Combined vector velocity and spectral Doppler imaging for improved imaging of complex blood flow in the carotid arteries

A Transverse Velocity Spectral Estimation Method for Ultrafast Ultrasound Doppler Imaging

A novel transverse velocity spectral estimation method is proposed to estimate the velocity component in the direction transverse to the beam axis for ultrafast imaging. The transverse oscillation was introduced by filtering the envelope data after the axial oscillation was removed. The complex transverse oscillated signal was then used to estimate the transverse velocity spectrum and mean velocity. In simulations, both steady flow with a parabolic flow profile and temporally varying flow were simulated to investigate the performance of the proposed method. Next, the proposed approach was used to estimate the flow velocity in a phantom with pulsatile flow, and finally, this method was applied in vivo in a small animal model. Results of the simulation study indicate that the proposed method provided an accurate velocity spectrogram for beam-to-flow angles from 45° to 90°, without significant performance degradation as the angle decreased. For the simulation of temporally varying flow, the proposed method had a reduced bias ( % versus 73.3%) and higher peak-to-background ratio (PBR) (>15.6 versus 10.5 dB) compared to previous methods. Results in a vessel phantom show that the temporally varying flow velocity can be estimated in the transverse direction obtained using the spectrogram produced by the proposed method operating on the envelope data. Finally, the proposed method was used to map the microvascular blood flow velocity in the mouse spinal cord, demonstrating the estimation of pulsatile blood flow in both the axial and transverse directions in vivo over several cardiac cycles.

Blood Flow Turbulence Quantification of Carotid Artery With a High-Frame Rate Vector Flow Imaging

OBJECTIVES

To assess the feasibility and performance of Turbulence (Tur) index as a quantitative tool for carotid artery flow turbulence; to detect and compare the blood flow patterns of common carotid artery (CCA) and carotid bulb (CB) at different ages and cardiac phases in healthy adults, and thus interpret the evolvement of etiology difference between CCA and CB.

METHODS

Carotid flow characteristics of 40 healthy volunteers were evaluated quantitatively by a high-frame rate vector flow imaging. Three types of flow patterns were defined depending on the distributive range of complex flow during systole in CB. Comparison of mean Tur value in CCA and CB at different age groups and cardiac phases was performed. And the correlation between Tur value and the diameter ratio of proximal internal carotid artery to common carotid artery (DRpro-ica/cca) was tested.

RESULTS

Mean Tur values in CB were remarkably higher than that in CCA, whether during systole or diastole (P < .001). Meanwhile Tur values in CB during systole were significantly higher than that during diastole (P < .001). Flow complexity of CB showed variations among 40 participants especially in systole, whereas the flow pattern of CCA was relatively consistent. Mean Tur values were positively correlated with DRpro-ica/cca in CB (ρ = 0.69, P < .05).

CONCLUSIONS

V Flow imaging provided a reliable method-Tur, for quantitative analysis of carotid blood flow. It had potential to be further applied in distinguishing complex hemodynamic characteristics in high-risk people of carotid diseases for the risk stratification of cardiovascular events.

Advances in vascular anatomy and pathophysiology using high resolution and multiparametric sonography

B-mode and Color Doppler are the first-line imaging modalities in cardiovascular diseases. However, conventional ultrasound (US) provides a lower spatial and temporal resolution (70-100 frames per second) compared to ultrafast technology which acquires several thousand frames per second. Consequently, the multiparametric ultrafast platforms manage new imaging algorithms as high-frequency ultrasound, contrast-enhanced ultrasound, shear wave elastography, vector flow, and local pulse wave imaging. These advances allow better ultrasound performances, more detailed blood flow visualization and vessel walls' characterization, and many future applications for vascular viscoelastic properties evaluation.In this paper, we provide an overview of each new technique's principles and concepts and the real or potential applications of these modalities on the study of the artery and venous anatomy and pathophysiology of the upper limb before and after creating a native or prosthetic arterio-venous fistula. In particular, we focus on high-frequency ultrasound that could predict cannulation readiness and its potential role in the venous valvular status evaluation before vascular access creation; on contrast-enhanced ultrasound that could improve the peri-operative imaging evaluation during US-guided angioplasty; on shear wave elastography and local pulse wave imaging that could evaluate preoperative vessels stiffness and their potential predictive role in vascular access failure; on vector flow imaging that could better characterize the different components of the vascular access complex flow.

Real-Time High Frame Rate Color Flow Mapping System

Plane wave (PW) transmission (TX) can be profitably used to improve the performance of color flow mapping (CFM) systems by increasing the autocorrelation ensemble length (EL) and/or the frame rate (FR). Although high-end scanners tend to include imaging schemes using PW TX and parallel receive beams, high frame rate (HFR) CFM has been so far experimentally implemented mostly through research platforms that transmit PWs and beamform/process the received channel data off-line. In this article, full real-time implementation of PW CFM with continuous-time clutter filtering and extended FR/EL is reported. The field-programmable gate arrays (FPGAs) and digital signal processors (DSPs) onboard the ULA-OP 256 research scanner were programmed to perform high-speed parallel beamforming and autocorrelation-based CFM processing, respectively. Different strategies were tested, in which the TX of PWs for CFM is either continuous or interleaved with the TX of packets of B-mode pulses. A fourth-order Chebyshev continuous-time high-pass filter with programmable cutoff frequency was implemented and its clutter rejection performance was positively compared with that obtained when operating on packet data. CFM FRs up to 575 were obtained. The possibility of programming the autocorrelation EL up to 64 permitted to detect flow with high sensitivity and accuracy (average relative errors down to 0.4% ± 8.4%). In vivo HFR movies are presented, showing the dynamics of flow in the common carotid artery, which highlight the presence of secondary flow components.

