Cardiac function and vascular function are closely related to the flow of blood within. The flow velocities in these larger cavities easily reach 1 m/s, and generally complex spatiotemporal flow patterns are involved, especially in a non-physiologic state. Visualization of such flow patterns using ultrasound can be greatly enhanced by administration of contrast agents. Tracking the high-velocity complex flows is challenging with current clinical echographic tools, mostly because of limitations in signal-to-noise ratio; estimation of lateral velocities; and/or frame rate of the contrast-enhanced imaging mode. This review addresses the state of the art in 2-D high-frame-rate contrast-enhanced echography of ventricular and deep-vessel flow, from both technological and clinical perspectives. It concludes that current advanced ultrasound equipment is technologically ready for use in human contrast-enhanced studies, thus potentially leading to identification of the most clinically relevant flow parameters for quantifying cardiac and vascular function.

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 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. High frame rate ePIV was successfully performed in the AA of healthy volunteers and shows promise for future studies in patients.

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 ow 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.

Introduction: To improve carotid artery stenting (CAS), more information about the functioning of the stent is needed. Therefore, a method that can image the flow near and around a stent is required. The aim of this study was to evaluate the performance of high-frame-rate contrast-enhanced ultrasound (HFR CEUS) in the presence of a stent. Methodology: HFR CEUS acquisitions of a carotid artery phantom, a silicone tube with diameter 8 mm, with and without a stent were acquired at transmit voltages of 2V, 4V and 10V using a Verasonics ultrasound system and C5-2 probe. Different concentrations of ultrasound contrast agent (UCA) were tested in a blood mimicking fluid (BMF). Particle image velocimetry (PIV) analysis was performed on Singular Value Decomposition (SVD) filtered images. Mean and peak velocities, and correlation coefficients were compared between stented and non-stented regions. Also, experimental results were compared with theoretical and numerical models. Results: The averaged experimental mean velocity (0.113 m/s) was significant lower than the theoretical and numerical mean velocity (0.129 m/s). The averaged experimental peak velocity (0.152 m/s) was significant lower than the theoretical and numerical peak velocity (0.259 m/s). Correlation coefficients and averaged mean velocity values were lower (difference of 0.022 m/s) in stented regions compared to non-stented regions. Conclusion: In vitro experiments showed an underestimation of mean and peak velocities in stented regions compared to non-stented regions. However, the microbubbles can be tracked efficiently and the expected laminar flow profile can be quantified using HFR CEUS near and around a stent.

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 framesper-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.