A STATISTICAL-ANALYSIS OF THE RECEIVED SIGNAL FROM BLOOD DURING LAMINAR-FLOW

In order to determine the limiting velocity resolution that can be ach ieved using ultrasound, and to provide a model which can be generalize d for the analysis of disturbed flow, a theoretical and experimental e valuation of the statistics of the received signal from laminar flow f ollowing the transmission of a train of short pulses is presented. We derive the autocorrelation function and determine the length of the co rrelated signal for various flow rates, comparing experimental measure ments to theoretical predictions. High resolution experimental RF M-mo de images are used to verify the theoretical model. Using a fluid with a density, viscosity, volume concentration, particle size, and speed of sound which is similar to that of blood, we show that the signal re mains correlated for a long interval under many conditions of clinical interest. Including a comparison with experimental data, the effect o f the lateral transit time through the sample volume and the axial vel ocity spread within the sample volume on the correlation of the receiv ed signal is evaluated. When a significant range of velocity component s is present within the sample volume, this range is the limiting fact or in the length of the correlated signal interval. Therefore, the use of a wideband signal, which reduces the sample volume size, produces a returned signal that may be correlated for a larger number of pulses , or for a longer time.