Progressive attenuation of the longitudinal kinetics in the common carotid artery: preliminary in vivo assessment

The influence of physical activity and sex on carotid artery longitudinal wall motion in younger healthy adults

Carotid artery longitudinal wall motion (CALM) is a novel preclinical marker for atherosclerosis that describes the axial anterograde and retrograde motion of the intima-media complex. While regular physical activity and sex are known to independently influence arterial stiffness, their roles on axial arterial wall behaviour are unknown. The purpose of this study is to examine whether physical activity and sex impact CALM. We hypothesized that CALM retrograde displacement and total amplitude would be greater in females and active individuals, as a function of arterial stiffness. Fifty-seven young healthy adults (30 females; aged 22 ± 3 years) were evaluated for CALM outcomes and arterial stiffness and grouped by physical activity based on active (V̇O2 = 44.2 ± 8.9 mL/kg/min) or sedentary (V̇O2 = 33.7 ± 6.7 mL/kg/min) lifestyles defined by the Canadian 24-Hour Movement Guidelines. Arterial stiffness and CALM were measured by carotid-femoral pulse wave velocity (cfPWV) and vascular ultrasound at the right common carotid artery with speckle tracking analysis, respectively. cfPWV was greater in males (p < 0.01) with no interaction between sex and physical activity (p = 0.90). CALM anterograde displacement was greater in males (p = 0.03) resulting in a forward shift in total CALM pattern, which became less prominent when controlling for mean arterial pressure (p = 0.06). All other CALM outcomes were not different between activity and sex. V̇O2max was not correlated to any CALM outcome (all p > 0.05). Apparent sex differences in vascular function extend to novel CALM outcomes but may be confounded by blood pressure. We recommend sex-balanced design and reporting in future studies due to possible anterograde-shifted CALM patterns in healthy males.

The impact of geometry, intramural friction, and pressure on the antegrade longitudinal motion of the arterial wall: A phantom and finite element study

Longitudinal motion of the carotid arterial wall, as measured with ultrasound, has shown promise as an indicator of vascular health. The underlying mechanisms are however not fully understood. We have found, in in vivo studies, that blood pressure has a strong relation to the antegrade longitudinal displacement in early systole. Further, we have identified that a tapered geometry and the intramural friction in-between two parts of a vessel wall influence the longitudinal displacement. We therefore studied the interaction between pressure, vessel geometry and intramural friction, tapered and straight ultrasound phantoms in a paralleled hydraulic bench study and corresponding numerical models. Profound antegrade longitudinal motion was induced in the innermost part of both tapered phantoms and the numerical models, but to a lesser extent when intramural friction was increased in the simulations. Strong correlations (R = 0.82-0.96; p < 1e-3; k = 9.3-14 μm/mmHg) between longitudinal displacement and pulse pressure were found in six of seven regions of interest in tapered phantoms. The motion of the straight phantom and the corresponding numerical model was smaller, on average zero or close to zero. This study demonstrates that tapering of the lumen, low intramural friction, and pressure might be important conducive features to the antegrade longitudinal motion of the arterial wall in vivo.

The influence of respiration, neck flexion, and arterial segment on carotid artery longitudinal wall motion

Although carotid artery longitudinal wall motion (CALM) has been highly detailed in cross-sectional studies, there is little evidence to explain population interindividual variability. This study was conducted to investigate how common external factors impact CALM. Twenty-one young healthy adults (11 females, aged 22 ± 2 yr) underwent three within-subject protocols. To evaluate probe positioning, vascular ultrasound was performed at a proximal and distal location along the common carotid artery. To evaluate neck angle, scans were acquired with the neck positioned at 70°, 90°, maximum extension (112 ± 9°), and maximum flexion (51 ± 7°). For the respiratory cycle condition, scans were taken during 7 s of inhalation, 7 s of exhalation, and 7 s of breath hold. CALM was evaluated for anterograde, retrograde, and maximum displacements, as well as radial-axial displacement. CALM was greater at proximal versus distal locations (retrograde = 1.14 ± 0.62 vs. 0.63 ± 0.24 mm, maximal = 1.32 ± 0.59 vs. 0.73 ± 0.24 mm; all P < 0.05). Minimum neck angles had greater motion than maximum angles (maximum displacement = 1.03 ± 0.43 vs. 0.77 ± 0.23 mm, P < 0.05). Without correcting breathing bias, retrograde displacement was greater during inspiration versus expiration (1.06 ± 0.34 vs. 0.58 ± 0.24 mm) and breath hold (1.06 ± 0.34 vs. 0.58 ± 0.24 mm), diastolic CALM was greater during expiration versus breath hold (1.10 ± 0.44 vs. 0.76 ± 0.33 mm), and maximum CALM was smaller during breath hold versus expiration (0.89 ± 0.31 vs. 1.21 ± 0.39 mm) and inspiration (0.89 ± 0.31 vs. 1.41 ± 0.70 mm). We recommend scanning 1-2 cm proximal to the carotid bifurcation, maintaining a neutral neck angle (70°-90°) for optimal CALM data collection in humans.NEW & NOTEWORTHY Carotid artery longitudinal wall motion (CALM) provides unique cardiovascular health information, yet a standardized approach to measurement is nonexistent. We tested CALM during manipulation of common external factors including probe position, neck angle, and breathing. All three conditions were found to alter CALM with drift in the breathing condition correctable by use of a linear bias correction. Consistent techniques should be used in CALM acquisition to reduce variability between individuals and population groups.

