P2.28 Development and Validation in Flow-Phantoms of a Simple Ultrasound-Based Method for Estimation of Wall Shear Stress in-Vivo

Background Wall shear stress (WSS) is an important measure of endothelial function however there are few clinical studies due to lack of a widely available measurement technique. Aim To develop and validate a simplified technique for estimation of WSS. Methods The Womersley equations were used; these describe pulsatile blood flow. With input of the vessel diameter (from B-mode ultrasound) and the centre-line blood velocity waveform (from Doppler ultrasound), the equations provided velocity profile as a function of time. Wall shear rate was estimated from the velocity gradient at the vessel wall. WSS was estimated from WSR*(viscosity), with an assumed viscosity of 4mPas. The technique was validated in a pulsatile flow phantom for vessels of physiological depth, diameter and flow-waveform. Results Estimated mean WSS was in error by 9±1% for brachial, 7±1% for carotid, 22±4% for femoral and 17±10% for fetal aorta. Discussion The errors are comparable with those obtained using dedicated WSS measuring systems. The method assumes that the vessel is rigid, straight and that flow is fully developed. There are several arteries where these flow conditions might hold, in health and in early disease where lumen diameter is preserved through outward remodelling. In-vivo validation is needed, possibly against a ‘gold-standard’ of MRI and computational fluid dynamics. Conclusion A simple method for estimation of WSS has been developed which is suitable for clinical studies. Acknowledgement The work has been published (Ultrasound Med Biol 2008, 34, 760-774) and previously presented (British Medical Ultrasound Society meeting, UK, December 2007)