Effects of aggregation of red cells and linear velocity gradients on the correlation-based method for quantitative IVUS blood flow at 20 MHz.

Recent computer simulations suggest that the presence of aggregates of red blood cells (RBCs), at random angles and lengths, does not affect the measurements of blood flow transverse to the ultrasound (US) beam direction using a correlation-based method and an intravascular (IV) US array catheter. However, in case of aggregates of RBCs aligned with the flow, measurements of simulated blood velocity are affected. Blood velocity gradients were also shown not to influence the correlation-based method for blood velocity estimation. The objective of this study was to quantify the influence of aggregates of RBCs and blood velocity gradients on the correlation-based method during in vitro experiments. For this purpose, measurements were performed on washed RBCs (no aggregation), normal human blood, and two types of diseased blood in which a lower or a higher level of aggregation was present. The decorrelation pattern of a circular US transducer as a function of transverse blood flow was studied using a Couette system. Changing the shear rate of the Couette system modified the aggregation level of RBCs and the velocity gradient. With the exception of the results at low shear rates and abnormally high aggregation levels, agreements were found between the autoconvolution of the acoustical beam (reference curve) and the radiofrequency (RF) decorrelation patterns. For the high shear rate present in coronary arteries, the correlation-based method for blood flow estimation should not be influenced by these phenomena.