Physiological basis for longitudinal motion of the arterial wall

As opposed to arterial distension in the radial plane, longitudinal wall motion (LWM) is a multiphasic and bidirectional displacement of the arterial wall in the anterograde (i.e., in the direction of blood flow) and retrograde (i.e., opposing direction of blood flow) directions. While initially disregarded as imaging artifact, LWM has been consistently reported in ultrasound investigations in the last decade and is reproducible beat-to-beat, albeit with large inter-individual variability across healthy and diseased populations. Emerging literature has sought to examine the mechanistic control of LWM to explain the shape and variability of the motion pattern but lacks considerations for key foundational vascular principles at the level of the arterial wall ultrastructure. The purpose of this review is to summarize the potential factors that underpin the causes and control of arterial LWM, spanning considerations from the arterial extracellular matrix to systems-level integrative theories. First, an overview of LWM and relevant aspects wall composition will be discussed, including major features of the multiphasic pattern, arterial wall extracellular components, tunica fiber orientations, and arterial longitudinal pre-stretch. Second, current theories on the systems-level physiological mechanisms driving LWM will be discussed in the context of available evidence including experimental human research, porcine studies, and mathematical models. Throughout, we discuss implications of these observations with suggestions for future priority research areas.

Simulation of Ultrasound RF Signals Backscattered from a 3D Model of Pulsating Artery Surrounded by Tissue

Arterial stiffness is an independent predictor of cardiovascular events. The motion of arterial tissues during the cardiac cycle is important as a mechanical deformation representing vessel elasticity and is related to arterial stiffness. In addition, arterial pulsation is the main source of endogenous tissue micro-motions currently being studied for tissue elastography. Methods based on artery motion detection are not applied in clinical practice these days, because they must be carefully investigated in silico and in vitro before wide usage in vivo. The purpose of this paper is to propose a dynamic 3D artery model capable of reproducing the biomechanical behavior of human blood vessels surrounded by elastic tissue for endogenous deformation elastography developments and feasibility studies. The framework is based on a 3D model of a pulsating artery surrounded by tissue and simulation of linear scanning by Field II software to generate realistic dynamic RF signals and B-mode ultrasound image sequential data. The model is defined by a spatial distribution of motions, having patient-specific slopes of radial and longitudinal motion components of the artery wall and surrounding tissues. It allows for simulating the quantified mechanical micro-motions in the volume of the model. Acceptable simulation errors calculated between modeled motion patterns and those estimated from simulated RF signals and B-scan images show that this approach is suitable for the development and validation of elastography algorithms based on motion detection.

