Accurate blood peak velocity estimation using spectral models and vector doppler

Ultrasound blood peak velocity estimates are routinely used for diagnostics, such as the grading of a stenosis. The peak velocity is typically assessed from the Doppler spectrum by locating the highest frequency detectable from noise. The selected frequency is then converted to velocity by the Doppler equation. This procedure contains several potential sources of error: the frequency selection is noise dependent and sensitive to the spectral broadening, which, in turn, is affected by the Doppler angle uncertainty. The result is, often, an inaccurate estimate. In this work we propose a new method that removes the aforementioned errors. The frequency is selected by exploiting a mathematical model of the Doppler spectrum that has recently been introduced. When a very large sample volume is used, which includes all the vessel section, the model is capable of predicting the exact threshold to be used without the need of broadening compensation. The angle ambiguity is solved by applying the threshold to the Doppler spectra measured from two different directions, according to the vector Doppler technique. The proposed approach has here been validated through Field II simulations, phantom experiments, and tests on volunteers by using defocused waves to insonify a large region from a linear array probe. A mean error lower than 1% and a mean coefficient of variability lower than 5% were measured in a variety of experimental conditions.

Thomson's multitaper approach combined with coherent plane-wave compounding to reduce speckle in ultrasound imaging

In ultrasound imaging, the speckle pattern limits the image quality. Spatial and frequency compounding are commonly used to reduce speckle noise or improve the contrast. Although recent implementations can preserve a frame rate that is compatible with real-time imaging (e.g., synthetic aperture compounding), most classic compounding techniques are based on the coherent combination of several radiofrequency images from the same investigated area, which reduces the frame rate. Furthermore, Thomson's multitaper approach aims to smooth the speckle by incoherently combining the obtained B-mode images after applying different apodization windows on the same original data. With only one acquisition, the frame rate remains high, but the spatial resolution is decreased. To improve the resolution and contrast while reducing the speckle noise, this paper proposes combining the coherent plane-wave compounding technique (CPWC) with Thomson's multitaper method. The resulting multitaper coherent plane-wave compounding (MCPWC) takes advantage of coherent and incoherent approaches. Simulations and experimental results demonstrate that in terms of the signal-to-noise ratio, contrast, and resolution, the image quality is increased using plane wave emissions at approximately ten steering angles with three Thomson's tapers. Outside the focal area, the lateral resolution is improved by a factor of 2, and the contrast is increased by approximately 2dB compared with images obtained using a single focalization technique and Thomson's multitaper approach.

An improved Doppler model for obtaining accurate maximum blood velocities

Maximum blood velocity estimates are frequently required in diagnostic applications, including carotid stenosis evaluation, arteriovenous fistula inspection, and maternal-fetal examinations. However, the currently used methods for ultrasound measurements are inaccurate and often rely on applying heuristic thresholds to a Doppler power spectrum. A new method that uses a mathematical model to predict the correct threshold that should be used for maximum velocity measurements has recently been introduced. Although it is a valuable and deterministic tool, this method is limited to parabolic flows insonated by uniform pressure fields. In this work, a more generalized technique that overcomes such limitations is presented. The new approach, which uses an extended Doppler spectrum model, has been implemented in an experimental set-up based on a linear array probe that transmits defocused steered waves. The improved model has been validated by Field II simulations and phantom experiments on tubes with diameters between 2mm and 8mm. Using the spectral threshold suggested by the new model significantly higher accuracy estimates of the peak velocity can be achieved than are now clinically attained, including for narrow beams and non-parabolic velocity profiles. In particular, an accuracy of +1.2±2.5 cm/s has been obtained in phantom measurements for velocities ranging from 20 to 80 cm/s. This result represents an improvement that can significantly affect the way maximum blood velocity is investigated today.

High-frame-rate 2-D vector blood flow imaging in the frequency domain

Conventional ultrasound Doppler techniques estimate the blood velocity exclusively in the axial direction to produce the sonograms and color flow maps needed for diagnosis of cardiovascular diseases. In this paper, a novel method to produce bi-dimensional maps of 2-D velocity vectors is proposed. The region of interest (ROI) is illuminated by plane waves transmitted at the pulse repetition frequency (PRF) in a fixed direction. For each transmitted plane wave, the backscattered echoes are recombined offline to produce the radio-frequency image of the ROI. The local 2-D phase shifts between consecutive speckle images are efficiently estimated in the frequency domain, to produce vector maps up to 15 kHz PRF. Simulations and in vitro steady-flow experiments with different setup conditions have been conducted to thoroughly evaluate the method's performance. Bias is proved to be lower than 10% in most simulations and lower than 20% in experiments. Further simulations and in vivo experiments have been made to test the approach's feasibility in pulsatile flow conditions. It has been estimated that the computation of the frequency domain algorithm is more than 50 times faster than the computation of the reference 2-D cross-correlation algorithm.

