To determine the feasibility of quantitative ultrasonic techniques to define the composition of atherosclerotic plaques, samples of freshly excised human aortas were sewn to a sample holder and immersed in a saline bath for ultrasonic interrogation. Integrated backscatter of a 2-microsecond portion of the backscattered radiofrequency signal was measured with a 10 MHz focused transducer. A noninvasive, quantitative method is needed for ultimate application to longitudinal assess-L ment of the temporal evolution of atherosclerosis and the efficacy of therapy designed to retard or reverse atherogenesis. Although conventional ultrasonic methods are suitable for characterization of flow patterns and dimensions of vessels (pulsed and continuous Doppler and B-mode ultrasound), 1 -2 they do not characterize structural alterations of the vessel wall such as fibrosis, calcification, or deposition of lipid. Furthermore, quantification of present B-mode ultrasonic images lacks standardization and is hampered by subjectivity with which gain and energy settings are selected by the operator.We have shown that selected quantitative ultrasonic indexes can identify structural features of tissue manifesting diverse pathophysiological processes.3 For example, integrated backscatter, a measure of the energy efficiency of acoustic backscatter from a tissue sample, readily differentiated acutely ischemic. from normal myocardium. 4 In addition, backscatter analysis identified focal scarring and calcification in myocardium from cardiomyopathic Syrian hamsters.5 Thus, integrated backscatter provides a sensitive index of altered tissue structure.Because it entails reflected rather than transmitted ultrasound, integrated backscatter can be utilized to evaluate the physical properties of arterial walls in a