Spontaneous extension wave for in vivo assessment of arterial wall anisotropy

Another type of natural wave, traced from longitudinal wall motion and propagation along the artery, is observed in our in vivo human carotid artery experiments. We coin it as extension wave (EW) and hypothesize that EW velocity (EWV) is associated with arterial longitudinal stiffness. The EW is thus assumed to complement the pulse wave (PW), whose velocity (PWV) is tracked from the radial wall displacement and linked to arterial circumferential stiffness through the Moens-Korteweg equation, as indicators for arterial mechanical anisotropy quantification by noninvasive high-frame-rate ultrasound. The relationship between directional arterial stiffnesses and the two natural wave speeds was investigated in wave theory, finite-element simulations based on isotropic and anisotropic arterial models, and in vivo human common carotid artery (n = 10) experiments. Excellent agreement between the theory and simulations showed that EWV was 2.57 and 1.03 times higher than PWV in an isotropic and an anisotropic carotid artery model, respectively, whereas in vivo EWV was consistently lower than PWV in all 10 healthy human subjects. A strong linear correlation was substantiated in vivo between EWV and arterial longitudinal stiffness quantified by a well-validated vascular-guided wave imaging technique (VGWI). We thereby proposed a novel index calculated as EWV²/PWV² as an alternative to assess arterial mechanical anisotropy. Simulations and in vivo results corroborated the effect of mechanical anisotropy on the propagation of spontaneous waves along the arterial wall. The proposed anisotropy index demonstrated the feasibility of the concurrent EW and PW imaged by high frame-rate ultrasound in grading of arterial wall anisotropy.NEW & NOTEWORTHY An extension wave formed by longitudinal wall displacements was observed by high-frame-rate ultrasound in the human common carotid artery in vivo. A strong correlation between extension wave velocity and arterial longitudinal stiffness complements the well-established pulse wave, which is linked to circumferential stiffness, to noninvasively assess direction-dependent wall elasticity of the major artery. The proposed anisotropy index, which directly reflects arterial wall microstructure and function, might be a potential risk factor for screening (sub-) clinical cardiovascular diseases.

Comparison of the multi-phasic longitudinal displacement of the left and right common carotid artery in healthy humans

BACKGROUND

During the cardiac cycle, there is a multi-phasic bidirectional longitudinal movement (LMov) of the intima-media complex of large arteries, i.e. along the arteries. On the left side the common carotid artery (CCA) arises directly from the aortic arc, whereas on the right side the CCA originate from the innominate artery.

AIM

The aim of this study was to compare LMov of the left and right CCA of healthy subjects to investigate whether the difference in anatomy is of importance for LMov.

MATERIAL AND METHODS

The CCA's of 93 healthy subjects were investigated using in-house developed ultrasound methods.

RESULTS

Although the basic pattern were the same in the majority of subjects, several phases of LMov were significantly larger on the left side (the first retrograde phase, p = 0.0006; the second antegrade, "returning" phase, p < 0.00001; and the rapid retrograde phase of movement at the end of the cardiac cycle, p < 0.000001). In contrast, no significant side-difference in the amplitude of the first antegrade movement was seen. The maximal (peak-to-peak) LMov was significantly larger on the left side (p = 0.002).

DISCUSSION AND CONCLUSION

The side-differences found in LMov may be related to the difference in anatomy, including possible difference in distance to the heart and especially the presence of an extra bifurcation on the right side. Our data provide an important base for the further study of the relation between LMov and cardiovascular risk factors and atherosclerosis.

Estimating Arterial Wall Deformations from Automatic Key-Point Detection and Matching

Assessing arterial-wall motion and deformations may reveal pathologic alterations in biomechanical properties of the parietal tissues and, thus, contribute to the detection of vascular disease onset. Ultrasound image sequences allow the observation of this motion and many methods have been developed to estimate temporal changes in artery diameter and wall thickness and to track 2-D displacements of selected points. Some methods enable the assessment of shearing or stretching within the wall, but none of them can estimate all these deformations simultaneously. The method herein proposed was devised to simultaneously estimate translation, compression, stretching and shearing of the arterial wall in ultrasound B-mode image sequences representing the carotid artery longitudinal section. Salient blob-like patterns, called key points, are automatically detected in each frame and matched between successive frames. A robust estimator based on an affine transformation model is then used to assess frame-to-frame motion explaining at best the key-point matches and to reject outliers. Realistic simulated image sequences were used to evaluate the accuracy and robustness of the method against ground truth. The method was also visually assessed on clinical image sequences, for which true deformations are unknown.

