Echo Particle Image Velocimetry Research Articles

Validation of ultrasound velocimetry and computational fluid dynamics for flow assessment in femoral artery stenotic disease.

To investigate the accuracy of high-framerate echo particle image velocimetry (ePIV) and computational fluid dynamics (CFD) for determining velocity vectors in femoral bifurcation models through comparison with optical particle image velocimetry (oPIV). Separate femoral bifurcation models were built for oPIV and ePIV measurements of a non-stenosed (control) and a 75%-area stenosed common femoral artery. A flow loop was used to create triphasic pulsatile flow. In-plane velocity vectors were measured with oPIV and ePIV. Flow was simulated with CFD using boundary conditions from ePIV and additional duplex-ultrasound (DUS) measurements. Mean differences and 95%-limits of agreement (1.96*SD) of the velocity magnitudes in space and time were compared, and the similarity of vector complexity (VC) and time-averaged wall shear stress (TAWSS) was assessed. Similar flow features were observed between modalities with velocities up to 110 and in the control and the stenosed model, respectively. Relative to oPIV, ePIV and CFD-ePIV showed negligible mean differences in velocity (), with limits of agreement of (control) and (stenosed). CFD-DUS overestimated velocities with limits of agreements of and for the control and stenosed model, respectively. VC showed good agreement, whereas TAWSS showed similar trends but with higher values for ePIV, CFD-DUS, and CFD-ePIV compared to oPIV. EPIV and CFD-ePIV can accurately measure complex flow features in the femoral bifurcation and around a stenosis. CFD-DUS showed larger deviations in velocities making it a less robust technique for hemodynamical assessment. The applied ePIV and CFD techniques enable two- and three-dimensional assessment of local hemodynamics with high spatiotemporal resolution and thereby overcome key limitations of current clinical modalities making them an attractive and cost-effective alternative for hemodynamical assessment in clinical practice.

The Effect of Spatial Velocity Gradients on Block-Matching Accuracy for Ultrasound Velocimetry

Block matching serves as the foundation for ultrasound velocimetry techniques such as blood speckle tracking and echo-particle image velocimetry. Any spatial velocity gradients (SVGs) inside a block-matching pair will result in tracking error, due to both the finite block size and the ultrasound point-spread-function. We assess, using an in silico sinusoidal flow phantom, the effect of SVG magnitude and beam-to-flow angle on block-matching bias and precision. Secondarily we assess the effect that SVGs have on velocimetry bias when using angled plane-wave compounding. The magnitude and angle of SVGs were varied by adjusting the wavelength and direction of a sinusoidal flow profile. Scatterers displaced by this flow profile were used for simulating ultrasound radio frequency data at discrete time points. After beamforming, the 2-D flow field was estimated using block matching. Two imaging sequences were tested, a single plane-wave and a three-angled plane-wave. Smaller sinusoidal flow wavelengths resulted in increased bias and reduced precision, revealing an inverse relationship between sinusoidal flow wavelength and tracking error, with median errors ranging from 69%-90% for the smallest flow wavelengths (highest SVGs) down to 3%-5% for the largest (lowest SVGs). The SVG angle was also important, in which lateral SVGs (with axially oriented flows) resulted in significant speckle decorrelation and high tracking errors in regions with high SVGs. Conversely, axial SVGs (laterally oriented flow) experienced higher bias in the peak velocity regions of the flow profile. Coherent compounding resulted in higher velocity errors than using a single transmission for lateral SVGs but not for axial SVGs. The highest SVGs that could be measured with ≤10% error was when the sinusoidal flow wavelength was less than 20 times the ultrasound pulse wavelength. The clinical significance is that the high SVGs present in high kinetic energy flows, such as severe carotid stenosis and aortic regurgitation, will limit the ability to accurately quantify the velocities in these flow structures.

