Ronny X. Li

Pulse wave imaging of the human carotid artery: an in vivo feasibility study

Noninvasive quantification of regional arterial stiffness, such as measurement of the pulse wave velocity (PWV), has been shown to be of high clinical importance. Pulse wave imaging (PWI) has been previously developed by our group to visualize the propagation of the pulse wave along the aorta and to estimate the regional PWV. The objective of this paper is to determine the feasibility of PWI in the human carotid artery in vivo. The left common carotid arteries of eight (n = 8) healthy volunteers (male, age 27 + 4 years old) were scanned in a long-axis view, with a 10-MHz linear-array transducer. The beam density of the scan was reduced to 16 beams within an imaging width of 38 mm. The frame rate of ultrasound imaging was therefore increased to 1127 Hz at an image depth of 25 mm. The RF ultrasound signals were then acquired at a sampling rate of 40 MHz and used to estimate the velocity of the arterial wall using a 1-D cross-correlation-based speckle tracking method. The sequence of the wall velocity images at different times depicts the propagation of the pulse wave in the carotid artery from the proximal to distal sides. The regional PWV was estimated from the spatiotemporal variation of the wall velocities and ranged from 4.0 to 5.2 m/s in eight (n = 8) normal subjects, in agreement with findings reported in the literature. PWI was thus proven feasible in the human carotid artery, and may be proven useful for detecting vascular disease through mapping the pulse wave and estimating the regional PWV in the carotid artery.

Pulse-Wave Propagation in Straight-Geometry Vessels for Stiffness Estimation: Theory, Simulations, Phantoms and In Vitro Findings

Pulse wave imaging (PWI) is an ultrasound-based method for noninvasive characterization of arterial stiffness based on pulse wave propagation. Reliable numerical models of pulse wave propagation in normal and pathological aortas could serve as powerful tools for local pulse wave analysis and a guideline for PWI measurements in vivo. The objectives of this paper are to (1) apply a fluid-structure interaction (FSI) simulation of a straight-geometry aorta to confirm the Moens-Korteweg relationship between the pulse wave velocity (PWV) and the wall modulus, and (2) validate the simulation findings against phantom and in vitro results. PWI depicted and tracked the pulse wave propagation along the abdominal wall of canine aorta in vitro in sequential Radio-Frequency (RF) ultrasound frames and estimates the PWV in the imaged wall. The same system was also used to image multiple polyacrylamide phantoms, mimicking the canine measurements as well as modeling softer and stiffer walls. Finally, the model parameters from the canine and phantom studies were used to perform 3D two-way coupled FSI simulations of pulse wave propagation and estimate the PWV. The simulation results were found to correlate well with the corresponding Moens-Korteweg equation. A high linear correlation was also established between PWV² and E measurements using the combined simulation and experimental findings (R² =  0.98) confirming the relationship established by the aforementioned equation.

Mapping the longitudinal wall stiffness heterogeneities within intact canine aortas using Pulse Wave Imaging (PWI) ex vivo

The aortic stiffness has been found to be a useful independent indicator of several cardiovascular diseases such as hypertension and aneurysms. Existing methods to estimate the aortic stiffness are either invasive, e.g. catheterization, or yield average global measurements which could be inaccurate, e.g., tonometry. Alternatively, the aortic pulse wave velocity (PWV) has been shown to be a reliable marker for estimating the wall stiffness based on the Moens-Korteweg (M-K) formulation. Pulse Wave Imaging (PWI) is a relatively new, ultrasound-based imaging method for noninvasive and regional estimation of PWV. The present study aims at showing the application of PWI in obtaining localized wall mechanical properties by making PWV measurements on several adjacent locations along the ascending thoracic to the suprarenal abdominal aortic trunk in its intact vessel form. The PWV estimates were used to calculate the regional wall modulus based on the M-K relationship and were compared against conventional mechanical testing. The findings indicated that for the anisotropic aortic wall, the PWI estimates of the modulus are smaller than the circumferential modulus by an average of -32.22% and larger than the longitudinal modulus by an average of 25.83%. Ongoing work is focused on the in vivo applications of PWI in normal and pathological aortas with future implications in the clinical applications of the technique.

