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Fig. 1 —Sample image acquisition of phantom (CDRAD 2.0, Artinis Medical Systems). Phantom contains circular holes logarithmically sized from 0.3 to 8.0 mm in both diameter and depth. For 4 mm or smaller holes, additional hole of matching diameter and depth is located at one of four corners. Presence of these additional holes allows four-alternative forced-choice experiment in which observer must select location of low-contrast objects among four possible corners.