FLATTENING OF THE CONTRAST-DETAIL CURVE FOR LARGE LESIONS ON LIVER CTIMAGES

This study evaluated the relative roles of physical and perceptual fac tors in flattening the contrast-detail (CD) curve on liver CT scans. T o estimate the role of physical factors, the theoretical CD curve for a calculated theoretical observer (i.e., a nonprewhitening matched fil ter) was predicted using the measured noise power spectrum and measure d modulation transfer function of the CT system. Another theoretical C D curve was also produced from the output of the same calculated obser ver after taking the human visual response function (VRF) into account . Perceptual factors were evaluated by analyzing human observers' repl icated ratings of the visibility of details superimposed on liver CT s cans. The CD curve for the calculated theoretical observer was below t he CD curve actually measured for nine human observers and showed no f lattening. With the VRF included, flattening of the theoretical CD cur ves was only produced by fixed image viewing distances of less than 30 cm, a reading style not employed by the human observers. Correlated R OC analysis of observers' replicated ratings indicated that while rand om, intraobserver variation was present, the magnitude of this so-call ed observer noise was insufficient to explain the flattening of CD cur ves. Use of narrow display windows did not eliminate this flattening e ffect. The main reason for human observers' inefficient detection of l arge, low contrast liver lesions appears to be a consistent misuse of the image information.