Flow Dynamics in Children With Bicuspid Aortic Valve: A Blood Speckle Tracking Study

OBJECTIVE

Bicuspid aortic valve (BAV) is associated with progressive aortic dilation. Although the etiology is complex, altered flow dynamics is thought to play an important role. Blood speckle tracking (BST) allows for visualization and quantification of complex flow, which could be useful in identifying patients at risk of root dilation and could aid in surgical planning. The aims of this study were to assess and quantify flow in the aortic root and left ventricle using BST in children with bicuspid aortic valves.

METHODS AND RESULTS

A total of 38 children <10 y of age were included (24 controls, 14 with BAV). Flow dynamics were examined using BST in the aortic root and left ventricle. Children with BAV had altered systolic flow patterns in the aortic root and higher aortic root average vorticity (25.9 [23.4-29.2] Hz vs. 17.8 [9.0-26.2] Hz, p < 0.05), vector complexity (0.17 [0.14-0.31] vs. 0.05 [0.02-0.13], p < 0.01) and rate of energy loss (7.9 [4.9-12.1] mW/m vs. 2.7 [1.2-7.4] mW/m, p = 0.01). Left ventricular average diastolic vorticity (20.9 ± 5.8 Hz vs. 11.4 ± 5.2 Hz, p < 0.01), kinetic energy (0.11 ± 0.05 J/m vs. 0.04 ± 0.02 J/m, p < 0.01), vector complexity (0.38 ± 0.1 vs. 0.23 ± 0.1, p < 0.01) and rate of energy loss (11.1 ± 4.8 mW/m vs. 2.7 ± 1.9 mW/m, p < 0.01) were higher in children with BAV.

CONCLUSION

Children with BAV exhibit altered flow dynamics in the aortic root and left ventricle in the absence of significant aortic root dilation. This may represent a substrate and potential predictor for future dilation and diastolic dysfunction.

Retrospective Ultrasound Doppler Quantification Using a Single Acquisition in Healthy Adults

OBJECTIVE

Using an experimental tool for retrospective ultrasound Doppler quantification-with high temporal resolution and large spatial coverage-simultaneous flow and tissue measurements were obtained. We compared and validated these experimental values against conventional measurements to determine if the experimental acquisition produced trustworthy tissue and flow velocities.

METHODS

We included 21 healthy volunteers. The only exclusion criterion was the presence of an irregular heartbeat. Two ultrasound examinations were performed for each participant, one using conventional and one using experimental acquisition. The experimental acquisition used multiple plane wave emissions combined with electrocardiography stitching to obtain continuous data with over 3500 frames per second. With two recordings covering a biplane apical view of the left ventricle, we retrospectively extracted selected flow and tissue velocities.

RESULTS

Flow and tissue velocities were compared between the two acquisitions. Statistical testing showed a small but significant difference. We also exemplified the possibility of extracting spectral tissue Doppler from different sample volumes in the myocardium within the imaging sector, showing a decrease in the velocities from the base to the apex.

CONCLUSION

This study demonstrates the feasibility of simultaneous, retrospective spectral and color Doppler of both tissue and flow from an experimental acquisition covering a full sector width. The measurements were significantly different between the two acquisitions but were still comparable, as the biases were small compared to clinical practice, and the two acquisitions were not done simultaneously. The experimental acquisition also enabled the study of deformation by simultaneous spectral velocity traces from all regions of the image sector.

Assessment of Early Diastolic Intraventricular Pressure Difference in Children by Blood Speckle-Tracking Echocardiography

BACKGROUND

The lack of reliable echocardiographic techniques to assess diastolic function in children is a major clinical limitation. Our aim was to develop and validate the intraventricular pressure difference (IVPD) calculation using blood speckle-tracking (BST) and investigate the method's potential role in the assessment of diastolic function in children.

METHODS

Blood speckle-tracking allows two-dimensional angle-independent blood flow velocity estimation. Blood speckle-tracking images of left ventricular (LV) inflow from the apical 4-chamber view in 138 controls, 10 patients with dilated cardiomyopathies (DCMs), and 21 patients with hypertrophic cardiomyopathies (HCMs) <18 years of age were analyzed to study LV IVPD during early diastole. Reproducibility of the IVPD analysis was assessed, IVPD estimates from BST and color M mode were compared, and the validity of the BST-based IVPD calculations was tested in a computer flow model.

