Arterial Wall Motion Research Articles

Compliance of abdominal aortic aneurysms evaluated by tissue Doppler imaging: Correlation with aneurysm size

Several studies have shown that an increase in abdominal aortic aneurysm (AAA) growth rate occurs when the diameter reaches 40 to 50 mm. AAA expansion is related to remodeling of the parietal extracellular matrix. The parietal mechanisms involved in this critical phase of sudden increase remain unexplained. Analysis of AAA wall movements and determination of AAA compliance may provide information about the constitution of the arterial wall. If a change in parietal wall motion somewhere between 40 and 50 mm could be shown, this would contribute to the understanding of the growth of AAA. Furthermore, it would provide a valuable additional parameter for AAA monitoring. This study had two aims: first, to evaluate the relationship between AAA compliance and maximum diameter using the tissue Doppler imaging system; and second, to test the hypothesis of a change in AAA behavior at around 45 mm in diameter. Fifty-six patients with AAA (mean diameter, 39 mm) were prospectively investigated using the tissue Doppler imaging system, which provides information concerning arterial wall motion. Maximum mean segmental dilation (MMSD), segmental compliance, pressure strain elastic modulus (Ep), and stiffness were determined and related to the maximum diameter of AAA. Results After natural log transformation of all variables, there was a significant positive linear relationship between maximum diameter and both MMSD (P < .001) and segmental compliance (P < .001) but not with Ep or stiffness (P = .37 and .22, respectively). MMSD and segmental compliance were significantly higher in AAA > or = 45 mm than in AAA < 45 mm (P < .0002 and <. 004, respectively). Ep and stiffness tended to decrease in larger AAAs, but this was not statistically significant (P < .43 and .24, respectively). Dispersion of Ep and stiffness values seemed to be wider among AAA < 45 mm compared with those > or = 45 mm. Compliance parameters can easily be measured during routine AAA ultrasound monitoring using the tissue Doppler imaging system. The study showed an increase in MMSD and segmental compliance as well as a nonsignificant trend toward increased distensibility (decreased Ep and stiffness) with increased AAA diameter. A change in dispersion of AAA distensibility may appear around 45 mm in diameter, but a larger study will be needed to clarify this.

The identification of reversible dysfunctional myocardium is influenced by the severity of contractile dysfunction and by the length of follow-up

To evaluate the influence of the severity of regional myocardial dysfunction and of the length of follow-up on the identification of myocardial viability with rest-redistribution Tl single photon emission computed tomography (SPECT) and low-dose dobutamine echocardiography (LDDE). Twenty-six patients with chronic coronary artery disease and wall motion abnormalities, candidates for revascularization, were included in this study. All patients underwent, in the same week, Tl SPECT and LDDE for pre-revascularization evaluation of myocardial viability. Reversibility of regional dysfunction was assessed by two-dimensional echocardiography, 40+/-20 days (early follow-up) and 12+/-5 months (late follow-up) after revascularization. In a/dyskinetic segments, Tl SPECT showed similar values of sensitivity (78% vs. 71%, P=NS) and slightly higher values of specificity (43% vs. 18%, P<0.01) compared to hypokinetic segments, in predicting functional recovery at early follow-up. No significant changes were observed in the diagnostic accuracy of Tl SPECT at late follow-up. On the contrary, LDDE provided significantly lower values of sensitivity (56% vs. 94%, P<0.05) and higher values of specificity (73% vs. 9%, P<0.01) in a/dyskinetic compared to hypokinetic segments. Specificity of LDDE in a/dyskinetic segments significantly increased from early (73%) to late follow-up (95%; P<0.05). Similarly, positive predictive value in a/dyskinetic segments significantly increased from early (69%) to late follow-up (96%; P<0.05). The severity of regional dysfunction and the length of follow-up significantly influence the diagnostic accuracy of LDDE but not of rest-redistribution Tl SPECT in the identification of myocardial viability.

Fluid Dynamic Analysis in a Human Left Anterior Descending Coronary Artery with Arterial Motion

A computational fluid dynamic (CFD) analysis is pre sented to describe local flow dynamics in both 3-D spatial and 4-D spatial and temporal domains from reconstructions of intravascular ultrasound (IVUS) and bi-plane angiographic fusion images. A left anterior descending (LAD) coronary artery segment geometry was accurately reconstructed and subsequently its motion was incorporated into the CFD model. The results indicate that the incorporation of motion had appreciable effects on blood flow patterns. The velocity profiles in the region of a stenosis and the circumferential distribution of the axial wall shear stress (WSS) patterns in the vessel are altered with the wall motion introduced in the simulation. The time-averaged axial WSS between simulations of steady flow and unsteady flow without arterial motion were comparable (-0.3 to 13.7 Pa in unsteady flow versus -0.2 to 10.1 Pa in steady flow) while the magnitudes decreased when motion was introduced (0.3-4.5 Pa). The arterial wall motion affects the time-mean WSS and the oscillatory shear index in the coronary vessel fluid dynamics and may provide more realistic predictions on the progression of atherosclerotic disease.

