- Pictorial Review
- Open Access
High-frame rate vector flow imaging of the carotid bifurcation
© The Author(s) 2017
- Received: 7 March 2017
- Accepted: 13 April 2017
- Published: 12 May 2017
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.
• 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.
- Vector flow imaging
- Plane wave imaging
- Carotid arteries
Carotid artery atherosclerotic disease is still a significant cause of cerebrovascular morbidity and mortality .
Blood flow alteration and inflammation, in addition to systemic risk factors, are considered possible causes for the development of atherosclerotic lesions . Some studies, performed to analyze the flow in the carotid arteries, showed that the development of arterial plaques is more frequent in the presence of a vortex flow [3, 4]. Color Doppler (CD), for evaluation of flow patterns, and spectral Doppler analysis (PW), for measurement of blood velocities, have been used to detect flow disturbances in the carotid bifurcation [5–9]. Although these studies showed that complex flow patterns are detectable, CD and PW are angle-dependent and only estimate the axial component of blood flow velocity; consequently, the quantification of complex flow is not achievable with conventional ultrasound (US) systems. Moreover, CD is also affected by a limited frame rate, allowing low temporal resolution; PW displays the complete spectrum of velocities through the cardiac cycle, but related to a small sample volume and along a single line only. These limitations explain why, in the last decades, the flow complexity analysis was not used for clinical diagnosis or for long-term prognosis and the Doppler evaluation of abnormal flow velocity has been restricted to the grading of vessel stenosis only .
In recent years, some manufacturers tried to introduce different techniques with the aim to better describe the flow complexity in the carotid artery and other vessels. A new angle-independent technique, measuring and visualizing blood flow velocities in all directions, called vector flow imaging (VFI), has been proposed . VFI is an operator-independent technique that can provide more intuitive and quantitative imaging of vortex formation, which is not clearly distinguishable in the CD images. Various VFI methods of estimation principles can be used [12–23]. Among the various methods of estimation suggested, the one based on phase shift estimation with transverse oscillations (TO)  and the other based on plane-wave imaging (PWI) [19–23] were implemented on commercial systems, thus ready for clinical application. Almost all vendors are rapidly equipping their US scanners with VFI.
The PWI methods estimate the 2D vector velocity of the flow at higher frame rate than the TO method, allowing better depiction of the complex flows. This educational material was collected by using a system equipped with VFI based on a multi-angle transmission plane waves method, which allows a very high frame rate of about 500 Hz . Such a high frame rate offers a detailed visualization of complex flow by showing even transient events, otherwise undetectable.
This pictorial review will consider which new information VFI adds for the knowledge of hemodynamics in comparison to the conventional US techniques.
High-frame rate VFI
The high-frame rate VFI is based on PWI. Acquisition of flow vector information at high frame rates is obtained by performing multi-directional transmissions of plane waves; after a single plane wave transmission, multiple image receiving lines are obtained . It allows calculation of the true velocity vectors at any location in a vessel. The dynamic flow is obtained by continuously updating the target’s position according to the calculated velocity. The interleaved transmissions ensure both a highly sensitive vector flow image and a high-resolution B-mode images.
The flow is analyzed by the system for 1.5 s at a pulse repetition frequency (PRF) of 10–15 kHz and at a very high frame rate of 400–600 Hz, depending on the used PRF, allowing to study at least one cardiac cycle. The data are reprocessed automatically by the US system in a 35–36-s clip, generating a sequence of about 600–900 images that can be displayed at a frame rate of 20–30 Hz. The acquired data can be further evaluated in the saved video. Such a high frame rate allows a detailed analysis of hemodynamics. V Flow detects the speed and direction of all blood cells flowing through every point of the region of interest (ROI). There are low-speed cells, high-speed blood cells, and reverse cells flowing through a point in a short moment. It means that the speed measured and displayed by V Flow in a point is the average speed of all blood cells in a precise short moment. Spatiotemporal characteristics of flow can be evaluated visually and quantitatively to asses the specific flow pattern. VFI shows velocity vectors, streamline distribution and vorticity distribution. The streamline distribution uses arrows to indicate the flow direction. The color and length of the arrows show the flow velocity, magnitude and direction (green means low velocities, yellow and orange medium velocities and red higher velocities; the longer the arrows the faster the flow). For quantitative evaluation, velocity curves are available: the maximum velocity vector point curve, automatically detected by the system, and the user-defined vector point curve. Both are displayed at the bottom of the image and show the fluctuating velocities of the flow varying in subsequent cardiac cycles.