RESULTS

Mean IVPD was significantly higher in controls (-2.28 ± 0.62 mm Hg) compared with in DCM (-1.21 ± 0.39 mm Hg, P < .001) and HCM (-1.57 ± 0.47 mm Hg, P < .001) patients. Feasibility was 88.3% in controls, 80% in DCM patients, and 90.4% in HCM patients. The peak relative negative pressure occurred earlier at the apex than at the base and preceded the peak E-wave LV filling velocity, indicating that it represents diastolic suction. Intraclass correlation coefficients for intra- and interobserver variability were 0.908 and 0.702, respectively. There was a nonsignificant mean difference of 0.15 mm Hg between IVPD from BST and color M mode. Estimation from two-dimensional velocities revealed a difference in peak IVPD of 0.12 mm Hg (6.6%) when simulated in a three-dimensional fluid mechanics model.

CONCLUSIONS

Intraventricular pressure difference calculation from BST is highly feasible and provides information on diastolic suction and early filling in children with heart disease. Intraventricular pressure difference was significantly reduced in children with DCM and HCM compared with controls, indicating reduced early diastolic suction in these patient groups.

Pulmonary Hypertension in Children is Associated With Abnormal Flow Patterns in the Main Pulmonary Artery as Demonstrated by Blood Speckle Tracking

Background

Paediatric pulmonary arterial hypertension (PAH) is characterized by increased pulmonary vascular resistance resulting in increased pulmonary artery (PA) and right ventricular pressure (RV). This is associated with disturbed flow dynamics in the PA and RV that are not well characterized. We aimed to compare flow dynamics in children with PAH compared with healthy controls using blood speckle tracking echocardiography.

Methods

Patients <10 years of age with PAH and healthy controls were included. We examined flow dynamics in the main PA (MPA) and right ventricle based on acquisition blood speckle tracking images obtained from the RV and PA. Qualitative and quantitative analyses were performed.

Results

Eighteen subjects were included in each group. A diastolic vortex in the MPA was identified in 16 of the patients with PAH, but not in controls. Significantly higher MPA systolic (4.84 vs 2.42 mW/m; P = 0.01) and diastolic (0.69 vs 0.14 mW/m; P = 0.01) energy loss, as well as increased vector complexity (systole: 0.21 vs 0.04, P = 0.003; diastole: 0.13 vs 0.05, P = 0.04) and diastolic vorticity (15.2 vs 4.4 Hz; P = 0.001), were noted in PAH compared with controls.

Conclusion

This study demonstrates the presence of abnormal flow patterns in the MPA with diastolic vortex formation in most patients with PAH. This diastolic vortex likely results from reflected waves from the distal pulmonary bed. Our data indicate that the diastolic vortex could potentially be used in the diagnosis of PAH. The clinical significance of the energy loss findings warrants further investigation in a larger cohort of patients with PAH.

Intraventricular Vector Flow Imaging with Blood Speckle Tracking in Adults: Feasibility, Normal Physiology and Mechanisms in Healthy Volunteers

This study examines the feasibility of blood speckle tracking for vector flow imaging in healthy adults and describes the physiologic flow pattern and vortex formation in relation to the wall motion in the left ventricle. The study included 21 healthy volunteers and quantified and visualized flow patterns with high temporal resolution down to a depth of 10-12 cm without the use of contrast agents. Intraventricular flow seems to originate during the isovolumetric relaxation with a propagation of blood from base to apex. With the E-wave, rapid inflow and vortex formation occurred on both sides of the valve basally. During diastasis the flow gathers in a large vortex before the pattern from the E-wave repeats during the A-wave. In isovolumetric contraction, the flow again gathers in a large vortex that seems to facilitate the flow out in the aorta during systole. No signs of a persistent systolic vortex were visualized. The geometry of the left ventricle and the movement of the AV-plane is important in creating vortices that are favorable for the blood flow and facilitate outflow. The quantitative measurements are in concordance with these findings, but the clinical interpretation must be evaluated in future clinical studies.

Right Ventricular Flow Dynamics in Dilated Right Ventricles: Energy Loss Estimation Based on Blood Speckle Tracking Echocardiography-A Pilot Study in Children

Using blood speckle tracking (BST) based on high-frame-rate echocardiography (HFRE), we compared right ventricle (RV) flow dynamics in children with atrial septal defects (ASDs) and repaired tetralogy of Fallot (rTOF). Fifty-seven children with rTOF with severe pulmonary insufficiency (PI) (n = 21), large ASDs (n = 11) and healthy controls (CTL, n = 25) were included. Using a flow phantom, we studied the effects of imaging plane and smoothing parameters on 2-D energy loss (EL). RV diastolic EL was similar in ASD and rTOF, but both were greater than in CTL. Locations of high EL were similar in all groups in systole, occurring in the RV outflow tract and around the tricuspid valve leaflets in early diastole. An additional apical early diastolic area of EL was noted in rTOF, corresponding to colliding tricuspid inflow and PI. The flow phantom revealed that EL varied with imaging plane and smoothing settings but that the EL trend was preserved if kept consistent.