Carotid Wall Longitudinal Motion in Ultrasound Imaging: An Expert Consensus Review

Motion extracted from the carotid artery wall provides unique information for vascular health evaluation. Carotid artery longitudinal wall motion corresponds to the multiphasic arterial wall excursion in the direction parallel to blood flow during the cardiac cycle. While this motion phenomenon has been well characterized, there is a general lack of awareness regarding its implications for vascular health assessment or even basic vascular physiology. In the last decade, novel estimation strategies and clinical investigations have greatly advanced our understanding of the bi-axial behavior of the carotid artery, necessitating an up-to-date review to summarize and classify the published literature in collaboration with technical and clinical experts in the field. Within this review, the state-of-the-art methodologies for carotid wall motion estimation are described, and the observed relationships between longitudinal motion-derived indices and vascular health are reported. The vast number of studies describing the longitudinal motion pattern in plaque-free arteries, with its putative application to cardiovascular disease prediction, point to the need for characterizing the added value and applicability of longitudinal motion beyond established biomarkers. To this aim, the main purpose of this review was to provide a strong base of theoretical knowledge, together with a curated set of practical guidelines and recommendations for longitudinal motion estimation in patients, to foster future discoveries in the field, toward the integration of longitudinal motion in basic science as well as clinical practice.

Longitudinal Movement of the Common Carotid Artery Wall: New Information on Cardiovascular Aging

Putative changes in the multiphasic pattern of longitudinal movement of the common carotid artery wall in the normal aging process are unknown. The aim of this study was to explore the phases, and resulting patterns, of the longitudinal movement of the intima-media complex of the human common carotid artery with respect to age and gender. One hundred thirty-five healthy non-smoking patients of different ages were investigated using in-house-developed ultrasound methods. The patterns of longitudinal movement seen in middle-aged and older patients were markedly different from those commonly seen in young patients, including the appearance of two additional phases of motion and, thus, new complex patterns. The displacement and maximum velocity of one of the phases, occurring at the time of aortic valve closure, increased quadratically with age in both men and women.

A combination of parabolic and grid slope interpolation for 2D tissue displacement estimations

Parabolic sub-sample interpolation for 2D block-matching motion estimation is computationally efficient. However, it is well known that the parabolic interpolation gives a biased motion estimate for displacements greater than |y.2| samples (y = 0, 1, …). Grid slope sub-sample interpolation is less biased, but it shows large variability for displacements close to y.0. We therefore propose to combine these sub-sample methods into one method (GS15PI) using a threshold to determine when to use which method. The proposed method was evaluated on simulated, phantom, and in vivo ultrasound cine loops and was compared to three sub-sample interpolation methods. On average, GS15PI reduced the absolute sub-sample estimation errors in the simulated and phantom cine loops by 14, 8, and 24% compared to sub-sample interpolation of the image, parabolic sub-sample interpolation, and grid slope sub-sample interpolation, respectively. The limited in vivo evaluation of estimations of the longitudinal movement of the common carotid artery using parabolic and grid slope sub-sample interpolation and GS15PI resulted in coefficient of variation (CV) values of 6.9, 7.5, and 6.8%, respectively. The proposed method is computationally efficient and has low bias and variance. The method is another step toward a fast and reliable method for clinical investigations of longitudinal movement of the arterial wall.

Principal Component Analysis of the Longitudinal Carotid Wall Motion in Association with Vascular Stiffness: A Pilot Study

The longitudinal motion of the carotid wall during a heart cycle has a multiphasic waveform. Recent studies have examined the amplitude of this motion. Instead of amplitude measurements, we focus on making a detailed characterization of the motion waveform. Two-minute carotid ultrasound videos were obtained for 19 healthy volunteers, and a speckle tracking algorithm was used to measure the motion of the carotid wall. Principal component analysis revealed the characteristic features of wall motion and their relation to known arterial stiffness indices. By estimating two principal components, we could account for more than 92% of the variation in the motion graphs. The first principal component derived from the longitudinal motion curves was significantly correlated to pulse pressure, indicating that the main dominant base waveform of the longitudinal motion was related to blood pressure. The second principal component derived from the longitudinal motion curves had multiple significant correlations to known stiffness indices, indicating that the stronger biphasic structure of the motion curve, especially on the adventitia layer, was associated with higher distensibility and compliance, as well as reduced carotid artery stiffness. According to this study, the second principal component of the longitudinal motion may be a useful parameter reflecting vascular health.

