Improved accuracy of vascular wall shear rate measurements

Wall shear rate (WSR) is the derivative of blood velocity with respect to vessel radius. Low and oscillating WSR has been identified as a necessary factor in increasing endothelial cell (EC) permeability leading to arterial wall remodeling and atherosclerosis. Accurate WSR estimates require both high spatial resolution and high sensitivity to slow-flow velocities. Conventional velocity estimators involve narrowband pulses with autocorrelation estimators or broadband pulses with cross correlation (CC) estimators. Both approaches yield noisy or biased WSR estimates. We propose a time-domain method designed to simultaneously achieve high spatial resolution and high sensitivity for slow flow by transmitting a phase-modulated code to increase echo signal-to-noise ratio (eSNR). Regularized CC estimators are used to estimate velocity and thus WSR. One dimensional simulation results and experimental data from a flow phantom show coded pulses generate the least WSR bias (5%) and lowest estimation variance compared to other uncoded pulses (16% bias for narrowband and 32% bias for broadband) under noise-limited conditions. Implementation of coded excitation techniques make it possible to achieve precise and accurate WSR measurements with higher center frequency transmissions. This may extend the use of noninvasive ultrasound to new areas in the study of atherosclerosis.