Influence of the Acoustic Lens on Vector Doppler Measurements Based on Plane Wave Transmission: a Simulation Study

We recently showed that vector Doppler velocity measurements based on the transmission of plane waves by linear array probes may result strongly underestimated at great investigation depths. This paper investigates the contribution to such underestimation by the divergence of the beam due to the acoustic lens of the probe. Two identical 128-element linear arrays were simulated, one equipped with an acoustic lens focused at 20 mm and the second with a flat lens. Such probes were used to estimate the velocities of laminar flows located at different depths. The results show that the velocity profiles estimated for the array with the focused lens are accurate at the focal distance, but poor at greater depths. Conversely, estimations based on the array with a flat lens are less depth-dependent. For example, at 100 mm depth, the velocity estimates based on the unfocused lens are ≈18% more accurate. The shapes of the simulated one-way fields on the elevation plane highlight that the beam associated to the flat lens is significantly more uniform than that obtained with the focused lens. These results suggest that the lens should be properly designed when targeting accurate blood flow estimates over an extended range of interest.