Left ventricular high frame rate echo-particle image velocimetry: clinical application and comparison with conventional imaging

BackgroundEcho-Particle Image Velocimetry (echoPIV) tracks speckle patterns from ultrasound contrast agent(UCA), being less angle-sensitive than colour Doppler. High frame rate (HFR) echoPIV enables tracking of high velocity flow in the left ventricle (LV). We aimed to demonstrate the potential clinical use of HFR echoPIV and investigate the feasibility and accuracy in patients.MethodsNineteen patients admitted for heart failure were included. HFR contrast images were acquired from an apical long axis view (ALAX), using a fully-programmable ultrasound system. A clinical UCA was continuously infused with a dedicated pump. Additionally, echocardiographic images were obtained using a clinical system, including LV contrast-enhanced images and pulsed-wave (PW) Doppler of the LV inflow and outflow in ALAX. 11 patients underwent CMR and 4 cardiac CT as clinically indicated. These CMR and CT images were used as reference. In 10 patients with good echoPIV tracking and reference imaging, the intracavitary flow was compared between echoPIV, conventional and UCA echocardiography.ResultsEchoPIV tracking quality was good in 12/19 (63%), moderate in 2/19 (10%) and poor in 5/19 (26%) subjects. EchoPIV could determine inflow velocity in 17/19 (89%), and outflow in 14/19 (74%) patients. The correlation of echoPIV and PW Doppler was good for the inflow (R2 = 0.77 to PW peak; R2 = 0.80 PW mean velocity) and moderate for the outflow (R2 = 0.54 to PW peak; R2 = 0.44 to PW mean velocity), with a tendency for echoPIV to underestimate PW velocities. In selected patients, echoPIV was able in a single acquisition to demonstrate flow patterns which required multiple interrogations with classical echocardiography. Those flow patterns could also be linked to anatomical abnormalities as seen in CMR or CT.ConclusionHFR echoPIV tracks multidirectional and complex flow patterns which are unapparent with conventional echocardiography, while having comparable feasibility. EchoPIV tends to underestimate flow velocities as compared to PW Doppler. It has the potential to provide in one acquisition all the functional information obtained by conventional imaging, overcoming the angle dependency of Doppler and low frame rate of classical contrast imaging.

Left ventricular high frame rate echo-particle image velocimetry: feasibility and comparison with conventional echocardiography

Abstract Funding Acknowledgements Type of funding sources: None. Introduction Echo-Particle Image Velocimetry (echoPIV) tracks speckle patterns from ultrasound contrast agent (UCA) microbubbles injected intravenously, being less angle-sensitive than colour Doppler. One limitation of conventional echoPIV is the inability to accurately resolve high velocities, because of relatively low frame rates. In contrast, high frame rate (HFR) echoPIV enables tracking of fast flow in the left ventricle (LV). Purpose To investigate the feasibility and precision of HFR echoPIV in patients. Methods 19 heart failure patients were included. UCA was infused with a dedicated pump. HFR contrast images were acquired, in apical long axis view (ALAX, ensuring simultaneous visualization of LV inflow and outflow), using a fully-programmable research ultrasound system, with a phased array probe. In the same session, complete echocardiographic studies were obtained using a clinical ultrasound system, with a matrix array probe, including LV UCA. Non-contrast pulsed-wave (PW) Doppler were also obtained in ALAX (Figure 1) from the mitral valve tips (inflow) and the LV outflow tract (outflow). HFR echoPIV image quality and tracking were assessed offline by two independent observers. The peak velocity of the inflow and outflow were determined by the automated tracking algorithm of the HFR echoPIV, and measured by the peak modal velocity of the conventional PW. These velocities were compared using Pearson’s correlations and Bland-Altman plots. All patients gave their informed consent. The study was approved by the institutional review board. Results Conventional echo image quality was good in 12 (63%), medium in 5 (26%) and bad in 2 (11%). EchoPIV tracking was good in 12 (63%), medium in 2 (10%) and bad in 5 (26%). In the 12 patients where echoPIV tracking was good, the direction and velocity of intracavitary vortices could be visualized (Figure 1). The inflow velocity could be determined by echoPIV in 17/19 (89%), and outflow in 14/19 (74%). EchoPIV tended to underestimate the maximal velocity as determined by PW (Figure 2), with a bias of 0.19 m/s (inflow) and 0.28 m/s (outflow). This negative bias is expected as the PW is assessing maximum velocity in the interrogation kernel whereas echoPIV returns the mean velocity. The correlation of the two methods was good for the inflow (R2 = 0.77, p < 0.001) and moderate for the outflow (R2 = 0.54, p < 0.001). This may be explained by the position of the LV outflow tract deeper in the image, leading to increased attenuation, clutter and reduced lateral resolution. Conclusion HFR echoPIV has comparable feasibility to routine echocardiography, and the ability to correctly estimate intraventricular flow velocity. It can provide in one acquisition all the functional information that can be detected by routine echocardiography, PW and color Doppler, as well as contrast. It succeeds in surpassing the shortcomings of Doppler (angle dependency) and classical contrast imaging (low frame rate). Abstract Figure 1: HFR echoPIV in study patients Abstract Figure 2: study results

