Doppler ultrasound has been shown to be a useful clinical tool in the noninvasive detection of vascular stenoses, occlusion, or plaques. The presence of atherosclerotic lesions in blood vessel walls can distort the Doppler ultrasonic signal, producing inverted or biphasic tracings. This observation, in fact, has been used frequently as a criterion for the diagnosis of plaque formation and calcification of these lesions. To define the source of this distortion, we have examined the effects of various types of atherosclerotic lesions on the transmitted ultrasonic beam generated by a continuous wave (CW) Doppler ultrasonic probe. These results are compared to the Doppler flow tracings obtained by a CW ultrasonic Doppler flowmeter from blood vessels with overlaying lesions in a mock flow system. It is concluded that severe distortion in the form of beam refraction or diffraction can be produced by calcified lesions, resulting in biphasic or inverted flow signals. Indexing Words: Doppler * Atherosclerosis Beam distortion Doppler ultrasound has long been recognized as an extremely useful clinical tool for the noninvasive diagnosis of vascular diseases such as stenosis, occlusion, or plaquing of the blood vessels and the presence of air embolisms or thrombo~es.~" Continuous wave (CW) Doppler ultrasound has been used to characterize the degree of calcification of plaques formed in the carotid artery3,' based on the quality, frequency, direction, and amplitude of the returning Doppler signal. A working hypothesis has been developed by Spencer and Reid3 to correlate the composition of the atherosclerotic lesions with the characteristics of the Doppler signal. They postulated that medium-dense and dense deposits of calcium granules give rise to biphasic or inverted Doppler signals, whereas a solid calcified plaque produces no Doppler signal because of the total absorption and reflection of the sound beam by the plaque. This implies that the direction of the sound beam would be so refracted or scattered after passing through the calcium deposits that part of or the From the Bioengineering Program,