Adaptive Spectral Doppler Estimation Based on the Modified Amplitude Spectrum Capon

In conventional ultrasound systems, the compromise between frequency and temporal resolution limits the quality of the spectrograms and the ability to track fast blood flows. The main objective of this study was to identify a method that could reduce spectral broadening over time by reducing the observations and improving the spectral resolution and contrast. This problem is more pronounced in the process of imaging at higher blood velocities when using a short Doppler signal observation window (OW) in adaptive methods. The proposed adaptive technique, which is based on the covariance matrix Eigen space and the amplitude spectrum Capon (ASC) algorithm, managed to improve the spectral resolution and contrast compared with other adaptive algorithms within a shorter observation time, and it offered a narrower power spectrum and a more accurate spectrogram over time in combination with a coherence-based post filter. All methods were tested through various simulations. First, an analysis was carried out by simulating the femoral artery flow and the time-independent parabolic flow using the Field II simulator. Then, the performance of the proposed method was evaluated under more realistic conditions using a computational fluid dynamics simulation of complex flow fields in a carotid bifurcation model. Afterward, in vivo clinical data on the hepatic vein were used to validate the proposed method. Finally, the accuracy of the velocity estimated by different methods was evaluated through a mean-square-error assessment. Not only could the proposed method show significant improvements using extreme small OWs, N=[2, 4] , in the simulated data in terms of frequency resolution and contrast, but it also managed to offer an improvement of 74%, 73.3%, 22.2%, and 50% in frequency resolution, and an increase of 96.5, 90.2, 49, and 31.5 dB in contrast using in vivo clinical data compared with the Capon, amplitude and phase estimation (APES), projection-based Capon, and projection-based APES, respectively, for the OW of N=4 .

Tapered Vector Doppler for Improved Quantification of Low Velocity Blood Flow

A new vector velocity estimation scheme is developed, termed tapered vector Doppler (TVD), aiming to improve the accuracy of low velocity flow estimation. This is done by assessing the effects of singular value decomposition (SVD) and finite impulse response (FIR) filters and designing an estimator which accounts for signal loss due to filtering. Synthetic data created using a combination of in vivo recordings and flow simulations were used to investigate scenarios with low blood flow, in combination with true clutter motion. Using this approach, the accuracy and precision of TVD was investigated for a range of clutter-to-blood and signal-to-noise ratios. The results indicated that for the investigated carotid application and setup, the SVD filter performed as a frequency-based filter. For both SVD and FIR filters, suppression of the clutter signal resulted in large bias and variance in the estimated blood velocity magnitude and direction close to the vessel walls. Application of the proposed tapering technique yielded significant improvement in the accuracy and precision of near-wall vector velocity measurements, compared to non-TVD and weighted least squares approaches. In synthetic data, for a blood SNR of 5 dB, and in a near-wall region where the average blood velocity was 9 cm/s, the use of tapering reduced the average velocity magnitude bias from 26.3 to 1.4 cm/s. Complex flow in a carotid bifurcation was used to demonstrate the in vivo performance of TVD, and it was shown that tapering enables vector velocity estimation less affected by clutter and clutter filtering than what could be obtained by adaptive filter design only.

Innovations in Vascular Ultrasound

There are several vascular ultrasound technologies that are useful in challenging diagnostic situations. New vascular ultrasound applications include directional power Doppler ultrasound, contrast-enhanced ultrasound, B-flow imaging, microvascular imaging, 3-dimensional vascular ultrasound, intravascular ultrasound, photoacoustic imaging, and vascular elastography. All these techniques are complementary to Doppler ultrasound and provide greater ability to visualize small vessels, have higher sensitivity to detect slow flow, and better assess vascular wall and lumen while overcoming limitations color Doppler. The ultimate goal of these technologies is to make ultrasound competitive with computed tomography and magnetic resonance imaging for vascular imaging.

Spectral Doppler Measurements With 2-D Sparse Arrays

The 2-D sparse arrays, in which a few hundreds of elements are distributed on the probe surface according to an optimization procedure, represent an alternative to full 2-D arrays, including thousands of elements usually organized in a grid. Sparse arrays have already been used in B-mode imaging tests, but their application to Doppler investigations has not been reported yet. Since the sparsity of the elements influences the acoustic field, a corresponding influence on the mean frequency (Fm), bandwidth (BW), and signal-to-noise ratio (SNR) of the Doppler spectra is expected. This article aims to assess, by simulations and experiments, to what extent the use of a sparse rather than a full gridded 2-D array has an impact on spectral Doppler measurements. Parabolic flows were investigated by a 3 MHz, 1024-element gridded array and by a sparse array; the latter was obtained by properly selecting a subgroup of 256 elements from the full array. Simulations show that the mean Doppler frequency does not change between the sparse and the full array while there are significant differences on the BW (average reduction of 17.2% for the sparse array, due to different apertures of the two probes) and on the signal power (Ps) (22 dB, due to the different number of active elements). These results are confirmed by flow phantom experiments, which also highlight that the most critical difference between sparse and full gridded array in Doppler measurements is in terms of SNR (-16.8 dB).

