In vitro and preliminary in vivo validation of echo particle image velocimetry in carotid vascular imaging

Abstract

Noninvasive, easy-to-use and accurate measurements of wall shear stress (WSS) in human blood vessels have always been challenging in clinical applications. Echo particle image velocimetry (Echo PIV) has shown promise for clinical measurements of local hemodynamics and wall shear rate. Thus far, however, the method has only been validated under simple flow conditions. In this study, we validated Echo PIV under in vitro and in vivo conditions. For in vitro validation, we used an anatomically correct, compliant carotid bifurcation flow phantom with pulsatile flow conditions, using optical particle image velocimetry (optical PIV) as the reference standard. For in vivo validation, we compared Echo PIV-derived 2-D velocity fields obtained at the carotid bifurcation in five normal subjects against phase-contrast magnetic resonance imaging (PC-MRI)-derived velocity measurements obtained at the same locations. For both studies, time-dependent, 2-D, two-component velocity vectors; peak/centerline velocity, flow rate and wall shear rate (WSR) waveforms at the common carotid artery (CCA), carotid bifurcation and distal internal carotid artery (ICA) were examined. Linear regression, correlation analysis and Bland-Altman analysis were used to quantify the agreement of different waveforms measured by the two techniques. In vitro results showed that Echo PIV produced good images of time-dependent velocity vector maps over the cardiac cycle with excellent temporal (up to 0.7 ms) and spatial (∼0.5 mm) resolutions and quality, comparable with optical PIV results. Further, good agreement was found between Echo PIV and optical PIV results for velocity and WSR measurements. In vivo results also showed good agreement between Echo PIV velocities and phase contrast MRI velocities. We conclude that Echo PIV provides accurate velocity vector and WSR measurements in the carotid bifurcation and has significant potential as a clinical tool for cardiovascular hemodynamics evaluation.

Figure 1

Schematic of the human carotid artery model showing the nomenclature used in the text and measurement positions. (1) CCA- common carotid artery; (2) carotid bifurcation; (3) ICA-p – proximal internal carotid artery; ICA-d – distal internal carotid artery.

Figure 2

Schematic of the experimental setup. Red arrows represent the flow direction.

Figure 3

Three-dimensional plot of axial flow velocity in the CCA vs. radius and time: (a) optical PIV measurement and (b) Echo PIV measurement.

Figure 4

Comparison of peak velocity measurements in the CCA by Echo and optical PIV: (A) peak velocity vs. time; (B) correlation analysis; (C) Bland-Altman analysis. The error bars in (A) represent the standard deviations of Echo PIV measurement from multiple cardiac cycles. This is the same for all subsequent plots.

Figure 5

Comparison of centerline velocity measurements at the bifurcation (at plane 2 in Figure 1) by Echo and optical PIV: (A) centerline velocity vs. time; (B) correlation analysis; (C) Bland-Altman analysis.

Figure 6

Comparison of peak velocity measurements in ICA by Echo and optical PIV: (A) peak velocity vs. time; (B) correlation analysis; (C) Bland-Altman analysis.

Figure 7

Radial velocity profiles at plane 1 (Fig. 1) at eight time points during one cardiac cycle, measured by both Echo (solid line) and optical PIV (dash line). The x axis represents radial position (in mm), with negative values representing radial locations toward the near wall and positive toward the far wall; the y axis represents velocity (in cm/s).

Figure 8

Comparison of flow rate measurement in the CCA by Echo PIV and optical PIV: (A) flow rate versus time; (B) correlation analysis; (C) Bland-Altman analysis.

Figure 9

Velocity vector and streamline plots in the carotid bifurcation region at different time points during one cardiac cycle: the plots in each part are optical streamline, echo streamline, optical velocity vector and echo velocity vector. Note that a phase of 360 degrees corresponds here to a cardiac cycle of 0.8 s. For visualization purposes, the density of velocity vectors in longitudinal direction was reduced by 2.

Figure 10

Wall shear rate measurements by both Echo and optical PIV in the CCA: (A) at the near wall; (B) at the far wall. (a) Time-dependent WSR waveforms measured by Echo and optical PIV, error bars denote standard deviations; (b) Linear regression between measurements from the two techniques; (c) Bland-Altman analysis, with dashed lines representing ±2SD deviation from the mean difference. SEE denotes the standard error of the estimate.

Figure 11

Temporal-mean wall shear rate along the carotid sinus wall (from A to B in Figure 1) measured by both Echo and optical PIV.

Figure 12

Microbubble images (A) and velocity vector map (B) from right carotid artery of one subject. The red lines indicate the auto-detected vessel boundaries; colors in velocity vector map denote the velocity magnitude; the streamlines were superimposed on velocity vector map at carotid bulb region.

Figure 13

Three representative comparisons of velocity and flow rate waveforms obtained in human carotid vessels by Echo PIV and PC-MRI.