Transverse Doppler spectral analysis for a correct interpretation of flow sonograms

Real-Time 3-D Spectral Doppler Analysis With a Sparse Spiral Array

2-D sparse arrays may push the development of low-cost 3-D systems, not needing to control thousands of elements by expensive application-specific integrated circuits (ASICs). However, there is still some concern about their suitability in applications, such as Doppler investigation, which inherently involve poor signal-to-noise ratios (SNRs). In this article, a novel real-time 3-D pulsed-wave (PW) Doppler system, based on a 256-element 2-D spiral array, is presented. Coded transmission (TX) and matched filtering were implemented to improve the system SNR. Standard sonograms as well as multigate spectral Doppler (MSD) profiles, along lines that can be arbitrarily located in different planes, are presented. The performance of the system was assessed quantitatively on experimental data obtained from a straight tube flow phantom. An SNR increase of 11.4 dB was measured by transmitting linear chirps instead of standard sinusoidal bursts. For a qualitative assessment of the system performance in more realistic conditions, an anthropomorphic phantom of the carotid arteries was used. Finally, real-time B-mode and MSD images were obtained from healthy volunteers.

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).

Ultrasound Vector Flow Imaging-Part I: Sequential Systems

This paper gives a review of the most important methods for blood velocity vector flow imaging (VFI) for conventional sequential data acquisition. This includes multibeam methods, speckle tracking, transverse oscillation, color flow mapping derived VFI, directional beamforming, and variants of these. The review covers both 2-D and 3-D velocity estimation and gives a historical perspective on the development along with a summary of various vector flow visualization algorithms. The current state of the art is explained along with an overview of clinical studies conducted and methods for presenting and using VFI. A number of examples of VFI images are presented, and the current limitations and potential solutions are discussed.

Comparison of carotid artery blood velocity measurements by vector and standard Doppler approaches

Although severely affected by the angle dependency, carotid artery peak systolic velocity measurements are widely used for assessment of stenosis. In this study, blood peak systolic velocities in the common and internal carotid arteries of both healthy volunteers and patients with internal carotid artery stenosis were measured by two vector Doppler (VD) methods and compared with measurements obtained with the conventional spectral Doppler approach. Although the two VD techniques were completely different (using the transmission of focused beams and plane waves, respectively), the measurement results indicate that these techniques are nearly equivalent. The peak systolic velocities measured in 22 healthy common carotid arteries by the two VD techniques were very close (according to Bland-Altman analysis, the average difference was 3.2%, with limits of agreement of ± 8.6%). Application of Bland-Altman analysis to comparison of either VD technique with the spectral Doppler method provided a 21%-25% average difference with ± 13%-15% limits of agreement. Analysis of the results obtained from 15 internal carotid arteries led to similar conclusions, indicating significant overestimation of peak systolic velocity with the spectral Doppler method. Inter- and intra-operator repeatability measurements performed in a group of 8 healthy volunteers provided equivalent results for all of the methods (coefficients of variability in the range 2.7%-6.9%), even though the sonographers were not familiar with the VD methods. The results of this study suggest that the introduction of vector Doppler methods in commercial machines may finally be considered mature and capable of overcoming the angle-dependent overestimation typical of the standard spectral Doppler approach.

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.

Progress in atherosclerotic plaque imaging

Cardiovascular diseases are the primary cause of mortality in the industrialized world, and arterial obstruction, triggered by rupture-prone atherosclerotic plaques, lead to myocardial infarction and cerebral stroke. Vulnerable plaques do not necessarily occur with flow-limiting stenosis, thus conventional luminographic assessment of the pathology fails to identify unstable lesions. In this review we discuss the currently available imaging modalities used to investigate morphological features and biological characteristics of the atherosclerotic plaque. The different imaging modalities such as ultrasound, magnetic resonance imaging, computed tomography, nuclear imaging and their intravascular applications are illustrated, highlighting their specific diagnostic potential. Clinically available and upcoming methodologies are also reviewed along with the related challenges in their clinical translation, concerning the specific invasiveness, accuracy and cost-effectiveness of these methods.

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%.