Data-Adaptive 2-D Tracking Doppler for High-Resolution Spectral Estimation

Spectral broadening in pulsed-wave Doppler caused by the transit-time effect deteriorates the frequency resolution and may cause overestimation of maximum velocities in high-velocity blood flow regions and for large beam-to-flow angles. Data-adaptive spectral estimators have been shown to provide improved frequency resolution, especially for small ensemble lengths, but offer little or no improvement when the transit-time effect dominates. In this work, a method is presented that combines a data-adaptive spectral estimation method, the power spectral Capon, and 2-D tracking Doppler to enable improved frequency resolution for both high and low velocities. For each velocity, a time signal is extracted by tracking scatterers over time and space to decrease the transit-time effect, and power spectral Capon is used for spectral estimation. The method is evaluated using simulations, flow phantom recordings, and recordings from healthy and stenotic carotid arteries. Simulation results showed that the spectral width was decreased by 60% compared to 2-D tracking Doppler for velocities around 2.3 m/s using 12 time samples. The reduction was estimated to be 66% using the flow phantom results for 0.85-m/s mean velocity. A 5-dB SNR gain was observed from the in vivo results compared with Welch's method. Computer simulations confirm that in the presence of velocity gradients or out-of-plane motion, the proposed method can be used to reduce spectral broadening by requiring shorter observation windows.

In Vivo Blood Velocity Vector Imaging Using Adaptive Velocity Compounding in the Carotid Artery Bifurcation

Visualization and quantification of blood flow are considered important for early detection of atherosclerosis and patient-specific diagnosis and intervention. As conventional Doppler imaging is limited to 1-D velocity estimates, 2-D and 3-D techniques are being developed. We introduce an adaptive velocity compounding technique that estimates the 2-D velocity vector field using predominantly axial displacements estimated by speckle tracking from dual-angle plane wave acquisitions. Straight-vessel experiments with a 7.8-MHz linear array transducer connected to a Verasonics Vantage ultrasound system revealed that the technique performed with a maximum velocity magnitude bias and angle bias of -3.7% (2.8% standard deviation) and -0.16° (0.41° standard deviation), respectively. In vivo, complex flow patterns were visualized in two healthy and three diseased carotid arteries and quantified using a vector complexity measure that increased with increasing wall irregularity. This measure could potentially be a relevant clinical parameter which might aid in early detection of atherosclerosis.

Portable Vector Flow Imaging Compared With Spectral Doppler Ultrasonography

In this study, a vector flow imaging (VFI) method developed for a portable ultrasound scanner was used for estimating peak velocity values and variation in beam-to-flow angle over the cardiac cycle in vivo on healthy volunteers. Peak-systolic velocity (PSV), end-diastolic velocity (EDV), and resistive index (RI) measured with VFI were compared to spectral Doppler ultrasonography (SDU). Seventeen healthy volunteers were scanned on the left and right common carotid arteries (CCAs). The standard deviation (SD) of VFI measurements averaged over the cardiac cycle was 7.3% for the magnitude and 3.84° for the angle. Bland-Altman plots showed a positive bias for the PSV measured with SDU (mean difference: 0.31 ms ^-1 ), and Pearson correlation analysis showed a highly significant correlation ( r = 0.6 ; ). A slightly positive bias was found for EDV and RI measured with SDU (mean difference: 0.08 ms ^-1 and -0.01 ms ^-1 , respectively). However, the correlation was low and not significant. The beam-to-flow angle was estimated over the systolic part of the cardiac cycle, and its variations were for all measurements larger than the precision of the angle estimation. The range spanned deviations from -25.2° (-6.0 SD) to 23.7° (4.2 SD) with an average deviation from -15.2° to 9.7°. This can significantly affect PSV values measured by SDU as the beam-to-flow angle is not constant and not aligned with the vessel surface. The study demonstrates that the proposed VFI method can be used in vivo for the measurement of PSV in the CCAs, and that angle variations across the cardiac cycle can lead to significant errors in SDU velocity estimates.

Blood flow reversal from the external into the internal carotid artery-New insights into the hemodynamics at the carotid bifurcation

BACKGROUND

Complex blood flow patterns are a well-known phenomenon at the carotid bifurcation. However, unlike for the descending aorta, a blood flow reversal has not been detected at the carotid bifurcation, so far.

METHODS

In 17 subjects, flow patterns with focus on blood flow reversal were examined at the carotid bifurcation with vector flow imaging.

RESULTS

We found a blood flow reversal from the external carotid artery (ECA) into the internal carotid artery (ICA) in 13 of 25 (52%) carotid bifurcations. The blood flow reversal ranged 5.3 ± 1.7 mm (range 2.6-8.3 mm) distally to the beginning of the ECA and lasted 105 ± 59 ms (range 32-225 ms). The mean peak systolic velocity within the blood flow reversal was 12.5 ± 4.6 cm/s (range 5-18 cm/s).