Novel Transcatheter Mitral Prosthesis Designed to Preserve Physiological Ventricular Flow Dynamics

BackgroundCurrent mitral bioprostheses are akin to the aortic valve and therefore abolish the left ventricular (LV) physiological vortex. We evaluated the hemodynamic performance and the effects on intraventricular flow dynamics (IFD) of a novel mitral bioprosthesis that presents an innovative design mimicking the native valve. MethodsA D-shaped self-expandable stent-bovine pericardium monoleaflet valve was designed to provide physiological asymmetric intraventricular flow. Transapical implantation was consecutively performed in 12 juvenile sheep. Postimplant studies using Doppler echocardiography and IFD using echo particle imaging velocimetry were obtained immediately after the implantation and at 3 months to assess the hemodynamic performance of the prostheses. ResultsThere were 3 deaths during follow-up, 1 due to valve misplacement because of poor imaging visualization and 2 not valve related. The mean transvalvular gradient and effective orifice area were 2.2 ± 1.2 mm Hg and 4.0 ± 1.1 cm2 after implantation and 3.3 ± 1.5 mm Hg and 3.5 ± 0.5 cm2 at 3 months, respectively. LV vortex dimension, orientation, and physiological anticlockwise rotation were preserved compared with preoperative normal LV flow pattern. One animal showed a moderate paravalvular leak, others mild or none. LV outflow tract obstruction, valve thrombosis, and hemolysis were not observed. ConclusionsOur preclinical in vivo results confirm the good hemodynamic performance of this new transcatheter bioprosthesis with preservation of the physiological IFD. Clinical studies are needed to document whether these characteristics will foster LV recovery and improve the clinical outcome of patients with mitral regurgitation.

Optimization of Microbubble Concentration and Acoustic Pressure for Left Ventricular High-Frame-Rate EchoPIV in Patients.

High-frame-rate (HFR) echo-particle image velocimetry (echoPIV) is a promising tool for measuring intracardiac blood flow dynamics. In this study, we investigate the optimal ultrasound contrast agent (UCA: SonoVue) infusion rate and acoustic output to use for HFR echoPIV (PRF = 4900 Hz) in the left ventricle (LV) of patients. Three infusion rates (0.3, 0.6, and 1.2 ml/min) and five acoustic output amplitudes (by varying transmit voltage: 5, 10, 15, 20, and 30 V-corresponding to mechanical indices of 0.01, 0.02, 0.03, 0.04, and 0.06 at 60-mm depth) were tested in 20 patients admitted for symptoms of heart failure. We assess the accuracy of HFR echoPIV against pulsed-wave Doppler acquisitions obtained for mitral inflow and aortic outflow. In terms of image quality, the 1.2-ml/min infusion rate provided the highest contrast-to-background ratio (CBR) (3-dB improvement over 0.3 ml/min). The highest acoustic output tested resulted in the lowest CBR. Increased acoustic output also resulted in increased microbubble disruption. For the echoPIV results, the 1.2-ml/min infusion rate provided the best vector quality and accuracy; mid-range acoustic outputs (corresponding to 15-20-V transmit voltages) provided the best agreement with the pulsed-wave Doppler. Overall, the highest infusion rate (1.2 ml/min) and mid-range acoustic output amplitudes provided the best image quality and echoPIV results.

In the Eye of the Storm: Echocardiographic Particle Image Velocimetry Analysis in a Patient with Takotsubo Syndrome.

Echo particle imaging velocimetry (Echo PIV) is a contrast-echo-based technique, used to evaluate the instantaneous vortical blood motion into the left ventricle (LV). Here, we report, for the first time, echo-PIV findings in a patient with Takotsubo syndrome (TTS). Vortex behavior suggested that TTS might present with peculiar PIV characteristics, including relatively preserved intra-ventricular pressure gradient and energy dissipation. Further studies are needed to elucidate whether the preservation of a more physiological vortex behavior could be related to the structural and functional recovery observed in TTS.

