images. The images featured two sizes of Gaussian targets in Gaussian lumpy backgrounds, with one target twice the size of the other. Pinhole size was a study variable. The VS observer performed both the localization and classification. Three human observers also participated in the study. The trends in localization performance as a function of pinhole size for the VS and average human results were in good agreement. For the classification task, the VS and human observers performed on par, but with substantial differences in how they were affected by pinhole size. The VS observer correctly classified the smaller target less often than the larger target even though both targets were correctly localized with the same frequency.
Lesion-detection studies that analyze a fixed target position are generally considered predictive of studies involving lesion search, but the extent of the correlation often goes untested. The purpose of this work was to develop a visual-search (VS) model observer for location-known tasks that, coupled with previous work on localization tasks, would allow efficient same-observer assessments of how search and other task variations can alter study outcomes. The model observer featured adjustable parameters to control the search radius around the fixed lesion location and the minimum separation between suspicious locations. Comparisons were made against human observers, a channelized Hotelling observer and a nonprewhitening observer with eye filter in a two-alternative forced-choice study with simulated lumpy background images containing stationary anatomical and quantum noise. These images modeled single-pinhole nuclear medicine scans with different pinhole sizes. When the VS observer’s search radius was optimized with training images, close agreement was obtained with human-observer results. Some performance differences between the humans could be explained by varying the model observer’s separation parameter. The range of optimal pinhole sizes identified by the VS observer was in agreement with the range determined with the channelized Hotelling observer.
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