Bicuspid aortic valve flow dynamics using blood speckle tracking in children

Funding Acknowledgements

Type of funding sources: Foundation. Main funding source(s): Labatt Family Heart Centre

Background

Bicuspid aortic valves (BAV) are the most common form of congenital heart disease with a prevalence of up to 2%. Fusion of the aortic cusps result in complex flow patterns in the aortic root which may contribute to the risk of progressive dilation. Imaging these patterns with conventional echocardiography is challenging due to the angle dependency of colour Doppler. Blood speckle tracking (BST) is a novel method based on high-frame rate ultrasound that is less angle-independent and developed to evaluate complex flow.

Purpose

The aims of this study were to 1) describe and quantify flow patterns in the aortic root using blood speckle tracking and 2) quantitatively compare flow parameters (energy loss, vorticity, kinetic energy) between paediatric patients with BAVs and those with normal aortic valves.

Methods

Clinical and routine echocardiographic parameters were collected for children with normal and BAVs.  Children were imaged using a commercially available clinical scanner with research software enabling the acquisition of high-frame rate imaging and blood speckle tracking. Aortic root energy loss, vorticity and kinetic energy were measured in the parasternal long axis view.

Results

Data from 46 patients were analyzed (31 normal aortic valves, 15 BAVs) with no significant difference in age, height, weight or BSA between groups. Nine patients (60%) had right-left fusion; 6 (40%) had right-non fusion. Qualitative flow assessment in aortic root demonstrated disturbed flow in the BAV group (see figure) generally directed towards the non-fused cusp (i.e non coronary cusp in right-left fusion). Patients with a BAV had higher aortic valve peak velocity (2.1 m/s vs 0.9 m/s, p&lt; 0.001);  aortic root systolic energy loss (4.29 mW/m vs 1.79 mW/m, p&lt; 0. 001), vorticity (38.7 1/s vs 31.1 1/s, p&lt; 0.01) and kinetic energy (34.5  mJ/m vs 24.5 mJ/m, p = 0.04) compared to those with normal aortic valves.

Conclusion(s)

Blood speckle tracking can demonstrate altered flow patterns in the aortic root in patients with BAV. These patients have increased energy losses, vorticity and kinetic energy when compared to age matched patients with normal aortic valves however the relationship of theses parameters to aortic root dilation remains to be demonstrated.

Abstract Figure. Flow patterns (A-C) and analysis (D-F)

Blood Speckle-Tracking Based on High-Frame Rate Ultrasound Imaging in Pediatric Cardiology

BACKGROUND

Flow properties play an important role in cardiac function, remodeling, and morphogenesis but cannot be displayed in detail with today's echocardiographic techniques. The authors hypothesized that blood speckle-tracking (BST) could visualize and quantify flow patterns. The aim of this study was to determine the feasibility, accuracy, and potential clinical applications of BST in pediatric cardiology.

METHODS

BST is based on high-frame rate ultrasound, using a combination of plane-wave imaging and parallel receive beamforming. Pattern-matching techniques are used to quantify blood speckle motion. Accuracy of BST velocity measurements was validated using a rotating phantom and by comparing BST-derived inflow velocities with pulsed-wave Doppler obtained in the left ventricles of healthy control subjects. To test clinical feasibility, 102 subjects (21 weeks to 11.5 years of age) were prospectively enrolled, including healthy fetuses (n = 4), healthy control subjects (n = 51), and patients with different cardiac diseases (n = 47).

RESULTS

The phantom data showed a good correlation (r = 0.95, with a tracking quality threshold of 0.4) between estimated BST velocities and reference velocities down to a depth of 8 cm. There was a good correlation (r = 0.76) between left ventricular inflow velocity measured using BST and pulsed-wave Doppler. BST displayed lower velocities (mean ± SD, 0.59 ± 0.14 vs 0.82 ± 0.21 m/sec for pulsed-wave Doppler). However, the velocity amplitude in BST increases with reduced smoothing. The clinical feasibility of BST was high, as flow patterns in the area of interest could be visualized in all but one case (>99%).