4-D Echo-Particle Image Velocimetry in a Left Ventricular Phantom

Left ventricular (LV) blood flow is an inherently complex time-varying 3-D phenomenon, where 2-D quantification often ignores the effect of out-of-plane motion. In this study, we describe high frame rate 4-D echocardiographic particle image velocimetry (echo-PIV) using a prototype matrix transesophageal transducer and a dynamic LV phantom for testing the accuracy of echo-PIV in the presence of complex flow patterns. Optical time-resolved tomographic PIV (tomo-PIV) was used as a reference standard for comparison. Echo-PIV and tomo-PIV agreed on the general profile of the LV flow patterns, but echo-PIV smoothed out the smaller flow structures. Echo-PIV also underestimated the flow rates at greater imaging depths, where the PIV kernel size and transducer point spread function were large relative to the velocity gradients. We demonstrate that 4-D echo-PIV could be performed in just four heart cycles, which would require only a short breath-hold, providing promising results. However, methods for resolving high velocity gradients in regions of poor spatial resolution are required before clinical translation.

Optimization of 3-D Divergence-Free Flow Field Reconstruction Using 2-D Ultrasound Vector Flow Imaging

3-D blood vector flow imaging is of great value in understanding and detecting cardiovascular diseases. Currently, 3-D ultrasound vector flow imaging requires 2-D matrix probes, which are expensive and suffer from suboptimal image quality. Our recent study proposed an interpolation algorithm to obtain a divergence-free reconstruction of the 3-D flow field from 2-D velocities obtained by high-frame-rate ultrasound particle imaging velocimetry (High Frame Rate echo-Particle Imaging Velocimetry, also known as HFR Ultrasound Imaging Velocimetry (UIV)), using a 1-D array transducer. The aim of this work was to significantly improve the accuracy and reduce the time-to-solution of our previous approach, thereby paving the way for clinical translation. More specifically, accuracy was improved by optimising the divergence-free basis to reduce Runge phenomena near domain boundaries, and time-to-solution was reduced by demonstrating that under certain conditions, the resulting system could be solved using widely available and highly optimised generalised minimum residual algorithms. To initially illustrate the utility of the approach, coarse 2-D subsamplings of an analytical unsteady Womersely flow solution and a steady helical flow solution obtained using computational fluid dynamics were used successfully to reconstruct full flow solutions, with 0.82% and 4.8% average relative errors in the velocity field, respectively. Subsequently, multiplane 2-D velocity fields were obtained through HFR UIV for a straight-tube phantom and a carotid bifurcation phantom, from which full 3-D flow fields were reconstructed. These were then compared with flow fields obtained via computational fluid dynamics in each of the two configurations, and average relative errors of 6.01% and 12.8% in the velocity field were obtained. These results reflect 15%–75% improvements in accuracy and 53- to 874-fold acceleration of reconstruction speeds for the four cases, compared with the previous divergence-free flow reconstruction method. In conclusion, the proposed method provides an effective and fast method to reconstruct 3-D flow in arteries using a 1-D array transducer.

Combined flow-based imaging assessment of optimal cardiac resynchronization therapy pacing vector: a\xa0case report

