The purpose of this work was to analyze the influence of background structure on the slopes of contrast-detail (CD)
curves in CT images acquired in uniform phantoms, anthropomorphic head phantoms, and in clinical head CT images.
Alternative forced-choice (AFC) studies were performed using CT images acquired in uniform (water) phantoms,
anthropomorphic (RANDO and ATOM) phantoms, and clinical head scans. The AFC experiments measure the lesion
contrast (I92%) that corresponds to 92% detection efficiency. The AFC experimental results were plotted as a function of
lesion size to produce CD curves, and the slopes of the curves determined when plotted on log-log axes. The Rose
model of detection predicts a slope of -1.0 for disk lesions in uniform backgrounds and white noise. CD curve slopes
showed a progression that depends on the complexity of the background structure in the CT images. For uniform water
phantom images, the slope averaged -0.9, which is close to that predicted by the Rose model. For the anthropomorphic
phantoms, the slope averaged -0.56, and for the patient scans the average slope was -0.20. The slope of CD curves
depends strongly on the background structure of the images in which the lesions are embedded, with increased
background structure leading to decreased CD curve slopes. The Rose model reasonably predicts the slopes for CD
curves acquired in uniform phantoms, but is a poor predictor of slopes in clinical head images.