Velocity magnitude estimation with linear arrays using Doppler bandwidth

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.

Effect of dual-frequency clutter suppression in harmonic Doppler detection

BACKGROUND

High-frequency Doppler imaging is highly potential for detection of blood flow in microcirculation. In a swept-scan system, however, the spectral broadening of tissue clutter limits the detectability of low-velocity flow signal. Conventionally, the scanning speed of transducer has to be reduced to alleviate the clutter interference but at the cost of imaging frame rate. For example, the blood velocity of 0.5mm/s becomes detectable only with a scanning speed lower than 1mm/s. In this study, an alternative method is examined by suppressing the clutter magnitude to reduce the interference to flow signal without sacrificing scanning speed.

METHODS

The method of third harmonic (3f(0)) transmit phasing can suppress the tissue harmonic clutter by transmitting at the fundamental and the additional 3f(0) frequencies to achieve mutual cancellation between the frequency-sum and the frequency-difference components of the second harmonic signal. With 3f(0) transmit phasing, the cut-off frequency of wall filtering can be reduced to preserve low-velocity flow without compromising the frame rate.

RESULTS

Our results indicate that the 3f(0) transmit phasing effectively reduces the harmonic clutter magnitude and thus improves the flow signal-to-clutter ratio. Compared to the conventional counterpart, the clutter-suppressed color flow and power Doppler images show fewer clutter artifacts and is capable of detecting more low-velocity flow of microbubbles. The resultant color-pixel-density also improves with clutter suppression.

CONCLUSION

For the swept-scan high-frequency (>20MHz) system, 3f(0) transmit phasing is capable of providing effective clutter suppression. With the same achievable scanning speed, the resultant Doppler image has higher sensitivity for low-velocity flow and is less susceptible to clutter artifacts.

Transit-time broadening in pulsed Doppler ultrasound: a generalized amplitude modulation model

In Doppler ultrasound, transit-time broadening arises from the finite scatterer transit time through the sample volume. As a unifying description of this broadening mechanism, a generalized amplitude modulation signal model was developed to collectively account for the transit-time effects of the ultrasound beam geometry and the range gate characteristics. Simulations based on a pulsed linear-array system also were performed to study the broadening extent for different scatterer flow lines. With our signal model and simulation results, some generalized insights were obtained on the characteristics of transit-time broadening. First, as consistent with previous findings, we found that, for scatterers passing though the center of the sample volume, the broadening extent mainly depends on beam-forming characteristics at higher beam-flow angles, but it is more dependent on range gate parameters at smaller angles. Second, for the central flow line, a transition angle exists in which a significant change occurs in the governing parameters of transit-time broadening. Third, for the general case in which scatterers undertake an off-central path through the sample volume, the broadening extent depends on both the beam geometry and the range gate. Bandwidth skewing and further spectral broadening also can be seen for these off-central flow lines.

Effect of vessel curvature on Doppler derived velocity profiles and fluid flow

Side-branches and curvatures in the arterial tree yield deviations from the axial oriented velocity. Velocity or volume flow estimates based on the assumption that flow is axially oriented are of limited value at these sites. This article evaluates information obtainable by using a multigate Doppler ultrasound (US) instrument used with curved phantoms, which resemble the human coronary arteries. The comparison of experimental velocity data with data provided by an accurate computational fluid dynamics (CFD) method shows differences in the range of 4 to 11% for four curvatures with different radii. Multigate data are also used to estimate the volume flow in the curved segments at different experimental conditions. An error lower than 15% is obtained, to be compared with a 24% error obtained by assuming a parabolic velocity profile. In particular, it is shown that the residual error is not related to the small deviation of the velocity vectors from the axial direction due to the presence of secondary velocity components, which are found to be of magnitude less than 10% with respect to the axial velocity component.

Sample volume shape for pulsed-flow velocity estimation using a linear array

Various definitions of the sample volume (SV) shape have been proposed, but they are mostly based on transducers with axisymmetrical geometry. We have defined the SV as that spatial region in which scatterers contribute a component to the total gated received-signal energy above a defined threshold. This definition is consistent with modern pulsed transducer arrays and accounts for the need to impose a signal/noise threshold. Based on this definition, SVs for a typical linear phased-array transducer were simulated using custom-designed software. The effects of different transmit pulses, receive gates, apertures, SV depths and lateral foci were studied using a one-dimensional (1-D) beam-forming array, with a fixed lens in the elevation direction. Based on a simplified method of analysis, the features of the beam-steered SV are qualitatively similar to those of the nonsteered SV, when compared at the same beam-flow angle. These studies have helped provide a clearer understanding of the manner in which the SV energy distribution is affected by various parameters. The results can have potentially significant implications in the use of ultrasound (US) for blood velocity estimation, specifically with respect to locating the SV within the blood vessel and the origin of the velocity spectrum.