BackgroundThere are still many pendent issues about the effective evaluation of cardiac resynchronization therapy impact on functional mitral regurgitation. In order to reduce the intrinsic difficulties of quantification of functional mitral regurgitation itself, an automatic quantification of real-time three-dimensional full-volume color Doppler transthoracic echocardiography was proposed as a new, rapid, and accurate method for the assessment of functional mitral regurgitation severity. Recent studies suggested that images of left ventricle flow by echo-particle imaging velocimetry could be a useful marker of synchrony. Echo-particle imaging velocimetry has shown that regional anomalies of synchrony/synergy of the left ventricle are related to the alteration, reduction, or suppression of the physiological intracavitary pressure gradients.Case summaryWe describe a case in which the two technologies are used in combination during acute echocardiographic optimization of left pacing vector in a 63-year-old man, Caucasian, who showed worsening heart failure symptoms a few days after an implant, and the effect of the device’s optimization at 6-month follow-up.DiscussionThe degree of realignment of hemodynamic forces, with quantitative analysis of the orientation of blood flow momentum (φ), can represent improvement of fluid dynamics synchrony of the left ventricle, and explain, with a new deterministic parameter, the effects of cardiac resynchronization therapy on functional mitral regurgitation. Real-time three-dimensional color flow Doppler quantification is feasible and accurate for measurement of mitral inflow, left ventricular outflow stroke volumes, and functional mitral regurgitation severity.ConclusionThis clinical case offers an innovative and accurate approach for acute echocardiographic optimization of left pacing vector. It shows clinical utility of combined three-dimensional full-volume color Doppler transthoracic echocardiography/echo-particle imaging velocimetry assessment to increase response to cardiac resynchronization therapy, in terms of reduction of functional mitral regurgitation, improving fluid dynamics synchrony of the left ventricle.

High-Frame-Rate Contrast-enhanced US Particle Image Velocimetry in the Abdominal Aorta: First Human Results.

Purpose To study the feasibility of high-frame-rate (HFR) contrast material-enhanced (CE) ultrasound particle image velocimetry (PIV), or echo PIV, in the abdominal aorta. Materials and Methods Fifteen healthy participants (six men; median age, 23 years [age range, 18-34 years]; median body mass index, 20.3 kg/m2 [range, 17.3-24.9 kg/m2]) underwent HFR CE US. US microbubbles were injected at incremental doses (0.25, 0.5, 0.75, and 1.5 mL), with each dose followed by US measurement to determine the optimal dosage. Different US mechanical index values were evaluated (0.09, 0.06, 0.03, and 0.01) in a diverging wave acquisition scheme. PIV analysis was performed via pairwise cross-correlation of all captured images. Participants also underwent phase-contrast MRI. The echo PIV and phase-contrast MRI velocity profiles were compared via calculation of similarity index and relative difference in peak velocity. Results Visualization of the aortic bifurcation with HFR CE US was successful in all participants. Optimal echo PIV results were achieved with the lowest contrast agent dose of 0.25 mL in combination with the lowest mechanical indexes (0.01 or 0.03). Substantial bubble destruction occurred at higher mechanical indexes (≥0.06). Flow patterns were qualitatively similar in the echo PIV and MR images. The echo PIV and MRI velocity profiles showed good agreement (similarity index, 0.98 and 0.99; difference in peak velocity, 8.5% and 17.0% in temporal and spatial profiles, respectively). Conclusion Quantification of blood flow in the human abdominal aorta with US particle image velocimetry (echo PIV) is feasible. Use of echo PIV has potential in the clinical evaluation of aortic disease. © RSNA, 2018 Online supplemental material is available for this article.

High-Frame-Rate Contrast-Enhanced Ultrasound for Velocimetry in the Human Abdominal Aorta.

Treatment of abdominal aortic (AA) aneurysms and stenotic lesions may be improved by analyzing their associated blood-flow patterns. Angle-independent blood-flow patterns in the AA can be obtained by combining echo-particle image velocimetry (ePIV) with high-frame-rate (HFR) contrast-enhanced ultrasonography. However, ePIV performance is affected by ultrasound contrast agent (UCA) concentration, microbubble stability, and tissue clutter. In this study, we assessed the influence of acoustic pressure and UCA concentration on image quality for ePIV analysis. We also compared amplitude modulation (AM) and singular value decomposition (SVD) as tissue suppression strategies for ePIV. Fourteen healthy volunteers were imaged in the region of the distal AA. We tested four different UCA bolus volumes (0.25, 0.5, 0.75, and 1.5 mL) and four different acoustic output pressures (mechanical indices: 0.01, 0.03, 0.06, and 0.09). As image quality metrics, we measured contrast-to-background ratio, bubble disruption ratio, and maximum normalized cross-correlation value during ePIV. At mechanical indices ≥ 0.06, we detected severe bubble destruction, suggesting that very low acoustic pressures should be used for ePIV. SVD was able to suppress tissue clutter better than AM. The maximum tracking correlation was affected by both UCA concentration and flow rate, where at high flow rates, lower UCA concentrations resulted in slightly higher correlation values but more signal drop-outs during late diastole. HFR ePIV was successfully performed in the AA of healthy volunteers and shows promise for future studies in patients.