Accuracy and reproducibility of a novel dual-beam vector Doppler method

Conventional Doppler ultrasound (US) investigations are limited to detect only the axial component of the blood velocity vector. A novel dual-beam method has been recently proposed in which the Doppler angle is estimated through a reference US beam, and the velocity magnitude through a measuring US beam, respectively. In this study, the performance of such a method has been assessed quantitatively through in vitro and in vivo measurements made in different experimental conditions. In vitro, more than 300 acquisitions were completed using seven transducers to insonify a straight tube phantom at different Doppler angles. In steady laminar flow conditions, the velocity magnitude was measured with mean error of -1.9% (95% confidence interval: -2.33% to -1.47%) and standard deviation of 3.4%, with respect to a reference velocity. In pulsatile flow conditions, reproducibility tests of the entire velocity waveforms provided an average coefficient of variation (CV) of 6.9%. For peak velocity measurements made at five Doppler angles and three flow rates, the intrasession and intersession CVs were in the range 0.8-3.7% and 2.9-10.6%, respectively. The peak systolic velocities (PSVs) in the common carotid arteries of 21 volunteers were estimated with 95% limits of agreement of +/- 9.6 cm/s (intersession). This analysis shows that the proposed dual-beam method is capable of overcoming the Doppler angle ambiguity by producing reliable velocity measurements over a large set of experimental conditions.

Noninvasive simultaneous assessment of wall shear rate and wall distension in carotid arteries

A novel technique has been developed for the noninvasive real-time simultaneous assessment of both blood velocity profile and wall displacements in human arteries. The novel technique is based on the use of two ultrasound beams, one set at optimal angle for wall motion measurements and the other for blood velocity profile measurements. The technique was implemented on a linear array probe divided into two subapertures. A modified commercial ultrasound machine and a custom PC board based on a high-speed digital signal processor was used to process the quadrature demodulated echo signals and display results in realtime. Flow phantom experiments demonstrated the validity of the technique, providing wall shear rate (WSR) estimates within 10% of the theoretical values. The system was also tested in the common carotid arteries of 16 healthy volunteers (age 30 to 53 y). Results of simultaneous diameter distension and WSR measurements were in agreement with published data.

Accurate Doppler angle estimation for vector flow measurements

Traditional Doppler methods measure only the axial component of the velocity vector. The lack of information on the beam-flow angle creates an ambiguity that can lead to large errors in velocity magnitude estimates. Different triangulation techniques so far have been proposed, which basically perform multiple measurements of the Doppler frequency shift originating from the same region. In this work, an original approach is introduced, in which two ultrasound beams with known relative orientation are directed toward the same vessel, but only one of them is committed to perform a Doppler measurement; the second (reference) beam has the specific task of detecting the beam-flow angle. The latter goal is obtained by accurately identifying the achievement of the target 900 reference-beam-to-flow angle through the inspection of the backscattered Doppler signal spectrum. In transverse flow conditions, in fact, such spectrum is expected to be centered on the zero frequency, and even small deviations from the desired 900 orientation cause noticeable losses of spectral symmetry. Validation of the new method has been performed through experimental tests, which show that the beam-flow angle can be estimated with high accuracy (rms errors lower than 1 degree), and repeatable velocity magnitude measurements are possible. A procedure for automatically tracking the desired orientation by the reference beam is also introduced and shown suitable for implementation in steerable linear array transducers.

Clinical validation of common carotid artery wall distension assessment based on multigate Doppler processing

Mechanical properties of human large arteries result from the interaction between blood pressure, wall distensibility and shear stress. Both the arterial diameter changes through the cardiac cycle (distension) and blood flow velocities can be noninvasively investigated through Doppler ultrasound approaches. Recently, an integrated system processing in real-time all the echo signals produced along an M-line has been developed. This system has been so far demonstrated to be suitable for accurate hemodynamic studies through the detection of blood velocity profiles. This paper reports on the extension of its processing capabilities to the real-time measurement of arterial distension. Tissue motion estimation is based on a modified 2-D autocorrelation algorithm. A novel adaptive approach to track wall position over time using the sum of the high-pass filtered displacement waveform and the low-pass filtered wall position is described. By observing the blood velocity profile, a rapid and accurate positioning of the ultrasound probe and an inherent check on perpendicular observation are provided. First clinical results obtained by measuring the distension of common carotid arteries in a group of 41 volunteers are reported and measurements are validated against those provided by a dedicated wall-track reference system. Average measured distension and diameter were 499 +/- 188 microm and 6.90 +/- 0.66 mm and intraobserver intrasession reproducibility tests showed coefficients of variability of 8.5% and 5.9%, respectively. The agreement between the proposed system and the reference system, expressed as bias +/- 2 SD of the differences, was -34 +/- 141 microm for distension and 0.05 +/- 1.07 mm for diameter.

