Wall shear stress (WSS) is an important determinant of vascular endothelial function and vascular health. However, the in vivo measurement of WSS is, at present, difficult or impractical to perform. Echo Particle Image Velocimetry (Echo PIV) is a novel ultrasound-based technique capable of measuring detailed blood flow and hemodynamic information with high temporal and excellent spatial resolution. Echo PIV has been validated in vitro but its ability to measure WSS in vivo has not been established. PURPOSE: The purpose of this study was to compare WSS obtained by Echo PIV and by phase-contrast MRI in the common carotid artery of human subjects. METHODS: Right common carotid artery blood flow and hemodynamics were determined in 30 healthy volunteers (50.2±12.9 yrs, 17 men) using Echo PIV and MRI on separate occasions. Echo PIV is the method that combines conventional B-mode ultrasound (Sonix RP, Ultrasonix Medical Corporation, Canada), and contrast agent (SonoVue, Bracco Diagnostics, Italy) to generate 2-D velocity vector maps. Data obtained from 7-10 cardiac cycles were ensemble averaged using concurrent ECG, and were processed offline using a custom-designed program. RESULTS: Peak WSS (mean±SD, dyne/cm2) for Echo PIV and for MRI were 13.7±3.4 and 15.3±5.0, respectively. Mean WSS (averaged over the cardiac cycle) was 7.2±1.7 and 8.1±2.8 for Echo PIV and for MRI, respectively. The correlation coefficients between Echo PIV and MRI in peak WSS and mean WSS were 0.71 and 0.76, respectively (p<0.05). CONCLUSION: The association between WSS measured using Echo PIV and MRI was reasonable. Inherent differences between Echo PIV and MRI measurements may account for at least some of the discrepancies between the two methods, e.g. low temporal and spatial resolution of MRI. We conclude that Echo PIV may be a feasible method for the in vivo assessment of WSS in human arteries. Further studies are planned to determine the utility of Echo PIV for measuring WSS in other vascular beds, such as the brachial and femoral arteries. Supported by grants from NSF (CTS-0421461) and NIH (HL 67393 & 072738).