Measurement of Wall Shear Stress Exerted by Flowing Blood in the Human Carotid Artery: Ultrasound Doppler Velocimetry and Echo Particle Image Velocimetry

Vascular endothelial cells lining the arteries are sensitive to wall shear stress (WSS) exerted by flowing blood. An important component of the pathophysiology of vascular diseases, WSS is commonly estimated by centerline ultrasound Doppler velocimetry (UDV). However, the accuracy of this method is uncertain. We have previously validated the use of a novel, ultrasound-based, particle image velocimetry technique (echo PIV) to compute 2-D velocity vector fields, which can easily be converted into WSS data. We compared WSS data derived from UDV and echo PIV in the common carotid artery of 27 healthy participants. Compared with echo PIV, time-averaged WSS was lower using UDV (28 ± 35%). Echo PIV revealed that this was due to considerable spatiotemporal variation in the flow velocity profile, contrary to the assumption that flow is steady and the velocity profile is parabolic throughout the cardiac cycle. The largest WSS underestimation by UDV was found during peak systole (118 ± 16%) and the smallest during mid-diastole (4.3± 46%). The UDV method underestimated WSS for the accelerating and decelerating systolic measurements (68 ± 30% and 24 ± 51%), whereas WSS was overestimated for end-diastolic measurements (−44 ± 55%). Our data indicate that UDV estimates of WSS provided limited and largely inaccurate information about WSS and that the complex spatiotemporal flow patterns do not fit well with traditional assumptions about blood flow in arteries. Echo PIV-derived WSS provides detailed information about this important but poorly understood stimulus that influences vascular endothelial pathophysiology.

Optimized Time-Resolved Echo Particle Image Velocimetry- Particle Tracking Velocimetry Measurements Elucidate Blood Flow in Patients With Left Ventricular Thrombus.

Contrast ultrasound is a widely used clinical tool to obtain real-time qualitative blood flow assessments in the heart, liver, etc. Echocardiographic particle image velocimetry (echo-PIV) is a technique for obtaining quantitative velocity maps from contrast ultrasound images. However, unlike optical particle image velocimetry (PIV), routine echo images are prone to nonuniform spatiotemporal variations in tracer distribution, making analysis difficult for standard PIV algorithms. This study introduces optimized procedures that integrate image enhancement, PIV, and particle tracking velocimetry (PTV) to obtain reliable time-resolved two-dimensional (2D) velocity distributions. During initial PIV analysis, multiple results are obtained by varying processing parameters. Optimization involving outlier removal and smoothing is used to select the correct vector. These results are used in a multiparameter PTV procedure. To demonstrate their clinical value, the procedures are implemented to obtain velocity and vorticity distributions over multiple cardiac cycles using images acquired from four left ventricular thrombus (LVT) patients. Phase-averaged data elucidate flow structure evolution over the cycle and are used to calculate penetration depth and strength of left ventricular (LV) vortices, as well as apical velocity induced by them. The present data are consistent with previous time-averaged results for the minimum vortex penetration depth associated with LVT occurrence. However, due to decay and fragmentation of LV vortices, as they migrate away from the mitral annulus, in two cases with high penetration, there is still poor washing near the resolved clot throughout the cycle. Hence, direct examination of entire flow evolution may be useful for assessing risk of LVT relapse before prescribing anticoagulants.

High Frame Rate Ultrasound Particle Image Velocimetry for Estimating High Velocity Flow Patterns in the Left Ventricle.

Echocardiographic determination of multicomponent blood flow dynamics in the left ventricle remains a challenge. In this paper, we compare contrast enhanced, high frame rate (HFR) (1000 frames/s) echo-particle image velocimetry (ePIV) against optical particle image velocimetry (oPIV, gold standard), in a realistic left ventricular (LV) phantom. We find that ePIV compares well to oPIV, even for the high velocity inflow jet (normalized RMSE = 9% ± 1%). In addition, we perform the method of proper orthogonal decomposition, to better qualify and quantify the differences between the two modalities. We show that ePIV and oPIV resolve very similar flow structures, especially for the lowest order mode with a cosine similarity index of 86%. The coarser resolution of ePIV does result in increased variance and blurring of smaller flow structures when compared to oPIV. However, both modalities are in good agreement with each other for the modes that constitute the bulk of the kinetic energy. We conclude that HFR ePIV can accurately estimate the high velocity diastolic inflow jet and the high energy flow structures in an LV setting.