CONCLUSIONS

BST is highly feasible in fetal and pediatric echocardiography and provides a novel approach for visualizing blood flow patterns. BST provides accurate velocity measurements down to 8 cm, but compared with pulsed-wave Doppler, BST displays lower velocities. Studying blood flow properties may provide novel insights into the pathophysiology of pediatric heart disease and could become an important diagnostic tool.

4-D Intracardiac Ultrasound Vector Flow Imaging-Feasibility and Comparison to Phase-Contrast MRI

In vivo characterization of intracardiac blood velocity vector fields may provide new clinical information but is currently not available for bedside evaluation. In this paper, 4-D vector flow imaging for intracardiac flow assessment is demonstrated using a clinical ultrasound (US) system and a matrix array transducer, without the use of contrast agent. Two acquisition schemes were developed, one for full volumetric coverage of the left ventricle (LA) at 50 vps and a 3-D thick-slice setup with continuous frame acquisition (4000 vps), both utilizing ECG-gating. The 3-D vector velocity estimates were obtained using a novel method combining phase and envelope information. In vitro validation in a rotating tissue-mimicking phantom revealed velocity estimates in compliance with the ground truth, with a linear regression slope of 0.80, 0.77, and 1.03 for the , , and velocity components, and with standard deviations of 2.53, 3.19, and 0.95 cm/s, respectively. In vivo measurements in a healthy LV showed good agreement with PC-MRI. Quantitative analysis of energy loss (EL) and kinetic energy (KE) further showed similar trends, with peak KE at 1.5 and 2.4 mJ during systole and 3.6 and 3.1 mJ for diastole for US and PC-MRI. Similar for EL, 0.15- 0.2 and 0.7 mW was found during systole and 0.6 and 0.7 mW during diastole, for US and PC-MRI, respectively. Overall, a potential for US as a future modality for 4D cardiac vector flow imaging was demonstrated, which will be further evaluated in clinical studies.

In Vivo Intracardiac Vector Flow Imaging Using Phased Array Transducers for Pediatric Cardiology

Two-dimensional blood speckle tracking (ST) has shown promise for measuring complex flow patterns in neonatal hearts using linear arrays and high-frame-rate plane wave imaging. For general pediatric applications, however, the need for phased array probes emerges due to the limited intercostal acoustic window available. In this paper, a clinically approved real-time duplex imaging setup with phased array probes was used to investigate the potential of blood ST for the 2-D vector flow imaging of children with congenital heart disease. To investigate transmit beam pattern and tracking accuracy, straight tubes with parabolic flow were simulated at three depths (4.5, 7, and 9.5 cm). Due to the small aperture available, diffraction effects could be observed when approaching 10 cm, which limited the number of parallel receive beams that could be utilized. Moving to (slightly) diverging beams was shown to solve this issue at the expense of a loss in signal-to-noise ratio. To achieve consistent estimates, a forward-backward tracking scheme was introduced to avoid measurement bias occurring due to tracking kernel averaging artifacts at flow domain boundaries. Promising results were observed for depths <10 cm in two pediatric patients, where complex cardiac flow patterns could be estimated and visualized. As a loss in penetration compared with color flow imaging is expected, a larger clinical study is needed to establish the clinical feasibility of this approach.

Assessing the Performance of Ultrafast Vector Flow Imaging in the Neonatal Heart via Multiphysics Modeling and In Vitro Experiments

Ultrafast vector flow imaging would benefit newborn patients with congenital heart disorders, but still requires thorough validation before translation to clinical practice. This paper investigates 2-D speckle tracking (ST) of intraventricular blood flow in neonates when transmitting diverging waves at ultrafast frame rate. Computational and in vitro studies enabled us to quantify the performance and identify artifacts related to the flow and the imaging sequence. First, synthetic ultrasound images of a neonate's left ventricular flow pattern were obtained with the ultrasound simulator Field II by propagating point scatterers according to 3-D intraventricular flow fields obtained with computational fluid dynamics (CFD). Noncompounded diverging waves (opening angle of 60°) were transmitted at a pulse repetition frequency of 9 kHz. ST of the B-mode data provided 2-D flow estimates at 180 Hz, which were compared with the CFD flow field. We demonstrated that the diastolic inflow jet showed a strong bias in the lateral velocity estimates at the edges of the jet, as confirmed by additional in vitro tests on a jet flow phantom. Furthermore, ST performance was highly dependent on the cardiac phase with low flows (<5 cm/s), high spatial flow gradients, and out-of-plane flow as deteriorating factors. Despite the observed artifacts, a good overall performance of 2-D ST was obtained with a median magnitude underestimation and angular deviation of, respectively, 28% and 13.5° during systole and 16% and 10.5° during diastole.