CONCLUSION

A blood flow reversal from the ECA into the ICA during the systole is a frequent finding at the carotid bifurcation. Considering ischemic stroke, retrograde embolism from plaques in the proximal ECA into the ICA might play a role.

Evaluation of Peak Reflux Velocities with Vector Flow Imaging and Spectral Doppler Ultrasound in Varicose Veins

Purpose Spectral Doppler ultrasound (SDUS) is used for quantifying reflux in lower extremity varicose veins. The technique is angle-dependent opposed to the new angle-independent Vector Flow Imaging (VFI) method. The aim of this study was to compare peak reflux velocities obtained with VFI and SDUS in patients with chronic venous disease, i. e., pathological retrograde blood flow caused by incompetent venous valves. Materials and Methods 64 patients with chronic venous disease were scanned with VFI and SDUS in the great or the small saphenous vein, and reflux velocities were compared to three assessment tools for chronic venous disease. A flow rig was used to assess the accuracy and precision of the two methods. Results The mean peak reflux velocities differed significantly (VFI: 47.4 cm/s vs. SDUS: 62.0 cm/s, p<0.001). No difference in absolute precision (p=0.18) nor relative precision (p=0.79) was found. No correlation to disease severity, according to assessment tools, was found for peak reflux velocities obtained with either method. In vitro, VFI was more accurate but equally precise when compared to SDUS. Conclusion Both VFI and SDUS detected the pathologic retrograde flow in varicose veins but measured different reflux velocities with equal precision. VFI may play a role in evaluating venous disease in the future.

Respiratory variability of peak velocities in the common femoral vein estimated with vector flow imaging and Doppler ultrasound

Respiratory variability of peak velocities (RVPV) in the common femoral vein measured with ultrasound can reveal venous outflow obstruction. Pulse wave (PW) Doppler is the gold standard for venous velocity estimation of the lower extremities. PW Doppler measurements are angle dependent, whereas vector flow imaging (VFI) can yield angle-independent measures. The hypothesis of the present study was that VFI can provide RVPV estimations without the angle dependency of PW Doppler for an improved venous disease assessment. Sixty-seven patients with symptomatic chronic venous disease were included in the study. On average, VFI measured a lower RVPV than PW Doppler (VFI: 14.11 cm/s; PW: 17.32 cm/s, p = 0.002) with a non-significant improved precision compared with PW Doppler (VFI: 21.09%; PW: 26.49%, p = 0.08). In a flow phantom, VFI had improved accuracy (p < 0.01) and equal precision compared with PW Doppler. The study indicated that VFI can characterize the hemodynamic fluctuations in the common femoral vein.

High-Frame Rate Vector Flow Imaging of the Carotid Bifurcation in Healthy Adults: Comparison With Color Doppler Imaging

OBJECTIVES

To evaluate the carotid bifurcation in healthy adults using a commercial system equipped with high-frame rate vector flow imaging (VFI) based on the plane wave and to compare VFI with color Doppler imaging.

METHODS

Carotid bifurcation diameters and flow characteristics of 60 vessels in 60 healthy volunteers were evaluated quantitatively and qualitatively to assess complex flow patterns and their extension and duration.

RESULTS

Complex flow in the internal carotid artery (ICA) was associated with a statistically significant difference in the ΔICA sinus-to-common carotid artery (CCA) diameter ratio (the relative change in diameter between the CCA and ICA sinus.) Vector flow imaging and color Doppler imaging were in accordance when detecting complex flow in 96.7% of cases; in 3.3% of cases, only VFI identified small recirculation areas of short duration. Vector flow imaging highlighted a larger extension of the complex flow (mean ± SD, 47.7 ± 28.5 mm² ; median, 45.5 mm² ) compared with color Doppler imaging (mean, 29.2 ± 19.9 mm² ; median, 29.5 mm² ) and better depicted different complex flow patterns; a strong correlation (r = 0.84) was found between the ΔICA sinus-to-CCA diameter ratio and the complex flow extension. Vector flow imaging showed a longer duration of the flow disturbances (mean, 380 ± 218 milliseconds; median, 352.5 milliseconds) compared with color Doppler imaging (mean, 325 ± 206 milliseconds; median, 333 milliseconds), and there was a strong correlation (r = 0.92).

CONCLUSIONS

Vector flow imaging is as effective as color Doppler imaging in the detection of flow disturbances, but it is more powerful in the assessment of complex flow patterns.

Role of the CHADS2 Score in the Evaluation of Carotid Atherosclerosis in Patients with Atrial Fibrillation Undergoing Carotid Artery Ultrasonography

Objective

This study investigated the characteristics of carotid atherosclerosis in patients with atrial fibrillation (AF) and determined the feasibility and significance of the CHADS₂ score in predicting the degree of carotid atherosclerosis.

Methods

Consecutive patients (n = 109) with nonvalvular AF were registered and classified into two groups, the paroxysmal AF group (n = 59) and persistent AF group (n = 50). Fifty healthy patients, matched by sex and age, were considered the control group. All patients were examined using carotid ultrasound and velocity vector imaging (VVI).