Echo Particle Image Velocimetry for Estimation of Carotid Artery Wall Shear Stress: Repeatability, Reproducibility and Comparison with Phase-Contrast Magnetic Resonance Imaging

Measurement of hemodynamic wall shear stress (WSS) is important in investigating the role of WSS in the initiation and progression of atherosclerosis. Echo particle image velocimetry (echo PIV) is a novel ultrasound-based technique for measuring WSS in vivo that has previously been validated in vitro using the standard optical PIV technique. We evaluated the repeatability and reproducibility of echo PIV for measuring WSS in the human common carotid artery. We measured WSS in 28 healthy participants (18 males and 10 females, mean age: 56 ± 12 y). Echo PIV was highly repeatable, with an intra-observer variability of 1.0 ± 0.1 dyn/cm2 for peak systolic (maximum), 0.9 dyn/cm2 for mean and 0.5 dyn/cm2 for end-diastolic (minimum) WSS measurements. Likewise, echo PIV was reproducible, with a low inter-observer variability (max: 2.0 ± 0.2 dyn/cm2, mean: 1.3 ± 0.1 dyn/cm2, end-diastolic: 0.7 dyn/cm2) and more variable inter-scan (test–retest) variability (max: 7.1 ± 2.3 dyn/cm2, mean: 2.9 ± 0.4 dyn/cm2, min: 1.5 ± 0.1 dyn/cm2). We compared echo PIV with the reference method, phase-contrast magnetic resonance imaging (PC-MRI); echo PIV-based WSS measurements agreed qualitatively with PC-MRI measurements (r = 0.89, p < 0.05). Significant differences were observed in some WSS measurements (echo PIV vs. PC-MRI): WSS at peak systole: 21 ± 7.0 dyn/cm2 vs. 15 ± 5.0 dyn/cm2; time-averaged WSS: 8.9 ± 3.0 dyn/cm2 vs. 7.1 ± 3.0 dyn/cm2 (p < 0.05); WSS at end diastole: 3.8 ± 2.8 dyn/cm2 vs. 3.9 ± 2 dyn/cm2 (p > 0.05). For the first time, we report that echo PIV can measure WSS with good repeatability and reproducibility in adult humans with a broad age range. Echo PIV is feasible in humans and offers an easy-to-use, ultrasound-based, quantitative technique for measuring WSS in vivo in humans with good repeatability and reproducibility.

In vitro post-stenotic flow quantification and validation using echo particle image velocimetry (Echo PIV)

Echo particle image velocimetry (Echo PIV) presents itself as an attractive in vivo flow quantification technique to traditional approaches. Promising results have been acquired; however, limited quantification and validation is available for post-stenotic flows. We focus here on the comprehensive evaluation of in vitro downstream stenotic flow quantified by Echo PIV and validated in relation to digital particle image velocimetry (DPIV). A Newtonian blood analog was circulated through a closed flow loop and quantified immediately downstream of a 50 % axisymmetric blockage at two Reynolds numbers (Re) using time-averaged Echo PIV and DPIV. Centerline velocities were in good agreement at all Re; however, Echo PIV measurements presented with elevated standard deviation (SD) at all measurements points. SD was improved using increased line density (LD); however, frame rate or field of view (FOV) is compromised. Radial velocity profiles showed close agreement with DPIV with the largest disparity in the shear layer and near-wall recirculation. Downstream recirculation zones were resolved by Echo PIV at both Re; however, magnitude and spatial coverage was reduced compared to DPIV that coincided with reduced contrast agent penetration beyond the shear layer. Our findings support the use of increased LD at a cost to FOV and highlight reduced microbubble penetration beyond the shear layer. High local SD at near-wall measurements suggests that further refinement is required before proceeding to in vivo quantification studies of wall shear stress in complex flow environments.

Quantitative analysis of intraventricular blood flow dynamics by echocardiographic particle image velocimetry in patients with acute myocardial infarction at different stages of left ventricular dysfunction.