Multi-transmit beam forming for fast cardiac imaging--experimental validation and in vivo application

High frame rate (HFR) echocardiography may be of benefit for functional analysis of the heart. In current clinical equipment, HFR is obtained using multi-line acquisition (MLA) which typically requires broadening of transmit beams. As this may result in a significant degradation of spatial resolution and signal-to-noise ratio (SNR), the capacity of MLA to obtain high quality HFR images remains limited. As an alternative, we have demonstrated by computer simulation that simultaneously transmitting multiple focused beams into different directions [multi-line transmit (MLT)], can increase the frame rate without significantly compromising the spatial resolution or SNR. This study aimed to experimentally verify these theoretical predictions both in vitro and in vivo to demonstrate, for the first time, that cardiac MLT imaging is feasible. Hereto, the ultrasound advanced open platform, equipped with a 2.0 MHz phased array, was programmed to interleave MLT and conventional single line transmit (SLT) beam forming. Using these two beam forming methods, images of phantoms and healthy volunteers were acquired and investigated both qualitatively and quantitatively. The results confirmed our simulations that image quality of a 4MLT imaging system with a Tukey apodization scheme is very competitive to that of SLT while providing a 4 times higher frame rate. It is also demonstrated that MLT can be combined with MLA to provide images at 12- to 16-fold frame rate (about 340-450 Hz) without significantly compromising spatial resolution and SNR. This is thus the first study to demonstrate that this new ultrasound imaging paradigm is viable which could have significant impact on future cardiac ultrasound systems.

Real-time vector velocity assessment through multigate Doppler and plane waves

Several ultrasound (US) methods have been recently proposed to produce 2-D velocity vector fields with high temporal and spatial resolution. However, the real-time implementation in US scanners is heavily hampered by the high calculation power required. In this work, we report a real-time vector Doppler imaging method which has been integrated in an open research system. The proposed approach exploits the plane waves transmitted from two sub-arrays of a linear probe to estimate the velocity vectors in 512 sample volumes aligned along the probe axis. The method has been tested for accuracy and reproducibility through simulations and in vitro experiments. Simulations over a 0° to 90° angle range of a 0.5 m/s peak parabolic flow have yielded 0.75° bias and 1.1° standard deviation for direction measurement, and 0.6 cm/s bias with 3.1% coefficient of variation for velocity assessment. In vitro tests have supported the simulation results. Preliminary measurements on the carotid artery of a volunteer have highlighted the real-time system capability of imaging complex flow configurations in an intuitive, easy, and quick way, as shown in a sample supplementary movie. These features have allowed reproducible peak velocity measurements to be obtained, as needed for quantitative investigations on patients.

Implementation of parallel transmit beamforming using orthogonal frequency division multiplexing--achievable resolution and interbeam interference

The speed of sound in the human body limits the achievable data acquisition rate of pulsed ultrasound scanners. To overcome this limitation, parallel beamforming techniques are used in ultrasound 2-D and 3-D imaging systems. Different parallel beamforming approaches have been proposed. They may be grouped into two major categories: parallel beamforming in reception and parallel beamforming in transmission. The first category is not optimal for harmonic imaging; the second category may be more easily applied to harmonic imaging. However, inter-beam interference represents an issue. To overcome these shortcomings and exploit the benefit of combining harmonic imaging and high data acquisition rate, a new approach has been recently presented which relies on orthogonal frequency division multiplexing (OFDM) to perform parallel beamforming in transmission. In this paper, parallel transmit beamforming using OFDM is implemented for the first time on an ultrasound scanner. An advanced open platform for ultrasound research is used to investigate the axial resolution and interbeam interference achievable with parallel transmit beamforming using OFDM. Both fundamental and second-harmonic imaging modalities have been considered. Results show that, for fundamental imaging, axial resolution in the order of 2 mm can be achieved in combination with interbeam interference in the order of -30 dB. For second-harmonic imaging, axial resolution in the order of 1 mm can be achieved in combination with interbeam interference in the order of -35 dB.

