Native blood speckle vs ultrasound contrast agent for particle image velocimetry with ultrafast ultrasound - in vitro experiments

Ultrafast contrast enhanced ultrasound, combined with echo particle image velocimetry (ePIV), can provide accurate, multidimensional hemodynamic flow field measurement. However, the use of ultrasound contrast agent (UCA) still prevents this method from becoming a truly versatile and non-invasive diagnostic tool. In this study, we investigate the use of native blood instead of UCA backscatter for ePIV measurements and compare their accuracy in vitro. Additionally, the effect of measurement depth is experimentally assessed. Blood mimicking fluid (BMF) was pumped through a 10 mm diameter tube producing parabolic flow profiles, adding UCA in the case of contrast imaging. Plane wave imaging at 5000 frames-per-second was performed with a Verasonics Vantage system and a linear array. The tube was imaged at three different depths: 25, 50 and 100 mm. Singular value decomposition (SVD) was assessed for clutter suppression against mean background subtraction. PIVlab was used as a PIV implementation. With SVD, BMF provided almost equal ePIV accuracy as UCA, except at 100 mm depth where UCA provided better accuracy. Use of clutter suppression greatly improved ePIV results, but minimal differences in ePIV accuracy were noted between mean and SVD filtered groups (BMF or UCA). Accuracy decreased with increasing depth, likely due to reduced elevation resolution, resulting in out-of-plane smoothing of velocity gradients.