Hemodynamic: Brief review
As demonstrated by basic laws of physics, the flow movement depends on various factors, such as: the pushing force, the pressure gradients and the frictional effect of the viscosity relative to the vessel boundaries and between streamlines sliding at different velocities over each other.
Laminar flow is widespread in the body circulation and is found in fairly large and straight arteries. In presence of laminar flow, the front wave is close to a parabolic profile and the directions of streamlines remain almost parallel to the boundaries.
In particular conditions, the laminar flow tends to become unstable, mixing with eddies and counter eddies: the initial phase of this transition stage consists of perturbations within the boundary layer interacting with shape discontinuities, in particular, changes of vessel lumen diameter, surface curvatures and roughness or velocity changes and relatively high flow velocity [24, 25]. As a consequence of the velocity difference between the flow and the wall boundary, the boundary layer adjacent to the tissue, also known as shear layer, develops vorticity. In other words, where streamlines separate from the wall, the fluid tends to curl into a vortex. This vortex formation process occurs in vessels presenting significant flow decelerations, like in the carotid sinus .
Various disturbed flow features were described: a) helical flow, which is a rotation around an axis of flow; b) recirculation, that consists in movement of streamlines from a forward stream back into a separation zone, for example, beyond a plaque; c) turbulence, meaning a regime characterized by randomly and rapidly fluctuating velocities, which may happen after a straight stenosis, creating unsteady vortices of many different sizes that increase friction and energy dissipation . In certain circumstances, these unusual hemodynamic conditions generate an abnormal biological response. Velocity profile skewing can, in fact, create pockets in which the direction of the wall shear stress oscillates, resulting in the development of atherosclerosis .
Blood flow visualization
Diagnostic US applied at the carotid bifurcation offers the possibility of analyzing both anatomy and hemodynamics.
B-mode US is considered the best method for demonstrating arterial wall thickness and plaques. In severe disease, cross-sectional images of the plaques are difficult to generate because of calcium shadowing or reverberations .
VFI laminar flow pattern
VFI disturbed flow pattern’s relationship with the vessel geometry
A specific feature of the carotid bifurcation is the anatomic sinus at the origin of the internal carotid artery. The anatomical variations of the bifurcation angle and curved vessels can also increase the amount of flow instability. These aspects can lead to the detachment of the layers forming the laminar flow resulting in complex flow. Though flow reversal may be considered a normal phenomenon as suggested by some authors , it is well-known that, beyond a certain limit, disturbed flow may increase plaque formation [3, 4]. Thus, a technique such as VFI, able to recognize disturbed flow, may shed new light on the differences between pathological and physiological conditions.
VFI disturbed flow pattern’s relationship with atherosclerosis
CD cannot quantify the velocity and precise direction of the streamlines, but is very useful in the detection of areas of abnormal blood flow, which are investigated further using the spectral Doppler technique. Nevertheless, in presence of a low-degree stenosis, a condition in which the increase of velocity is not relevant, the role of conventional US Doppler technique is less evident. However, considering that plaque formation and their progression may be a result of disturbed flow, further characterization of the complex flow patterns should be investigated.
Comparison of technical characteristics between color Doppler and high-frame rate VFI
High-frame rate VFI
Multiangle plane waves
Flow visualization frame rate
Flow direction estimation
True velocity magnitude
Velocity vector measurement
Axial & lateral components
Beams angle dependence
As any method based on pulse repetition frequencies, even VFI, suffers from aliasing, which limits the possibility to study high-grade stenosis. Velocity scale on the system must be adapted to the hemodynamic findings in order to limit the artifact (Fig. 10, Vid 10b, 10c).
At the moment, VFI, as a two-dimensional (2D) technique, does not allow a precise comprehension of the actual three-dimensional (3D) shape of the streamline. A 3D vector flow method, which allows very high temporal resolution, will be needed to completely elucidate the complexity of hemodynamic patterns.
Despite the quantitative information on the flow, related to the vector velocities calculation, the evaluation of complex flow with VFI is still visual, thus subjected to intra- and inter-observer variability. To overcome this limitation, some quantitative tools will be necessary. Vortex extension and duration must be taken into account; even vector concentration as suggested by Pedersen et al. should be considered .
Conventional US methods have the ability to measure blood velocities and flow direction on the basis of the Doppler principle. Doppler US is angle-dependent, only estimates the axial component of blood flow velocity and is limited by the vessel geometry. This issue represents a significant limitation when applied to a “non straight” vessel such as the carotid bifurcation, the site of a vorticity pattern in the majority of subjects. VFI, as a 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.
The authors thank Mindray Bio-Medical Electronic Co (Shenzhen, China) for a Resona 7 ultrasound system provided to support this work.
Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
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