Accuracy and reproducibility of a novel dynamic volume flow measurement method

In clinical practice, blood volume flow (BVF) is typically calculated assuming a perfect parabolic and axisymmetric velocity distribution. This simple approach cannot account for the complex flow configurations that are produced by vessel curvatures, pulsatility and diameter changes and, therefore, results in a poor estimation. Application of the Womersley model allows compensation for the flow distortion caused by pulsatility and, with some adjustment, the effects of slight curvatures, but several problems remain unanswered. Two- and three-dimensional approaches can acquire the actual velocity field over the whole vessel section, but are typically affected by a limited temporal resolution. The multigate technique allows acquisition of the actual velocity profile over a line intersecting the vessel lumen and, when coupled with a suitable wall-tracking method, can offer the ideal trade-off among attainable accuracy, temporal resolution and required calculation power. In this article, we describe a BVF measurement method based on the multigate spectral Doppler and a B-mode edge detector algorithm for wall-position tracking. The method has been extensively tested on the research platform ULA-OP, with more than 1700 phantom measurements at flow rates between 60 and 750 mL/min, steering angles between 10 ° and 22 ° and constant, sinusoidal or pulsed flow trends. In the averaged BVF measurement, we found an underestimation of about -5% and a coefficient of variability (CV) less than 6%. In instantaneous measurements (e.g., systolic peak) the CV was in the range 2%-8.5%. These results were confirmed by a preliminary test on the common carotid artery of 10 volunteers (CV = 2%-11%).

CREANUIS: a non-linear radiofrequency ultrasound image simulator

Nonlinear ultrasound methods are widely used in clinical applications for tissue or contrast harmonic imaging. Accurate non-linear imaging simulation tools are required in research studies for the development of new methods. However, in existing simulators, the possible inhomogeneity of the coefficient of non-linearity, which is generally observed in tissue and in particular when contrast agents are involved, has not yet been implemented. This article describes a new ultrasound simulator, called CREANUIS, devoted to the computation of B-mode images where both linear and non-linear propagation in media is considered, with a possible inhomogeneous coefficient of non-linearity. The resulting fundamental images, based on a spatially variant and non-linear point spread function, are in accordance with those obtained through the reference linear FieldII simulator, with computation time reduced by a factor of at least 1.8. Non-linear images of media exhibiting inhomogeneous coefficients of non-linearity are also provided. The simulation software can be freely downloaded from our website.

Finding the peak velocity in a flow from its Doppler spectrum

The signal backscattered by blood cells crossing a sample volume produces a Doppler power spectrum determined by the scatterers¿ velocity distribution. Because of intrinsic spectral broadening, the peak Doppler frequency observed does not correspond to the peak velocity in the flow. Several methods have been proposed for estimating the maximum velocity component--an important clinical parameter--but these methods are approximate, based on heuristic thresholds that can be inaccurate and strongly affected by noise. Reported here is a method of modeling the Doppler power spectrum of a flow, and from that model, determining what Doppler frequency on the descending slope of the power spectrum corresponds to the peak velocity in the insonated flow. It is shown that, for a fully insonated flow with a parabolic velocity distribution, the peak velocity corresponds to the Doppler frequency at the half-power point on that slope. The method is demonstrated to be robust with regard to the effects of noise and valid for a wide range of acquisition parameters. Experimental maximum velocity measurements on steady flows with rates between 100 and 300 mL/min (peak velocity range 6.6 cm/s to 19.9 cm/s) show a mean bias error that is smaller than 1%.

Correspondence - Nonlinear oscillations of deflating bubbles

Phospholipid-coated ultrasound contrast agents may deflate or even collapse because of stress resulting from ultrasound-induced oscillations. In this work, we investigate the behavior of isolated contrast agent microbubbles during prolonged ultrasound excitation. Isolated microbubbles placed in a thin capillary tube were excited with hundreds of ultrasound pulses at a low mechanical index, and their oscillations were recorded using the Brandaris-128 ultra-high-speed camera. Results show that microbubbles undergo an irreversible, non-destructive deflation process. Such deflation seems to occur in discrete steps rather than as a continuous process; furthermore, the dynamics of the bubble change during deflation: radial oscillations, both symmetric and asymmetric around the resting radius of the bubble, occur at various stages of the deflation process. Strongly asymmetric oscillations, such as compression-only and expansion-only behavior, were also observed: notably, expansion-only behavior is associated with a rapid size reduction, whereas compression-only behavior mostly occurs without a noticeable change of the bubble radius. We hypothesize that bubble deflation results from at least two distinct phenomena, namely diffusive gas loss and lipid material shedding from the encapsulating shell.