Noninvasive ultrasonic measurement of arterial wall motion in mice.

Despite the extensive use of genetically altered mice to study cardiovascular physiology and pathology, it remains difficult to quantify arterial function noninvasively in vivo. We have developed a noninvasive Doppler method for quantifying vessel wall motion in anesthetized mice. A 20-MHz probe was held by an alligator clip and positioned over the carotid arteries of 16 mice, including six 3- to 5-mo-old wild-type (WT), four 30-mo-old senescent (old), two apolipoprotein E null (ApoE), and four alpha-smooth muscle actin null (alpha-SMA) mice. Doppler signals were obtained simultaneously from both vessel walls and from blood flow. The calculated displacement signals from the near and far walls were subtracted to generate a diameter signal from which the excursion and an augmentation index were calculated. The excursion ranged between 13 microm (in ApoE) and 95 microm (in alpha-SMA). The augmentation index was lowest in the WT mice (0.06) and highest in the old mice (0.29). We conclude that Doppler signal processing may be used to measure vessel wall motion in mice with high spatial and temporal resolution and that diameter signals can replace pressure signals for calculating the augmentation index. This noninvasive method is able to identify and confirm characteristic changes in arterial properties previously associated with age, atherosclerosis, and the absence of vascular tone.

Quantification of vessel wall motion and cyclic strain using cine phase contrast MRI: in vivo validation in the porcine aorta.

Artery wall motion and strain play important roles in vascular remodeling and may be important in the pathogenesis of vascular disease. In vivo observations of circumferentially nonuniform wall motion in the human aorta suggest that nonuniform strain may contribute to the localization of vascular pathology. A velocity-based method to investigate circumferential strain variations was previously developed and validated in vitro; the current study was undertaken to determine whether accurate displacement and strain fields can be calculated from velocity data acquired in vivo. Wall velocities in the porcine thoracic aorta were quantified with PC-MRI and an implanted coil and were then time-integrated to compute wall displacement trajectories and cyclic strain. Displacement trajectories were consistent with observed aortic wall motion and with the displacements of markers in the aortic wall. The mean difference between velocity-based and marker-based trajectory points was 0.1 mm, relative to an average pixel size of 0.4 mm. Propagation of error analyses based on the precision of the computed displacements were used to demonstrate that 10% strain results in a standard deviation of 3.6%. This study demonstrates that it is feasible to accurately quantify strain from low wall velocities in vivo and that the porcine thoracic aorta does not deform uniformly.

M.501 M-mode ultrasound imaging of arterial wall movement and arterial wall thickness of distal carotid arteries in atherosclerosis studies in healthy adults

M.501 M-mode ultrasound imaging of arterial wall movement and arterial wall thickness of distal carotid arteries in atherosclerosis studies in healthy adults

M.501 M-mode ultrasound imaging of arterial wall movement and arterial wall thickness of distal carotid arteries in atherosclerosis studies in healthy adults

M.501 M-mode ultrasound imaging of arterial wall movement and arterial wall thickness of distal carotid arteries in atherosclerosis studies in healthy adults

A novel ultrasound instrument for investigation of arterial mechanics

The study of arterial mechanics concerns functional characteristics depending on wall elasticity and flow profile. Wall elasticity can be investigated through the estimation of parameters like the arterial distensibility, which is of high clinical interest because of its known correlation not only with the advanced atherosclerotic disease, but also with aging and major risk factors for cardiovascular disease. The flow velocity profile is also clinically relevant, because it modulates endothelial function and can be responsible for the development and distribution of atherosclerotic plaques. A clinically relevant variable extracted from the blood velocity profile is the wall shear rate (WSR), which represents the spatial velocity gradient near the vessel wall. This paper describes an integrated ultrasound system, capable of detecting both the velocity profile and the wall movements in human arteries. It basically consists of a PC add-on board including a single high-speed digital signal processor. This is dedicated to the analysis of echo-signals backscattered from 128 range cells located along the axis of the interrogating ultrasound (US) beam. Echoes generated from the walls (characterized by high amplitudes and low Doppler frequencies) and from red blood cells (characterized by low amplitudes and relatively high Doppler frequencies) are independently processed in real-time. Wall velocity is detected through the autocorrelation algorithm, while blood velocity is investigated through a complete spectral analysis of all signals backscattered by erythrocytes and WSR is extracted from the estimated velocity profile. Preliminary applications of the new system, including the simultaneous analysis of blood flow and arterial wall movement in healthy volunteers and in a diseased patient, are discussed, and first results are presented.

