QUANTITATIVE ASSESSMENT OF GRAY-LEVEL PERCEPTION - OBSERVERS ACCURACYIS DEPENDENT ON DENSITY DIFFERENCES

Sonographic image density represents tissue echogenicity, or the acous tic nature of scanned tissue. It has the potential to distinguish norm al from abnormal tissue. Humans are poor assessors of tissue echogenic ity, yet their abilities have not been quantitatively compared to more objective methods. We compared the accuracy of image density differen tiation by the human eye and by densitometry in the fetal liver model. A group of 60 observers was asked to compare the echogenicity of thre e pairs of images of a fetal liver. Twenty-three repeated the comparis on several weeks later. Image density was measured by electro-optical densitometry. The two-tailed signed rank test was used to compare the observers's perceptions with the density values and the repeated obser vations with the original responses. The density of each fetal liver i mage in pair A was the same. Image B1 had a 12.3% lower density than i mage B2, and image C1 had a 6.3% greater density than image C2. Observ ers were accurate more often when comparing images with large density differences (pair B) than they were when comparing images with the sam e density (pair A) or with small density differences (pair C). Accurac y was not related to being medically qualified or from an obstetric or gynecological department. Intraobserver variation was not significant . We conclude that the human eye is extremely inaccurate at discerning differences in echogenicity when images have small or no differences in optical density. Densitometry is, therefore, indispensable for accu rate image density assessment.