Left ventricular (LV) diastolic filling is characterized by the formation of a vortex that supports an efficient transit into systolic ejection. Aim of this study was to assess the intraventricular (IV) blood flow dynamics among patients with ST elevated myocardial infarction (STEMI) at different degrees of LV dysfunction, in the attempt to find novel indicators of LV pump efficiency. Sixty-four subjects, 34 consecutive STEMI patients and 30 healthy controls, underwent before hospital discharge 2D speckle tracking echocardiography to assess global longitudinal strain (GLS), and echo-particle image velocimetry analysis to assess flow energetic parameters. Left ventricular volumes ejection fraction (LVEF) and global wall motion score index (GWMSI) were evaluated by 3D echocardiography. ST elevated myocardial infarction patients were subdivided into three groups according to LVEF. Energy dissipation, vorticity fluctuation, and kinetic energy fluctuation indexes, which characterize the degree of disturbance in the flow, exhibit a biphasic behaviour in STEMI patients when compared with controls, with the highest values in patients with still preserved LV function and progressive lower values with LV function worsening. Significant linear correlations were found between energy dissipation index and both LVEF and GLS (r = 0.57, P < 0.001 and r = -0.61, P = 0.001, respectively). Kinetic energy fluctuation index significantly correlates with both LVEF (r = 0.75, P < 0.001) and GLS (-0.58, P = 0.002). Finally, a significant correlation was observed between GWMSI and energy dissipation index (-0.56, P = 0.008). The present study describes, for the first time, the progression of IV flow energetic properties in patients with acute myocardial infarction at different stages of LV dysfunction when compared with healthy controls. Further data are needed to assess the role of these parameters in the development and maintenance of LV dysfunction.

Abstract 246: Echo Particle Image Velocimetry Generates Wall Shear Stress Biomarkers with Higher Accuracy than Doppler: Clinical Studies on 24 Healthy and 12 Tia Subjects

Wall shear stress (WSS) is an important modulator of vascular disease. We assessed the hypothesis that WSS profiles derived from Echo Particle Image Velocimetry (EchoPIV) are characteristic of vascular conditions such as TIA with superior accuracy to Doppler. We obtained ultrasound images of CCA from 24 healthy and 12 TIA subjects. Doppler provided centerline peak velocity and WSS estimate based on parabolic velocity distribution. Velocity profiles measured by EchoPIV varied from patient to patient and within the cardiac cycle. Curve-fitting to a power law function with an exponent term (n-value) showed that in healthy carotid arteries, a blunt velocity profile is present during systole with an average n-value of 5, whereas during diastole a more parabolic profile with an n-value of 2 is present. Therefore, assuming a fully parabolic velocity distribution for WSS estimation, the basis for Doppler velocimetry, introduces a mean error of 48% during diastole and 98% during systole. EchoPIV offers characterization of WSS profiles; time waveform analysis revealed smoother and fewer number of peaks per cycle for healthy and increased flow pulsatality (reverberations) for TIA (Figure). Echo PIV generated means for phase averaged peak velocity (27.95 vs 16.13 cm/s), WSS (6.45 vs 4.65 Dynes/cm2) and flow rate (0.3179 vs 0.2215 L/min) with standard deviations 3.66 vs 2.35, 2.8 vs 1.74 and 0.0723 vs 0.0744 were significantly different between healthy and TIA subjects respectively (all p &lt; 0.01). Direct measurement of the velocity profile using Echo PIV provides superior estimates of WSS in carotid vessels. TIA populations have compromised hemodynamic variables compared to healthy subjects.

Plane wave echo particle image velocimetry

This paper deals with the application of topological imaging to ultrasonic echo-particle image velocimetry (Echo-PIV). Echo-PIV is a recent alternative to optical PIV for measuring the instantaneous velocity field of a fluid flow previously seeded with small particles. It consists in imaging the flow with a ultrasonic array at a high frame rate. Topological imaging is a method that benefits from the refocusing properties of the time-reversal principle in a systematic way, so that a single plane wave illumination of the medium leads to a fine resolution. Multiple insonifications are then possible at very high speed allowing not only static images of the medium but successive images of a moving medium. Experimental results are presented for a fluid seeded with stone powder. Two cases are studied: a vortex flow and the propagation of water surface waves.