A simplified approach for real-time detection of arterial wall velocity and distension

Arterial stiffness is known to increase with age and with many vascular diseases, but its noninvasive assessment in patients still represents a difficult task. The measurement of diameter change during the cardiac cycle (distension) has been proposed as a means to estimate arterial compliance and stiffness. Therefore, we have developed a simple PC-based device and algorithm for noninvasive quantification of vessel wall motion and diameter change in humans. This goal is achieved in real-time by processing the base-band signals from a commercial ultrasound Doppler system. Real-time operation is of crucial importance, because it allows a rapid achievement of optimal measurement conditions. The system was evaluated in a laboratory using a string phantom and was tested on the carotid arteries of 10 volunteers. Wall velocities from 0.05 to 600 mm/s and displacements lower than 2 microns were detected with phantoms. The measured carotid diameter change in the volunteers ranged from 7.5 to 11.8% (mean = 9.8%) and agrees closely with values reported in the literature. The difference between values taken one hour apart ranged from 0.2 to 0.5%. We conclude that the new system provides rapid, accurate, and repeatable measurements of vessel distension in humans.

Doppler angle and flow velocity estimations using the classic and transverse Doppler effects

Current clinical Doppler ultrasound systems could only measure the flow vector parallel to the ultrasound beam axis, and the knowledge of the Doppler angle (beam-to-flow angle) is needed to calculate the real flow velocity. Currently, the Doppler angle is determined visually by manually aligning a vessel axis marker along the blood vessel on the duplex scan image of the ultrasound. The application of this procedure is often limited by practical constraints; therefore, measurements are not reliable. In order to overcome this problem, the authors developed a simple Doppler angle and flow velocity estimation method using a combination of the classic and transverse Doppler effects. This method uses only a single focused annular array transducer to estimate the Doppler angle and the flow velocity. The authors have verified experimentally that this method is successful for measuring constant flow in a flow phantom between 45 degrees and 80 degrees Doppler angle. The standard deviation of the estimated Doppler angles is less than 4.5 degrees . This method could be implemented easily in medical Doppler ultrasound systems to automatically estimate the Doppler angle and the flow velocity.

An FFT-based flow profiler for high-resolution in vivo investigations

Pulsed Doppler spectral analysis is a well-established diagnostic technique in the assessment of arterial diseases. Because of hardware limitations, its use has been so far restricted to the analysis of a single sample volume located along the ultrasound beam axis. In this paper, we discuss the operation of a newly developed multigate instrument capable of performing, in real time, 64-point fast Fourier transforms of Doppler signals sampled from 64 different range cells. The new instrument is capable of accurately detecting the actual blood flow behavior in major human vessels. Significant examples of velocity profiles obtained in real time from carotid arteries in healthy subjects are reproduced here for the first time. Multigate extension of spectral analysis is demonstrated to be a suitable means for detailed in vivo investigation of blood flow dynamics.

Improved blood velocity estimation using the maximum Doppler frequency

In vessels whose diameter is smaller than the length of the range cell or measurement volume, the maximum blood velocity is often calculated from the maximum frequency of the Doppler spectrum, using the classical Doppler equation. It is shown that the accuracy of this procedure is significantly improved at large beam-to-flow angles, if a correction for transit time broadening is made. This finding is based on the demonstration that the maximum frequency of the Doppler spectrum depends only on the maximum velocity passing through the measurement volume, but in a manner which is a function both of the Doppler shift frequency as well as the transit time broadening associated with the passage of scatterers through the beam width.

Invariance of the Doppler bandwidth with range cell size above a critical beam-to-flow angle

For a sound beam impinging on a blood vessel, with a range cell much smaller than the vessel diameter, it is known that the breadth of the echo Doppler spectrum is proportional to the velocity of the flow through the range cell. As the range cell is lengthened to include a greater range of velocities, the spectrum is expected to widen proportionately. It is shown theoretically, and confirmed experimentally, that if the beam-to-flow angle is greater than a critical value, the Doppler spectrum bandwidth is independent of the length of the range cell, and depends only on the maximum velocity encompassed by it. This happens because for angles greater than the critical, the narrow spectra produced by lower velocity flows near the vessel walls are contained within the broader spectrum produced by the higher speed flow near the vessel axis. The critical angle is the angle at which the flow axis is normal to one of the beam edges.