Role of Ultrasonic Shear Rate Estimation Errors in Assessing Inflammatory Response and Vascular Risk

Atherosclerotic lesions preferentially originate in arterial regions that experience low wall shear stress (WSS) and reversing flow patterns. Therefore, routinely monitoring arterial WSS may help to identify the potential sites of early atherosclerosis. A new noninvasive ultrasonic method implemented with coded excitation techniques was utilized to improve WSS estimation accuracy and precision by providing high spatial and temporal resolution. WSS measurement errors were quantified in a model system by scanning a linearly varying WSS field (0.3 to 1.9 Pa) within a flow chamber. A 13-bit optimal code (Opt) was found to be most effective in reducing bias and standard deviation in WSS estimates down to ∼10% and ∼8%. The measurement errors slowly increased with input WSS for all imaging pulses. The expression of endothelial cellular adhesion molecules vascular cell adhesion molecule-1 (VCAM-1) and endothelial-leukocyte adhesion molecule-1 (E-selectin) was investigated over a similar shear range (0 to 1.6 Pa) to study the impact of relating shear-mediated cellular adhesion molecule (CAM) expression to inaccuracies in WSS measurements. We quantified this influence as the prediction error, which accounts for the ultrasonic measurement errors and the sensitivity of CAM expression within certain shear ranges. The highest prediction errors were observed at WSS <0.8 Pa, where CAM expression is most responsive to WSS. The results emphasize the importance of minimizing estimation errors, especially within low shear regions. Preliminary two-dimensional in vivo shear imaging is also presented to provide information about the spatial heterogeneity in arterial WSS distribution. (E-mail: mfi@uiuc.edu)