A noninvasive method to estimate wall shear rate using ultrasound

Abstract

Wall shear stress (blood viscosity × wall shear rate), imposed by the flowing blood, and blood pressure are the main mechanical forces acting on a blood vessel wall. Accurate measurement of wall shear stress is important when investigating the development of vascular disease, since both high and low wall shear stresses have been cited as factors leading to vessel wall anomalies. Furthermore, in vitro studies have shown that endothelial cells, which play a key role in the function of the underlying arterial wall, undergo a variety of structural and functional changes in response to imposed shear stress. However, there is practically no knowledge about the influence of wall shear stress on the arterial wall in vivo because of the difficulty in measuring this stress in terms of magnitude and time variation. The method presented in this article to measure the time-dependent wall shear rate in the main arteries is based on the evaluation of velocity profiles determined by means of ultrasound, using off-line signal processing. Pulsed ultrasound is well suited for this application since it is noninvasive. The processing performed in the radio-frequency (RF) domain consists of a mean frequency estimator preceded by an adaptive vessel wall filter. In a pilot study (30 measurements in the carotid artery of five healthy volunteers) we investigated the reproducibility of our method to estimate wall shear rate as compared with the reproducibility of the measurement of blood flow velocity in the middle of the vessel. The coefficient of variation was on the order of 9% for blood flow velocity estimation, and for wall shear rate estimation on the order of 5%.