Wall shear stress is typically obtained in-vivo by measuring axial velocities close to the vessel lumen and computing the wall shear rate (WSR), after which WSS is obtained by assuming high shear conditions for blood flow and using the Newtonian value for blood viscosity. Although the assumption of Newtonian rheology can be problematic for a shear-thinning fluid such as blood, it has been commonly used for flow in large- and medium-size blood vessels (pre arteriolar) and is generally well-accepted. Certainly, this approach is vastly superior to the current Doppler-based approach of simply measuring centerline velocity and assuming a parabolic velocity profile to calculate WSS (Gelfand et al. 2006
; Gnasso et al. 1996
; Nowak 2002
; Oshinski et al. 2006
). In this regard, phase-contrast magnetic resonance imaging (PC-MRI) has shown promise since it provides WSR accurately (Cheng et al. 2002
; Oyre et al. 1998
; Taylor et al. 2002
). However, PC-MRI is limited for routine WSR or WSS measurements due to relatively long data-collecting times (2~5 minutes for each slice), poor temporal resolution (15~30 msec), inherent cumbersomeness, and high cost.
In recent years, many novel ultrasound-based techniques have emerged to overcome some of the typical Doppler limitations such as angle dependence. Vennemann et al. (Vennemann et al. 2007
) and Hoskins (Hoskins 2010
) reviewed many of these methods. Since the time period covered by these reviews, there has been a few additional methods proposed. Hansen et al. and Udesen et al. (Hansen et al. 2009
; Udesen et al. 2008
) proposed a method called plane wave excitation (PWE), which applies speckle tracking algorithms (Crapper et al. 2000
) to detect 2D speckle displacements from two ultrasound images. The PWE method has been evaluated against phase-contrast MRI in measuring volumetric flow rate in a human carotid artery showing a mean underestimation of the PWE about 9% for flow measurement. One of the big limitations of the PWE method is its degraded contrast of B mode images compared to conventional B mode imaging, which makes it difficult to get reliable velocity measurements. Thus, temporal averaging of 40 images was necessary to improve the quality of velocity mapping, which decreased the temporal resolution to approximately 10 ms. Beulen et al. (Beulen et al. 2010
; Beulen et al. 2010
) proposed a technique termed ultrasonic perpendicular velocimetry (UPV), which applies cross-correlation on raw RF data to detect velocity components perpendicular to the ultrasound beam. The in-vitro validation on UPV against theoretical solutions and computational fluid dynamics (CFD) simulations in straight tube and curved vessels showed good accuracy of this technique. However, UPV uses a relatively large interrogation window size (about 4.4 mm) perpendicular to the beam direction, which limits its spatial resolution. Another limitation of UPV may be the low signal to noise ratio of speckles from red blood cells. The clinical feasibility of UPV remains unclear.
We have been working on the development and improvement of Echo particle image velocimetry (Echo PIV) over the last five years. Echo PIV enables the measurement of multi-dimensional and multi-component velocity vectors in opaque flows (Edmond et al. 1995
; Kim et al. 2004
; Liu et al. 2008
; Zhang et al. 2008
; Zheng et al. 2006
). Advantages of this technique include ease of use, simple implementation using commercially available ultrasound imaging systems and probes, low cost, high temporal resolution (up to 0.7 msec in the current system), and good spatial resolution (up to 0.5 mm in the current system). Echo PIV has shown its capability in quantifying flow patterns in human left ventricles (Hong et al. 2007
; Hong et al. 2007
; Kheradvar et al. 2010
; Sengupta et al. 2007
), however there is no or little validation on those measurements.
Certainly validation of Echo PIV is essential before it is applied clinically. In validation studies using simple in-vitro flow models, Echo PIV was shown to be accurate for velocity and WSR/WSS measurements (Kim et al. 2004
; Liu et al. 2008
; Zheng et al. 2006
). Although the flow models used in these studies were non-physiologic, results from these early validation studies provided confidence to begin considering Echo PIV for clinical measurements, specifically for measuring velocity vectors and WSR in blood vessels. The next step was assessing if Echo PIV could in fact provide accurate results using more anatomically-correct models and flow conditions. This paper presents the results of a study to further validate Echo PIV against the gold-standard of optical PIV using an anatomically-correct, elastic model of carotid bifurcation under pulsatile flow conditions. Before this study some changes were performed to the Echo PIV technique, which are summarized here. First, the Echo PIV algorithm was improved to enhance its accuracy and reliability. A customed RF filtering technique was employed to reduce the noise level of echo particle images, and advanced PIV algorithms, including adaptive window offset, sub-pixel interpolation and vector field filtering were introduced. The details were discussed in another manuscript (Zhang et al. 2010
). Second, this study uses an anatomically-correct carotid compliant model instead of the simplified and non-compliant flow models used in previous studies. The geometrical complexity of the model and its compliance lead to the generation of complicated flow fields in the carotid model, particularly in the bifurcation area. Third, our previous studies on Echo PIV mainly focused on validations of this technique in either temporal or 2D spatial domain. This study, for the first time, validated Echo PIV against Optical PIV in both temporal and 2D spatial domains simultaneously. Fourth, the compliant model in this study provided the opportunity to develop and validate segmentation techniques for tracking artery wall motion (Zhang et al. 2009
). Fifth, the Echo PIV system used in this study was different from those in previous studies. Specifically, the system was improved to provide better control of data acquisition parameters and enhanced temporal resolution (up to 0.7 msec).
With the confidence from in-vitro validation study on carotid artery model, we further explored the feasibility of Echo PIV in human subjects. We used Echo PIV to obtain detailed velocity vectors and flow rates from carotid artery in 5 normal human subjects, and compared these results to phase-contrast MRI (PC-MRI) measurements in the same subjects. Good agreement was found between Echo PIV and PC-MRI measurements.