Low Contrast Detectability and Contrast/Detail Analysis in Medical Ultrasound
The first- and second-order statistics of envelope detected ultrasound (US) B-mode images for the case of a scattering phantom with many scatterters per resolution cell have been previously derived. These characteristics are integrated over the region of a simulated focal (disk) lesion and the signal-to-noise ratio (SNR) for lesion detectability is obtained. This SNR requires the average number of independent speckle cells over the lesion area (analogous to the number of x-ray photons over the lesion area in incoherent light or x-ray imaging). This number is obtained from our autocorrelation analysis (second-order statistics). By setting the SNR expression equal to the threshold value SNRT required to detect a lesion in the presence of speckle noise, the dependence of lesion contrast on lesion diameter at threshold is found, i.e., the contrast/detail function. This is a simple inverse relation for ideal observers of US B-scans. It is also found that the contrast/detail results for envelope detection in diagnostic ultrasound are almost identical with the results for square law detection (the usual laser case) with the latter serving as an upper limit for performance in lesion detection. Finally, the results of human observer performance using a contrast/detail phantom are compared with the predictions for optimal or ideal performance. The results are comparable with results for photon imaging systems, with values of the SNR at threshold in the neighborhood of 2–3. Copyright © 1983 by The Institute of Electrical and Electronics Engineers, Inc.
