Effect of the beam-vessel angle on the received acoustic signal from blood

In order to explore the feasibility of algorithms to determine the three dimensional (3D) velocity magnitude from the received ultrasonic blood echo from a single line of sight, the signal from small sample volumes is studied as a function of beam-vessel angle. As opposed to previous treatments of the effect of the beam-vessel angle on the received acoustic signal, a wideband signal is transmitted and the returned signal in each sample volume is analyzed. High-resolution experimental M-mode images of radio-frequency (rf) echo signals are used to visualize the flow in individual regions of interest. These experiments confirm the predictions of a theoretical model for the signal and its second moment. It is shown that the two major effects limiting the correlated signal interval are the spread of axial velocities within the sample volume and the transit time across the lateral beam width. Particularly for small beam-vessel angles, the spread of velocities limits the correlated signal interval. In addition, the experimental results demonstrate that accurate velocity estimation for low volume flow rates and particularly for large beam-vessel angles may involve detection of changes in the correlation magnitude. For low volume flow rates, the shape of the correlation surface can be affected by small regions of blood with a strong scattering intensity located near the initial region of interest.<
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