Results

Compared with the control group, the mean intimal-medial thickness in the paroxysmal AF group (0.56 ± 0.11 versus 0.61 ± 0.10, respectively, P < 0.05) and the persistent AF group (0.56 ± 0.11 versus 0.64 ± 0.13, respectively, P < 0.001) was significantly increased. The plaque index (PI) in the persistent AF group was significantly higher than that observed in the paroxysmal AF group (1.05 ± 1.33 versus 1.42 ± 1.47, respectively, P < 0.001). Regarding the VVI indices, those reflecting the long-axis longitudinal motion function of carotid arteries were significantly decreased in both AF groups. Compared with the control group, a significantly lower total longitudinal displacement (tLoD) index was observed in the persistent AF group (0.73 ± 0.66 versus 0.31 ± 0.23, respectively, P < 0·0001) and the paroxysmal AF group (0.73 ± 0.66 versus 0.34 ± 0.17, P < 0·0001). The CHADS₂ score was related to indicators reflecting the structure and function of the carotid artery.

Conclusions

Carotid arterial structure and function were significantly altered in patients with AF. The degree of carotid atherosclerosis depended on the duration of AF. The CHADS₂ score may be useful as a predictor of the extent of carotid atherosclerosis in patients with AF.

Accuracy and Precision of a Plane Wave Vector Flow Imaging Method in the Healthy Carotid Artery

The objective of the study described here was to investigate the accuracy and precision of a plane wave 2-D vector flow imaging (VFI) method in laminar and complex blood flow conditions in the healthy carotid artery. The approach was to study (i) the accuracy for complex flow by comparing the velocity field from a computational fluid dynamics (CFD) simulation to VFI estimates obtained from the scan of an anthropomorphic flow phantom and from an in vivo scan; (ii) the accuracy for laminar unidirectional flow in vivo by comparing peak systolic velocities from VFI with magnetic resonance angiography (MRA); (iii) the precision of VFI estimation in vivo at several evaluation points in the vessels. The carotid artery at the bifurcation was scanned using both fast plane wave ultrasound and MRA in 10 healthy volunteers. The MRA geometry acquired from one of the volunteers was used to fabricate an anthropomorphic flow phantom, which was also scanned using the fast plane wave sequence. The same geometry was used in a CFD simulation to calculate the velocity field. Results indicated that similar flow patterns and vortices were estimated with CFD and VFI in the phantom for the carotid bifurcation. The root-mean-square difference between CFD and VFI was within 0.12 m/s for velocity estimates in the common carotid artery and the internal branch. The root-mean-square difference was 0.17 m/s in the external branch. For the 10 volunteers, the mean difference between VFI and MRA was -0.17 m/s for peak systolic velocities of laminar flow in vivo. The precision in vivo was calculated as the mean standard deviation (SD) of estimates aligned to the heart cycle and was highest in the center of the common carotid artery (SD = 3.6% for velocity magnitudes and 4.5° for angles) and lowest in the external branch and for vortices (SD = 10.2% for velocity magnitudes and 39° for angles). The results indicate that plane wave VFI measures flow precisely and that estimates are in good agreement with a CFD simulation and MRA.

Simultaneous Vascular Strain and Blood Vector Velocity Imaging Using High-Frequency Versus Conventional-Frequency Plane Wave Ultrasound: A Phantom Study

Plaque strain and blood vector velocity imaging of stenosed arteries are expected to aid in diagnosis and prevention of cerebrovascular disease. Ultrafast plane wave imaging enables simultaneous strain and velocity estimation. Multiple ultrasound vendors are introducing high-frequency ultrasound probes and systems. This paper investigates whether the use of high-frequency ultrafast ultrasound is beneficial for assessing blood velocities and strain in arteries. The performance of strain and blood flow velocity estimation was compared between a high-frequency transducer (MS250, fc = 21 MHz) and a clinically utilized transducer (L12-5, fc = 9 MHz). Quantitative analysis based on straight tube phantom experiments revealed that the MS250 outperformed the L12-5 in the superficial region: low velocities near the wall were more accurately estimated and wall strains were better resolved. At greater than 2-cm echo depth, the L12-5 performed better due to the high attenuation of the MS250 probe. Qualitative comparison using a perfused patient-specific carotid bifurcation phantom confirmed these findings. Thus, in conclusion, for strain and blood velocity estimation for depths up to ~2 cm, a high-frequency probe is recommended.

Reliability of forearm oxygen uptake during handgrip exercise: assessment by ultrasonography and venous blood gas