Embedded Doppler system for industrial in-line rheometry

Rheological fluid behavior characterization is crucial for the industrial production of cosmetics, food, pharmaceutics, adhesive, sealants, etc. For example, the measurement of specific rheological features at every step of the production chain is critical for product quality control. Such measurements are often limited to laboratory tests on product specimens because of technical difficulties. In this work, we present an embedded system suitable for in-line rheometric evaluation of highly filled polyurethane-based adhesives. This system includes an ultrasound front-end and a digital signal processing section integrated in a low-cost field-programmable gate array. The system measures the real-time velocity profile developed in the pipe by the fluid, employing a Doppler multigate technique. The high-resolution velocity profile, combined with a pressure drop measurement, allows an accurate evaluation of the flow consistency index, K, and the flow behavior index, n, of the interrogated fluid.

A reconfigurable and programmable FPGA-based system for nonstandard ultrasound methods

The availability of programmable and reconfigurable ultrasound (US) research platforms may have a considerable impact on the advancement of ultrasound systems technology; indeed, they allow novel transmission strategies or challenging processing methods to be tested and experimentally refined. In this paper, the ULtrasound Advanced Open Platform (ULA-OP), recently developed in our University laboratory, is shown to be a flexible tool that can be easily adapted to a wide range of applications. Five nonstandard working modalities are illustrated. Vector Doppler and quasi-static elastography applications emphasize the real-time potential and versatility of the system. Flow-mediated dilation, pulse compression, and high-frame-rate imaging highlight the flexibility of data access at different points in the reception chain. For each modality, the role played by the onboard programmable devices is discussed. Experimental results are reported, indicating the relative performance of the system for each application.

Frequency-domain-based strain estimation and high-frame-rate imaging for quasi-static elastography

In freehand elastography, quasi-static tissue compression is applied through the ultrasound probe, and the corresponding axial strain is estimated by calculating the time shift between consecutive echo signals. This calculation typically suffers from a poor signal-to-noise ratio or from the decorrelation between consecutive echoes resulting from an erroneous axial motion impressed by the operator. This paper shows that the quality of elastograms can be improved through the integration of two distinct techniques in the strain estimation procedure. The first technique evaluates the displacement of the tissue by analyzing the phases of the echo signal spectra acquired during compression. The second technique increases the displacement estimation robustness by averaging multiple displacement estimations in a high-frame-rate imaging system, while maintaining the typical elastogram frame-rate. The experimental results, obtained with the Ultrasound Advanced Open Platform (ULA-OP) and a cyst phantom, demonstrate that each of the proposed methods can independently improve the quality of elastograms, and that further improvements are possible through their combination.

Simultaneous ultrasound assessment of brachial artery shear stimulus and flow-mediated dilation during reactive hyperemia

In flow-mediated dilation (FMD) studies, brachial artery diameter changes due to reactive hyperaemia are typically measured through manual or automatic analysis of high resolution B-mode images while the stimulus of diameter change, i.e., the flow change, is qualitatively estimated by measuring the mean velocity in the vessel and assuming a parabolic velocity profile. This article describes an experimental approach to simultaneously measure the wall shear rate (WSR) and the diameter variations, through multigate spectral Doppler and B-mode image processing, respectively. By using an ultrasound advanced open platform (ULA-OP), experimental results from the brachial arteries of 15 presumed healthy volunteers have been obtained. The mean increments during reflow against baseline were 105% ± 22% for the peak WSR and 8% ± 3% for the FMD. The mean time interval between the WSR peak and the beginning of plateau of diameter waveform was 38 ± 8 s. The results confirm that in young healthy subjects the postischemic vasodilation of brachial artery is largely correlated to the WSR increase.