Carotid artery longitudinal wall motion is associated with local blood velocity and left ventricular rotational, but not longitudinal, mechanics

Recent studies have identified a predictable movement pattern of the common carotid artery wall in the longitudinal direction. While there is evidence that the magnitude of this carotid artery longitudinal wall motion (CALM) is sensitive to cardiovascular health status, little is known about the determinants of CALM. The purpose of this integrative study was to evaluate the contribution of left ventricular (LV) cardiac motion and local blood velocity to CALM. Simultaneous ultrasound measurements of CALM, common carotid artery mean blood velocity (MBV), and left ventricular motion were performed in ten young, healthy individuals (6 males; 22 ± 1 years). Peak anterograde CALM occurred at a similar time as peak MBV (18.57 ± 3.98% vs. 18.53 ± 2.81% cardiac cycle; t‐test: P = 0.94; ICC: 0.79, P < 0.01). The timing of maximum retrograde CALM displacement was different, but related, to both peak apical (41.00 ± 7.81% vs. 35.33 ± 5.79% cardiac cycle; t‐test: P < 0.01; ICC: 0.79, P < 0.01) and basal rotation (41.80 ± 6.12% vs. 37.30 ± 5.66% cardiac cycle; t‐test: P < 0.01; ICC: 0.74, P < 0.01) with peak cardiac displacements preceding peak CALM displacements in both cases. The association between basal rotation and retrograde CALM was further supported by strong correlations between their peak magnitudes (r = −0.70, P = 0.02), whereas the magnitude of septal longitudinal displacement was not associated with peak CALM (r = 0.11, P = 0.77). These results suggest that the rotational mechanical movement of the LV base may be closely associated with longitudinal mechanics in the carotid artery. This finding may have important implications for interpreting the complex relationship between ventricular and vascular function.

Carotid Atherosclerotic Plaque Alters the Direction of Longitudinal Motion in the Artery Wall

Longitudinal motion of the artery, a cyclical, bidirectional movement of the wall in the long axis of the artery, has recently gained interest in the characterization of artery function. The aim of this study was to evaluate longitudinal motion in patients with internal carotid atherosclerotic plaques. Speckle tracking ultrasound was used to assess common carotid artery wall motion in 12 patients with carotid plaque causing either moderate (50%-79%) or severe (80%-99%) stenosis based on the North American Carotid Endarterectomy Trial, and 23 healthy participants. Although healthy individuals were found to have a retrograde wall motion pattern, a distinct anterograde pattern was noted with plaque presence. Importantly, patients with severe plaque stenosis had greater anterograde motion (0.53 ± 0.36 mm) than those with moderate stenosis (0.17 ± 0.15 mm) (p < 0.05), likely owing to high wall shear stresses associated with greater peak systolic velocities at the site of stenosis (severe: 342.0 ± 99.4 cm/s, moderate: 177.5 ± 31.2 cm/s, p < 0.01). There were no differences in peak systolic velocities at plaque-free segments between plaque groups (severe: 80.2 ± 24.8 cm/s, moderate: 92.7 ± 23.0 cm/s). Blood flow at stenotic areas better predicted motion than plaque-free segments. We conclude that the presence of carotid plaque can have significant influence on longitudinal motion, with significantly greater anterograde displacements with increased stenosis. Future studies are needed to further investigate carotid artery wall mechanics.

Real-time ultrasound-tagging to track the 2D motion of the common carotid artery wall in vivo

PURPOSE

Tracking the motion of biological tissues represents an important issue in the field of medical ultrasound imaging. However, the longitudinal component of the motion (i.e., perpendicular to the beam axis) remains more challenging to extract due to the rather coarse resolution cell of ultrasound scanners along this direction. The aim of this study is to introduce a real-time beamforming strategy dedicated to acquire tagged images featuring a distinct pattern in the objective to ease the tracking.

METHODS

Under the conditions of the Fraunhofer approximation, a specific apodization function was applied to the received raw channel data, in real-time during image acquisition, in order to introduce a periodic oscillations pattern along the longitudinal direction of the radio frequency signal. Analytic signals were then extracted from the tagged images, and subpixel motion tracking of the intima-media complex was subsequently performed offline, by means of a previously introduced bidimensional analytic phase-based estimator.

RESULTS

The authors' framework was applied in vivo on the common carotid artery from 20 young healthy volunteers and 6 elderly patients with high atherosclerosis risk. Cine-loops of tagged images were acquired during three cardiac cycles. Evaluated against reference trajectories manually generated by three experienced analysts, the mean absolute tracking error was 98 ± 84 μm and 55 ± 44 μm in the longitudinal and axial directions, respectively. These errors corresponded to 28% ± 23% and 13% ± 9% of the longitudinal and axial amplitude of the assessed motion, respectively.

CONCLUSIONS

The proposed framework enables tagged ultrasound images of in vivo tissues to be acquired in real-time. Such unconventional beamforming strategy contributes to improve tracking accuracy and could potentially benefit to the interpretation and diagnosis of biomedical images.