Assessment of forearm oxygen uptake (V˙O2 ) during handgrip exercise is a keenly investigated concept for observing small muscle mass metabolism. Although a combination of Doppler ultrasound measurements of brachial artery blood flow (Q˙) and blood gas drawn from a deep forearm vein has been utilized to calculate forearm V˙O2 for more than two decades, the applicability of this experimental design may benefit from a thorough evaluation of its reliability during graded exercise. Therefore, we evaluated the reliability of this technique during incremental handgrip exercise in ten healthy young (24 ± 3(SD) years.) males. V˙O2 and work rate (WR) exhibited a linear relationship (1.0 W: 43.8 ± 10.1 mL·min-1 ; 1.5 W: 53.8 ± 14.1 mL·min-1 ; 2.0 W: 63.4 ± 16.3 mL·min-1 ; 2.5 W: 72.2 ± 17.6 mL·min-1 ; 3.0 W: 79.2 ± 18.6 mL·min-1 ; r = 0.65, P < 0.01). In turn, V˙O2 was strongly associated with Q˙ (1.0 W: 359 ± 86 mL·min-1 ; 1.5 W: 431 ± 112 mL·min-1 ; 2.0 W: 490 ± 123 mL·min-1 ; 2.5 W: 556 ± 112 mL·min-1 ; 3.0 W: 622 ± 131 mL·min-1 ; r = 0.96; P < 0.01), whereas arteriovenous oxygen difference (a-vO2diff ) remained constant following all WRs (123 ± 11-130 ± 10 mL·L-1 ). Average V˙O2 test-retest difference was -0.4 mL·min-1 with ±2SD limits of agreement (LOA) of 8.4 and -9.2 mL·min-1 , respectively, whereas coefficients of variation (CVs) ranged from 4-7%. Accordingly, test-retest Q˙ difference was 11.9 mL·min-1 (LOA: 84.1 mL·min-1 ; -60.4 mL·min-1 ) with CVs between 4 and 7%. Test-retest difference for a-vO2diff was -0.28 mL·dL-1 (LOA: 1.26mL·dL-1 ; -1.82 mL·dL-1 ) with 3-5% CVs. In conclusion, our results revealed that forearm V˙O2 determination by Doppler ultrasound and direct venous sampling is linearly related to WR, and a reliable experimental design across a range of exercise intensities.

Quantitative Doppler Analysis Using Conventional Color Flow Imaging Acquisitions

Interleaved acquisitions used in conventional triplex mode result in a tradeoff between the frame rate and the quality of velocity estimates. On the other hand, workflow becomes inefficient when the user has to switch between different modes, and measurement variability is increased. This paper investigates the use of power spectral Capon estimator in quantitative Doppler analysis using data acquired with conventional color flow imaging (CFI) schemes. To preserve the number of samples used for velocity estimation, only spatial averaging was utilized, and clutter rejection was performed after spectral estimation. The resulting velocity spectra were evaluated in terms of spectral width using a recently proposed spectral envelope estimator. The spectral envelopes were also used for Doppler index calculations using in vivo and string phantom acquisitions. In vivo results demonstrated that the Capon estimator can provide spectral estimates with sufficient quality for quantitative analysis using packet-based CFI acquisitions. The calculated Doppler indices were similar to the values calculated using spectrograms estimated on a commercial ultrasound scanner.

Vector flow imaging techniques: An innovative ultrasonographic technique for the study of blood flow

Doppler ultrasonography is routinely used to identify abnormal blood flow. Nevertheless, conventional Doppler can be used to determine only the axial component of blood flow velocity and is angle dependent. A new method of multidimensional angle-independent estimation of flow velocity, called Vector Flow Imaging (VFI), has been proposed. It quantitatively evaluates the true velocity vector's amplitude and direction at any location into a vessel and displays a more intuitive depiction of the flow movements. High frame rate VFI, based on plane wave imaging, allows a detailed dynamic visualization of complex flow by showing even transient events, otherwise undetectable. © 2017 Wiley Periodicals, Inc. J Clin Ultrasound 45:582-588, 2017.

A Vector Flow Imaging Method for Portable Ultrasound Using Synthetic Aperture Sequential Beamforming

This paper presents a vector flow imaging method for the integration of quantitative blood flow imaging in portable ultrasound systems. The method combines directional transverse oscillation (TO) and synthetic aperture sequential beamforming to yield continuous velocity estimation in the whole imaging region. Six focused emissions are used to create a high-resolution image (HRI), and a dual-stage beamforming approach is used to lower the data throughput between the probe and the processing unit. The transmit/receive focal points are laterally separated to obtain a TO in the HRI that allows for the velocity estimation along the lateral and axial directions using a phase-shift estimator. The performance of the method was investigated with constant flow measurements in a flow rig system using the SARUS scanner and a 4.1-MHz linear array. A sequence was designed with interleaved B-mode and flow emissions to obtain continuous data acquisition. A parametric study was carried out to evaluate the effect of critical parameters. The vessel was placed at depths from 20 to 40 mm, with beam-to-flow angles of 65°, 75°, and 90°. For the lateral velocities at 20 mm, a bias between -5% and -6.2% was obtained, and the standard deviation (SD) was between 6% and 9.6%. The axial bias was lower than 1% with an SD around 2%. The mean estimated angles were 66.70° ± 2.86°, 72.65° ± 2.48°, and 89.13° ± 0.79° for the three cases. A proof-of-concept demonstration of the real-time processing and wireless transmission was tested in a commercial tablet obtaining a frame rate of 27 frames/s and a data rate of 14 MB/s. An in vivo measurement of a common carotid artery of a healthy volunteer was finally performed to show the potential of the method in a realistic setting. The relative SD averaged over a cardiac cycle was 4.33%.

Vector velocity estimation of blood flow - A new application in medical ultrasound

Vector flow techniques in the field of ultrasound encompass different pulse emission and estimation strategies. Numerous techniques have been introduced over the years, and recently commercial implementations usable in the clinic have been made. A number of clinical papers using different vector velocity approaches have been published. This review will give an overview of the most significant in vivo results achieved with ultrasound vector flow techniques, and will outline some of the possible clinical applications for vector velocity estimation in the future.