Fundamental and second-harmonic ultrasound field computation of inhomogeneous nonlinear medium with a generalized angular spectrum method

The simulation of nonlinear propagation of ultrasound waves is typically based on the Kuznetsov-Zabolotskaya- Khokhlov equation. A set of simulators has been proposed in the literature but none of them takes into account a possible spatial 3-D variation of the nonlinear parameter in the investigated medium. This paper proposes a generalization of the angular spectrum method (GASM) including the spatial variation of the nonlinear parameter. The proposed method computes the evolution of the fundamental and second-harmonic waves in four dimensions (spatial 3-D and time). For validation purposes, the one-way fields produced by the GASM are first compared with those produced by established reference simulators and with experimental one-way fields in media with a homogeneous nonlinear parameter. The same simulations are repeated for media having an axial variation of the nonlinear parameter. The mean errors estimated in the focal region are less than 4.0% for the fundamental and 5.4% for the second harmonic in all cases. Finally, the fundamental and second-harmonic fields simulated for media having nonlinear parameter variations in the axial, lateral, and elevation directions, which cannot be simulated with other currently available methods, are presented. The new approach is also shown to yield a reduction in computation time by a factor of 13 with respect to the standard nonlinear simulator.

Extensions of nonlinear B/A parameter imaging methods for echo mode

This paper investigates nonlinear B/A parameter imaging in ultrasound echo mode. First, the B/A estimation methods which can be extended to echo mode are identified. The finite amplitude approaches are found to be excellent candidates to assess the nonlinear parameter because of their experimental simplicity, supported by a strong mathematical background. Second, three nonlinear coefficient measurement methods, thus far proposed for applications in homogeneous media, are extended to heterogeneous media. In particular, the simulations show that the extended comparative method (ECM) offers the best results when the probe diffraction effects are taken into consideration. The first experimental images obtained by applying the ECM for two different phantoms are also presented, showing the feasibility of B/A imaging.

Estimation of the viscoelastic properties of vessel walls using a computational model and Doppler ultrasound

Human arteries affected by atherosclerosis are characterized by altered wall viscoelastic properties. The possibility of noninvasively assessing arterial viscoelasticity in vivo would significantly contribute to the early diagnosis and prevention of this disease. This paper presents a noniterative technique to estimate the viscoelastic parameters of a vascular wall Zener model. The approach requires the simultaneous measurement of flow variations and wall displacements, which can be provided by suitable ultrasound Doppler instruments. Viscoelastic parameters are estimated by fitting the theoretical constitutive equations to the experimental measurements using an ARMA parameter approach. The accuracy and sensitivity of the proposed method are tested using reference data generated by numerical simulations of arterial pulsation in which the physiological conditions and the viscoelastic parameters of the model can be suitably varied. The estimated values quantitatively agree with the reference values, showing that the only parameter affected by changing the physiological conditions is viscosity, whose relative error was about 27% even when a poor signal-to-noise ratio is simulated. Finally, the feasibility of the method is illustrated through three measurements made at different flow regimes on a cylindrical vessel phantom, yielding a parameter mean estimation error of 25%.

An automatic angle tracking procedure for feasible vector Doppler blood velocity measurements

Two-dimensional angle-independent blood velocity estimates typically combine the Doppler frequencies independently measured by two ultrasound beams with known interbeam angle. A different dual-beam approach was recently introduced in which one (reference) beam is used to identify the flow direction, and the second (measuring) beam directly estimates the true flow velocity at known beam-flow angle. In this paper, we present a procedure to automatically steer the two beams along optimal orientations so that the velocity magnitude can be measured. The operator only takes care of locating the Doppler sample volume in the region of interest and, through the extraction of appropriate parameters from the Doppler spectrum, the reference beam is automatically steered toward right orientation to the flow. The velocity magnitude is thus estimated by the measuring beam, which is automatically oriented with respect to the (known) flow direction at a suitable Doppler angle. The implementation of the new angle tracking method in the ULtrasound Advanced Open Platform (ULA-OP), connected to a linear array transducer, is reported. A series of experiments shows that the proposed method rapidly locks the flow direction and measures the velocity magnitude with low variability for a large range of initial probe orientations. In vitro tests conducted in both steady and pulsatile flow conditions produced coefficients of variability (CV) below 2.3% and 8.3%, respectively. The peak systolic velocities have also been measured in the common carotid arteries of 13 volunteers, with mean CV of 7%.