AN UNSTEADY ANALYSIS OF NONLINEAR TWO‐LAYERED 2D MODEL OF PULSATILE FLOW THROUGH STENOSED ARTERIES

The effects of red cell concentration and peripheral layer viscosity on physiological characteristics of pulsatile flow in presence of mild stenosis are investigated. The flowing blood is represented by a two‐fluid model, consisting of a core region of suspension of all the erythrocytes assumed to be non‐Newtonian (inhomogeneous Newtonian) and a peripheral plasma layer free from cells of any kind as a Newtonian fluid. In the realm of the flow characteristics of blood the viscosity is taken to be a function of hematocrit in a manner that it varies radially only in the central core characterising its non‐Newtonian behaviour while it remains constant in the plasma region. The arterial wall motion and its effect on local fluid mechanics is also incorporated in the present theoretical study. Finite difference scheme has been used to solve the unsteady Navier‐Stokes equations in cylindrical coordinates assuming axial symmetry under laminar conditions, so that the problem effectively becomes two‐dimensional. The nonlinear terms appearing in the Navier‐Stokes equations governing blood flow are accounted for. Finally, the numerical illustration presented at the end of the paper provides an effective measure of the flux, the resistive impedance and the wall shear stress quantitatively in order to validate the applicability of the present model.

A new non-invasive ultrasonic method for simultaneous measurements of longitudinal and radial arterial wall movements: first in vivo trial.

During recent years, the radial movement of the arterial wall has been extensively studied, and measurements of the radial movement are now an important tool in cardiovascular research for characterizing the mechanical properties of the arterial wall. In contrast, the longitudinal movement of vessels has gained little or no attention as it has been presumed that this movement is negligible. With modern high-resolution ultrasound, it can, however, be seen that the intima-media complex of the arterial wall moves not only in the radial direction, but also in the longitudinal direction during pulse-wave propagation. This paper describes a new non-invasive ultrasonic method that is able to measure simultaneously two dimensionally arterial vessel wall movements. The method is demonstrated in a limited in vivo trial. Results from the in vivo trial show that, apart from the well-known radial movement, there is a distinct longitudinal movement in the human common carotid artery with, in this case, the intima-media complex moving substantially as compared with the region of the tunica adventitia. Two-dimensional evaluation of the vessel-wall movements, taking not only the radial movement, but also the longitudinal movement into account, may provide novel information of importance in the evaluation of vessel-wall function.

Carotid artery wall motion estimated from b-mode ultrasound using region tracking and block matching

The motion of the carotid atheromatous plaque relative to the adjacent wall may be related to the risk of cerebral events. A quantitative method for motion estimation was applied to analyse arterial wall movement from sequences of 2-D B-mode ultrasound (US) images. Image speckle patterns were tracked between successive frames using the correlation coefficient as the matching criterion. The size of the selected region-of-interest (ROI) was shown to affect the motion analysis results; an optimal size of 3.2 × 2.5 mm 2 was suggested for tracking a region at the wall-lumen interface and of 6.3 × 2.5 mm 2 for one within the tissue. The results showed expected cyclical motion in the radial direction and some axial movement of the arterial wall. The method can be used to study further the axial motion of the carotid artery wall and plaque and, thus, provide useful insight into the mechanisms of atherosclerosis.

Characterization of peripheral arterial wall motion by Doppler tissue echography: A validation study

Doppler tissue echography (DTE) has been proposed for arterial wall-motion analysis. However, it is not known if DTE gives precise measurements of arterial displacement. Therefore, we used a pig aorta, in vitro model to study arterial wall velocities. High resolution color B-mode DTE acquisitions gave accurate estimations of velocities as compared with those obtained by a referenced wall-tracking system, with a slight underestimation of values calculated from DTE (14.4 ± 0.1 for DTE vs 16.1 ± 0.2 at 5 L/min output, P > .05). Excellent correlations were obtained between both methods ( r 2 = 0.94, P < .001), and Bland-Altman analysis revealed no significant differences between measurements. We also compared common carotid arterial wall velocities obtained in 16 healthy volunteers by DTE and a wall-tracking system, which gave similar wave profiles. As expected, arterial distensions calculated from DTE were lower than those from a wall-tracking system (6.8 ± 1.7% vs 7.4 ± 2.1%, respectively, P = .02). These results indicate that DTE may be used to study arterial wall displacement. (J Am Soc Echocardiogr 2002;15:1218-25.)