High-frame rate vector flow imaging of the carotid bifurcation

Abstract

Carotid artery atherosclerotic disease is still a significant cause of cerebrovascular morbidity and mortality. A new angle-independent technique, measuring and visualizing blood flow velocities in all directions, called vector flow imaging (VFI) is becoming available from several vendors. VFI can provide more intuitive and quantitative imaging of vortex formation, which is not clearly distinguishable in the color Doppler image. VFI, as quantitative method assessing disturbed flow patterns of the carotid bifurcation, has the potential to allow better understanding of the diagnostic value of complex flow and to enhance risk stratification. This pictorial review article will show which new information VFI adds for the knowledge of hemodynamics in comparison to the conventional ultrasound techniques.

Teaching points

• VFI is an angle-independent technique measuring flow velocities in all directions.

• This kind of VFI is based on a plane wave multidirectional excitation technique.

• VFI allows quantitative assessment of carotid streamlines progression and visualizes vorticity.

• VFI does not allow a precise comprehension of streamlines’ 3D shape.

• VFI allows a better understanding of carotid artery complex flows.

Electronic supplementary material

The online version of this article (doi:10.1007/s13244-017-0554-5) contains supplementary material, which is available to authorized users.

Performance optimization of lateral displacement estimation with spatial angular compounding

Elastography provides tissue mechanical information to differentiate normal and disease states. Nowadays, axial displacement and strain are usually estimated in clinical practice whereas lateral estimation is rarely used given that its accuracy is typically one order of magnitude worse than that of axial estimation. To improve the performance of lateral estimation, spatial angular compounding of multiple axial displacements along ultrasound beams transmitting in different steering angles was previously proposed. However, few studies have been conducted to evaluate the influence of key factors such as grating lobe noise (GLN), the number of steering angles (NSA) and maximum steering angle (MSA) in terms of performance optimization. The aim of this study was thus to investigate the effects of these factors through both computer simulations and phantom experiments. Only lateral rigid motion was considered in this study to separate its effects from those of axial and lateral strains on lateral displacement estimation. The performance as indicated by the root mean square error (RMSE) and standard deviation (SD) of the estimated lateral displacements validates the capability of spatial angular compounding in improving the performance of lateral estimation. It is necessary to filter the GLN for better estimation, and better performance is associated with a larger NSA and bigger MSA in both simulations and experiments, which is in agreement with the theoretical analysis. As indicated by the RMSE and SD, two steering angles with a larger steering angle are recommended. These results could provide insights into the performance optimization of lateral displacement estimation with spatial angular compounding.

Extension of Fourier-Based Techniques for Ultrafast Imaging in Ultrasound With Diverging Waves

Ultrafast ultrasound imaging has become an intensive area of research thanks to its capability in reaching high frame rates. In this paper, we propose a scheme that allows the extension of the current Fourier-based techniques derived for planar acquisition to the reconstruction of sectorial scan with wide angle using diverging waves. The flexibility of the proposed formulation was assessed through two different Fourier-based techniques. The performance of the derived approaches was evaluated in terms of resolution and contrast from both simulations and in vitro experiments. The comparisons of the current state-of-the-art method with the conventional delay-and-sum technique illustrated the potential of the derived methods for producing competitive results with lower computational complexity.

Concepts and Tradeoffs in Velocity Estimation With Plane-Wave Contrast-Enhanced Doppler

While long Doppler ensembles are, in principle, beneficial for velocity estimates, short acoustic pulses must be used in microbubble contrast-enhanced (CE) Doppler to mitigate microbubble destruction. This introduces inherent tradeoffs in velocity estimates with autocorrelators, which are studied here. A model of the autocorrelation function adapted to the microbubble Doppler signal accounting for transit time, the echo frequency uncertainty, and contrast-agent destruction is derived and validated in vitro. It is further demonstrated that a local measurement of the center frequency of the microbubble echo is essential in order to avoid significant bias in velocity estimates arising from the linear and nonlinear frequency-dependent scattering of microbubbles and compensate for the inherent speckle nature of the received echo frequency. For these reasons, broadband Doppler estimators (2-D autocorrelator and Radon projection) are better suited than simpler narrow-band estimators (1-D autocorrelator and 1-D Fourier transform) for CE flow assessment. A case study of perfusion in a VX-2 carcinoma using CE plane-wave Doppler is also shown. We demonstrate that even when considering all uncertainties associated with microbubble-related decorrelation (destruction, pulse bandwidth, transit time, and flow gradient) and the need for real-time imaging, a coefficient of variation of 4% on the axial velocity is achievable with plane-wave imaging.

