Turbulence and circulating cerebral emboli detectable at Doppler ultrasonography: a differentiation study in a stenotic middle cerebral artery model.

BACKGROUND AND PURPOSE Blood flow within the middle cerebral artery can be monitored by transcranial Doppler (TCD) ultrasonography. Arterial stenosis can produce turbulent flow, but controversy remains regarding the degree of stenosis needed to cause TCD-detectable turbulence. Furthermore, cerebral emboli and turbulent flow may coexist in the same patient. The current study was designed to study the relationship between stenotic degree and TCD-detectable turbulent flow and its differentiation from cerebral emboli. METHODS A stenotic middle cerebral artery model was constructed to allow the study of different degrees of stenosis. TCD was used to detect the turbulent flow and embolic materials in the circulation model, and signal intensities were analyzed. RESULTS Turbulent flow was generated with a series of short, asymmetric stenoses in this test. Transition of turbulence from laminar flow began to occur at a moderate degree of stenosis (55% reduction of cross-sectional area) but became dampened at the high degrees of stenoses (>/=75%). Turbulence caused a notable increase in average Doppler signal intensity. However, the degree of stenosis was not associated with signal intensity at the region where turbulence existed, but rather was related to the mean flow velocity measured at the narrowed region. Embolic materials such as air bubbles (30 micro m) and platelet-rich clots (100 micro m) had notably higher Doppler signal intensity than that produced by turbulence. CONCLUSION TCD-detectable turbulence predominantly developed at a moderate arterial stenosis and its acoustic signature with TCD is different from that caused by air bubbles and platelet-rich clots.