Optimization of Condition of Ultrasonic Beam for Measurement of Small Change in Thickness of Arterial Wall

We previously developed a method for measuring small changes in thickness of the arterial wall during one cardiac cycle. Knowledge of this change in thickness is useful for in vivo assessment of the regional elasticity of the arterial wall. In this study, from computer simulations, it is found that measurement error depends on the distance of the ultrasonic beam from the center of the artery and it can be reduced by optimally setting the focal position. In basic experiments using a silicone rubber tube and in in vivo experiments with a human carotid artery, it is found that by optimizing the focal position, measurement of the change in thickness becomes more robust against mispositioning of the ultrasonic beam. From these results, it is demonstrated that optimum focal positioning provides more robustness in measurement, even if there is arterial wall motion causing the position of the ultrasonic beam to deviate from the center of the artery.

Blood flow and tissue motion with ultrasound for vascular applications

This paper provides a simple description of the fundamental principles of ultrasound vascular imaging. The mechanisms which govern the flow/pressure relationship are presented. The various components of the Doppler systems, namely continuous Doppler, pulse Doppler, color flow imaging and a potential new application called arterial wall motion are described. The time-domain approach to the Doppler effect is shown to be efficient for understanding of the signal processing path. The capabilities of the ultrasound system are illustrated with several clinical examples dealing with the particular problem of carotid stenoses.

Micromovements at the Aortic Aneurysm Neck Measured during Open Surgery with Close-Range Photogrammetry: Implications for Aortic Endografts

To describe the motion of abdominal aortic aneurysm (AAA) as a first step toward determining if stent-graft migration may be due to propulsive movements at the neck. In an initial laboratory validation test, a silicone AAA model was imaged with a stereo-pair configuration of calibrated digital cameras using retro-reflective and ink dot surface targets. This technique was then applied to 4 aneurysm necks imaged at open surgery. In each case the paired images were measured and analyzed to produce sequences of accurate three-dimensional (3-D) surface models of the anterior aspect of the aortic neck. Engineering techniques were used to visualize and identify differences between the generated surfaces, allowing changes in shape of the aortic wall to be determined. The correlation between stereoscopic digital imaging with electronic caliper measurements of linear dimensions was accurate, irrespective of the type of surface targets used (r2 = 0.98). The AAA neck demonstrated complex 3-D movements that ranged from 1.0 to 1.5 mm in the radial direction to 0.6 to 1.0 mm along the longitudinal axis, with a 0.44 to 0.59-mm rotational component (p < 0.05). This pilot study raises questions about the validity of accepted models of arterial wall movements and compliance in which rotation has not been described. The technique used to measure dynamic neck morphology appears promising as a means to further define the mechanical parameters of the human aorta, which may assist in designing stent-grafts.

Arterial Wall Motion Imaging: A New Approach for Vascular Characterization

Arterial Wall Motion Imaging: A New Approach for Vascular Characterization

Evaluating signal intensity of movement-impaired myocardial segments in MR delayed images after administration of Gd-DTPA. Correlation of regional increase in contraction after revascularization

The aim of the study was to correlate delayed contrast enhancement of dysfunctional regions of the myocardium after injection of Gd-DTPA with the improvement of regional contractility 3 months after revascularization. Eleven patients with coronary artery disease and wall motion abnormalities underwent MR imaging before and 3 months after revascularization therapy (PTCA or CABG). Short-axis images were acquired using a cine gradient echo sequence. After revascularization, a representative slice was analyzed 14 +/- 1 min after injection of 0.1 mmol/kg Gd-DTPA using a T1-weighted turbo fast low-angle shot sequence. Improved systolic wall thickening 3 months after revascularization served as criterion of viability and was correlated with delayed contrast enhancement patterns. After revascularization, 6 patients showed complete recovery of the dysfunctional area, 1 patient partial recovery, and 4 patients remained unchanged. All 4 patients with persisting wall motion abnormalities, 1/6 patients with recovery and the patient with partial recovery revealed a delayed enhancement. Three months after revascularization, delayed enhancement of dysfunctional myocardium is evident in patients without regional wall motion improvement, while the lack of delayed enhancement correlates with improvement of regional contractility.