Adaptive Spectral Estimation Methods in Color Flow Imaging

Clutter rejection for color flow imaging (CFI) remains a challenge due to either a limited amount of temporal samples available or nonstationary tissue clutter. This is particularly the case for interleaved CFI and B-mode acquisitions. Low velocity blood signal is attenuated along with the clutter due to the long transition band of the available clutter filters, causing regions of biased mean velocity estimates or signal dropouts. This paper investigates how adaptive spectral estimation methods, Capon and blood iterative adaptive approach (BIAA), can be used to estimate the mean velocity in CFI without prior clutter filtering. The approach is based on confining the clutter signal in a narrow spectral region around the zero Doppler frequency while keeping the spectral side lobes below the blood signal level, allowing for the clutter signal to be removed by thresholding in the frequency domain. The proposed methods are evaluated using computer simulations, flow phantom experiments, and in vivo recordings from the common carotid and jugular vein of healthy volunteers. Capon and BIAA methods could estimate low blood velocities, which are normally attenuated by polynomial regression filters, and may potentially give better estimation of mean velocities for CFI at a higher computational cost. The Capon method decreased the bias by 81% in the transition band of the used polynomial regression filter for small packet size ( N=8 ) and low SNR (5 dB). Flow phantom and in vivo results demonstrate that the Capon method can provide color flow images and flow profiles with lower variance and bias especially in the regions close to the artery walls.

Compressed Sensing Doppler Ultrasound Reconstruction Using Block Sparse Bayesian Learning

In this paper we propose a framework for using duplex Doppler ultrasound systems. These type of systems need to interleave the acquisition and display of a B-mode image and of the pulsed Doppler spectrogram. In a recent study (Richy , 2013), we have shown that compressed sensing-based reconstruction of Doppler signal allowed reducing the number of Doppler emissions and yielded better results than traditional interpolation and at least equivalent or even better depending on the configuration than the study estimating the signal from sparse data sets given in Jensen, 2006. We propose here to improve over this study by using a novel framework for randomly interleaving Doppler and US emissions. The proposed method reconstructs the Doppler signal segment by segment using a block sparse Bayesian learning (BSBL) algorithm based CS reconstruction. The interest of using such framework in the context of duplex Doppler is linked to the unique ability of BSBL to exploit block-correlated signals and to recover non-sparse signals. The performance of the technique is evaluated from simulated data as well as experimental in vivo data and compared to the recent results in Richy , 2013.

Quest for the Vulnerable Atheroma: Carotid Stenosis and Diametric Strain--A Feasibility Study

The Bernoulli effect may result in eruption of a vulnerable carotid atheroma, causing a stroke. We measured electrocardiography (ECG)-registered QRS intra-stenotic blood velocity and atheroma strain dynamics in carotid artery walls using ultrasonic tissue Doppler methods, providing displacement and time resolutions of 0.1 μm and 3.7 ms. Of 22 arteries, 1 had a peak systolic velocity (PSV) >280 cm/s, 4 had PSVs between 165 and 280 cm/s and 17 had PSVs <165 cm/s. Eight arteries with PSVs <65 cm/s and 4 of 9 with PSVs between 65 and 165 cm/s had normal systolic diametric expansion (0% and 7%) and corresponding systolic wall thinning. The remaining 10 arteries had abnormal systolic strain dynamics, 2 with diametric reduction (>-0.05 mm), 2 with extreme wall expansion (>0.1 mm), 2 with extreme wall thinning (>-0.1 mm) and 4 with combinations. Decreases in systolic diameter and/or extreme systolic arterial wall thickening may indicate imminent atheroma rupture.

Effects of reverberations and clutter filtering in pulsed Doppler using sparse sequences

Duplex ultrasound is a modality in which an ultrasound system is used for simultaneous acquisition of both B-mode images and velocity (Doppler) data. Conventional duplex sequences interleave packets of B-mode and Doppler transmissions, producing undesirable gaps during B-mode interruptions. In recent years, several techniques have been proposed for avoiding such gaps by using sparse sequences, in which velocity spectra are generated from nonuniformly sampled Doppler data containing frequent B-mode interruptions. In this work, two negative effects are discussed that may influence velocity estimation when using nonuniformly sampled sequences. First, it is shown that long reverberation times lead to discontinuities in the signal from stationary clutter after each B-mode interruption. Second, using frequency analysis, it is shown that clutter filtering of nonuniformly sampled data may introduce artifacts in the velocity spectrum, and also lead to significant bias in mean velocity estimates. Methods are presented for quantification of these effects, and utilized to analyze three types of sparse duplex sequences for blood velocity estimation. In particular, it is argued that the use of such sequences in cardiac applications is not recommended because of long reverberation time. Additionally, it is found that the use of regression filters to filter nonuniformly sampled data may produce significant artifacts in pulsed wave Doppler spectra, but is less significant for color Doppler imaging applications. In vitro and in vivo examples are included showing the presence and magnitude of these problems in clinically relevant applications.

Recent developments in vascular ultrasound technology

This article describes four technologies relevant to vascular ultrasound which are available commercially in 2015, and traces their origin back through the research literature. The technologies are 3D ultrasound and its use in plaque volume estimation (first described in 1994), colour vector Doppler for flow visualisation (1994), wall motion for estimation of arterial stiffness (1968), and shear wave elastography imaging of the arterial wall (2010). Overall these technologies have contributed to the understanding of vascular disease but have had little impact on clinical practice. The basic toolkit for vascular ultrasound has for the last 25 years been real-time B-mode, colour flow and spectral Doppler. What has changed over this time is improvement in image quality. Looking ahead it is noted that 2D array transducers and high frame rate imaging continue to spread through the commercial vascular ultrasound sector and both have the potential to impact on clinical practice.