Clutter filtering influence on blood velocity estimation using speckle tracking

Blood speckle tracking has shown potential for solving the angle-dependency limitation in color flow imaging. However, as clutter filtering is still Doppler-based, flow velocities at near-perpendicular beam-to-flow angles can be severely attenuated. It is shown that the clutter filter also alters the speckle appearance through a decrease in the lateral imaging bandwidth, leading to poorer lateral resolution and thus tracking performance. Interestingly, at perpendicular beam-to-flow angles lateral band-pass characteristics are inferred, and the resulting lateral amplitude modulation could help improve tracking estimates. Simulations and flow phantom experiments showed that substantially improved results could be achieved by utilizing time-variant clutter filters (e.g., polynomial regression filters) despite the inherent decorrelation inferred by these filters, but only for higher ensemble sizes (N > 36). We found that, compared with color flow imaging, speckle tracking could yield consistent estimates well below the clutter filter cutoff, but with a higher variance attributed to the low signalto- noise ratio inferred by filter attenuation. Overall, provided that a low f-number and high ensemble lengths (N approx. > 36) can be used, speckle tracking can consistently provide angle- independent flow velocity estimates, limited only by a lower bound on the flow velocity itself.

Robust angle-independent blood velocity estimation based on dual-angle plane wave imaging

Two-dimensional blood velocity estimation has shown potential to solve the angle-dependency of conventional ultrasound flow imaging. Clutter filtering, however, remains a major challenge for large beam-to-flow angles, leading to signal drop-outs and corrupted velocity estimates. This work presents and evaluates a compounding speckle tracking (ST) algorithm to obtain robust angle-independent 2-D blood velocity estimates for all beam-to-flow angles. A dual-angle plane wave imaging setup with full parallel receive beamforming is utilized to achieve high-frame-rate speckle tracking estimates from two scan angles, which may be compounded to obtain velocity estimates of increased robustness. The acquisition also allows direct comparison with vector Doppler (VD) imaging. Absolute velocity bias and root-mean-square (RMS) error of the compounding ST estimations were investigated using simulations of a rotating flow phantom with low velocities ranging from 0 to 20 cm/s. In a challenging region where the estimates were influenced by clutter filtering, the bias and RMS error for the compounding ST estimates were 11% and 2 cm/s, a significant reduction compared with conventional single-angle ST (22% and 4 cm/s) and VD (36% and 6 cm/s). The method was also tested in vivo for vascular and neonatal cardiac imaging. In a carotid artery bifurcation, the obtained blood velocity estimates showed that the compounded ST method was less influenced by clutter filtering than conventional ST and VD methods. In the cardiac case, it was observed that ST velocity estimation is more affected by low signal-to-noise (SNR) than VD. However, with sufficient SNR the in vivo results indicated that a more robust angle-independent blood velocity estimator is obtained using compounded speckle tracking compared with conventional ST and VD methods.

Shunt flow evaluation in congenital heart disease based on two-dimensional speckle tracking

High-frame-rate ultrasound speckle tracking was used for quantification of peak velocity in shunt flows resulting from septal defects in congenital heart disease. In a duplex acquisition scheme implemented on a research scanner, unfocused transmit beams and full parallel receive beamforming were used to achieve a frame rate of 107 frames/s for full field-of-view flow images with high accuracy, while also ensuring high-quality focused B-mode tissue imaging. The setup was evaluated in vivo for neonates with atrial and ventricular septal defects. The shunt position was automatically tracked in B-mode images and further used in blood speckle tracking to obtain calibrated shunt flow velocities throughout the cardiac cycle. Validation toward color flow imaging and pulsed wave Doppler with manual angle correction indicated that blood speckle tracking could provide accurate estimates of shunt flow velocities. The approach was less biased by clutter filtering compared with color flow imaging and was able to provide velocity estimates beyond the Nyquist range. Possible placements of sample volumes (and angle corrections) for conventional Doppler resulted in a peak shunt velocity variations of 0.49-0.56 m/s for the ventricular septal defect of patient 1 and 0.38-0.58 m/s for the atrial septal defect of patient 2. In comparison, the peak velocities found from speckle tracking were 0.77 and 0.33 m/s for patients 1 and 2, respectively. Results indicated that complex intraventricular flow velocity patterns could be quantified using high-frame-rate speckle tracking of both blood and tissue movement. This could potentially help increase diagnostic accuracy and decrease inter-observer variability when measuring peak velocity in shunt flows.