Mathematical model of pulsatile blood flow in a distensible aortic bifurcation subject to body acceleration

The present analytical investigation has been dealt with pulsatile flow characteristics of blood in a distensible bifurcated artery having a stenosis when it is subjected to whole body acceleration. The geometry of the bifurcated artery developed by the present first two authors (S. Chakravarty, P.K. Mandal, Int. J. Eng. Sci. 35 (1997) 409–422) has been used to carry out the present analysis. It incorporates the arterial wall motion by treating it as an anisotropic, linear viscoelastic incompressible material. The unsteady flow mechanism in the bifurcated artery subject to a pulsatile pressure gradient arising from the normal functioning of the heart as also the body acceleration is presented mathematically. The effect of the surrounding connective tissues on the motion of the arterial wall is also paid due consideration. The effect of the wall distensibility on the bifurcated flow phenomena has been accounted for through suitably formulated continuity conditions. The equations governing the motion of the system are sought in the Laplace transform space and their relevant solutions are obtained in the transformed domain by using an appropriate finite difference scheme. Their inversions to the physical domain lead to calculate the velocity profile of the flowing blood in both the parent and the daughter arteries together with the arterial wall displacements. From the computational results based on the short-time-range approximations, one may disclose that the body acceleration and the wall distensibility do not change the flow patterns in the parent aorta, but there is a drastic change in the daughter artery, which in turn, causes an appreciable increase in the shear stresses developed on both the parent and its daughter arterial walls. The pulsatile flow helps inducing the wall shear stress to change the direction over a cycle which may be important in the mechanism of atherosclerosis.

In vivo femoropopliteal arterial wall compliance in subjects with and without lower limb vascular disease

Objective: The purpose of this study was to further the development of a compliant vascular graft with a preliminary assessment of the elastic properties of the femoropopliteal artery in subjects with and without lower limb peripheral vascular disease. Methods: This prospective controlled study was set in a university department of surgery. Using an ultrasound scan wall tracking system with the simultaneous measurement of brachial blood pressure, measurements of femoropopliteal artery wall motion were undertaken in 11 patients with peripheral vascular disease (group 1), in 11 older control subjects who were matched for blood pressure, age, and sex (group 2), and in 12 younger control subjects (group 3). Diametrical compliance and stiffness index were determined for the common femoral artery, the proximal superficial femoral artery, the distal superficial femoral artery (DSFA), and the midgenicular popliteal artery. Results: All the arterial segments in group 1 showed a trend towards increased stiffness and less compliance than the group 2, age-matched control vessels, with significantly lower distensibility noted at the common femoral artery (mean compliance of 6.2% vs 14.1% mm Hg –1 × 10 –2, respectively; P < .05) and the DSFA (mean compliance of 2.2% vs 1.9% mm Hg –1 × 10 –2, respectively; P < .05). The popliteal artery segment in group 3 proved to be more compliant and less stiff than did the same vessel in group 2 (8.5% vs 4.7% mm Hg –1 × 10 –2, respectively; P < .01). In all three study groups, the DSFA was consistently noted to be the least distensible vessel segment. Conclusion: Lower limb peripheral vascular disease is associated with a reduction in femoropopliteal artery elasticity. Age alone appears to have a minimal effect on the compliance of the proximal half of the femoropopliteal segment. The elastic properties of the femoropopliteal vessel are subject to marked variation along its course. To minimize compliance mismatch, the degree of elasticity engineered into a vascular graft must reflect that observed in vivo. (J Vasc Surg 1999;30:936-45.)

Numerical simulation of pulsatile flow in a compliant curved tube model of a coronary artery.

The endothelial cells (ECs) lining a blood vessel wall are exposed to both the wall shear stress (WSS) of blood flow and the circumferential strain (CS) of pulsing artery wall motion. These two forces and their interaction are believed to play a role in determining remodeling of the vessel wall and development of arterial disease (atherosclerosis). This study focused on the WSS and CS dynamic behavior in a compliant model of a coronary artery taking into account the curvature of the bending artery and physiological radial wall motion. A three-dimensional finite element model with transient flow and moving boundaries was set up to simulate pulsatile flow with physiological pressure and flow wave forms characteristic of the coronary arteries. The characteristic coronary artery curvature and flow conditions applied to the simulation were: aspect ratio (lambda) = 10, diameter variation (DV) = 6 percent, mean Reynolds number (Re) = 150, and unsteadiness parameter (alpha) = 3. The results show that mean WSS is about 50 percent lower on the inside wall than the outside wall while WSS oscillation is stronger on the inside wall. The stress phase angle (SPA) between CS and WSS, which characterizes the dynamics of the mechanical force pattern applied to the endothelial cell layer, shows that CS and WSS are more out of phase in the coronaries than in any other region of the circulation (-220 deg on the outside wall, -250 deg on the inside wall). This suggests that in addition to WSS, SPA may play a role in localization of coronary atherosclerosis.