Pulse Wave Imaging (PWI) of the human carotid artery: An in vivo feasibility study

Noninvasive measurement of the pulse wave velocity (PWV) is of high clinical importance. Pulse Wave Imaging (PWI) has been previously developed by our group to visualize the propagation of the pulse wave and to estimate the regional PWV. The objective of this study was to determine the feasibility of PWI in the human carotid artery in vivo. The left common carotid artery of 8 healthy human subjects (27 ± 4 y.o.) was scanned in a long-axis view. The beam density of the 10 MHz linear array was equal to 16 beams so as to increase the frame rate to 1127 Hz for an imaging depth of 25 mm and width of 38 mm. The RF signals were acquired to estimate the velocity of the arterial wall using a 1D cross-correlation technique. Sequential wall velocity frames depicted the propagation of the pulse wave in the carotid artery within the field of view. Regional PWV was estimated from the spatiotemporal variation of the wall velocities and ranged from 4.0 to 5.2 m/s, in agreement with findings in the literature. PWI was thu...

An inverse approach to determining spatially varying arterial compliance using ultrasound imaging

The mechanical properties of arteries are implicated in a wide variety of cardiovascular diseases, many of which are expected to involve a strong spatial variation in properties that can be depicted by diagnostic imaging. A pulse wave inverse problem (PWIP) is presented, which can produce spatially resolved estimates of vessel compliance from ultrasound measurements of the vessel wall displacements. The 1D equations governing pulse wave propagation in a flexible tube are parameterized by the spatially varying properties, discrete cosine transform components of the inlet pressure boundary conditions, viscous loss constant and a resistance outlet boundary condition. Gradient descent optimization is used to fit displacements from the model to the measured data by updating the model parameters. Inversion of simulated data showed that the PWIP can accurately recover the correct compliance distribution and inlet pressure under realistic conditions, even under high simulated measurement noise conditions. Silicone phantoms with known compliance contrast were imaged with a clinical ultrasound system. The PWIP produced spatially and quantitatively accurate maps of the phantom compliance compared to independent static property estimates, and the known locations of stiff inclusions (which were as small as 7 mm). The PWIP is necessary for these phantom experiments as the spatiotemporal resolution, measurement noise and compliance contrast does not allow accurate tracking of the pulse wave velocity using traditional approaches (e.g. 50% upstroke markers). Results from simulations indicate reflections generated from material interfaces may negatively affect wave velocity estimates, whereas these reflections are accounted for in the PWIP and do not cause problems.

Performance assessment and optimization of Pulse Wave Imaging (PWI) in ex vivo canine aortas and in vivo normal human arteries

The amplitude, velocity, and morphology of the arterial pulse wave may all provide valuable diagnostic information for cardiovascular pathology. Pulse Wave Imaging (PWI) is an ultrasound-based method developed by our group to noninvasively visualize and map the spatio-temporal variations of the pulse wave-induced vessel wall motion. Because PWI is capable of acquiring multiple wall motion waveforms successively along an imaged arterial segment over a single cardiac cycle in vivo, the regional morphological changes, amplitudes, and velocity (i.e. pulse wave velocity, or PWV) of the pulse wave can all be evaluated. In this study, an ex vivo setup was used to assess the effects of varying PWI image acquisition variables (beam density/frame rate and scanning orientation) and signal processing methods (beam sweep compensation scheme and waveform feature tracking) on the PWV estimation in order to validate the optimal parameters. PWI was also performed on the carotid arteries and abdominal aortas of six healthy volunteers for identification of several salient features of the waveforms over the entire cardiac cycle that may aid in assessing the morphological changes of the pulse wave. The ex vivo results suggest that the PWI temporal resolution is more important for PWV estimation than the PWI spatial resolution, and also that the reverse scanning orientation (i.e. beam sweeping direction opposite the direction of fluid flow) is advantageous due to higher precision and less dependence on the frame rate. In the in vivo waveforms, the highest precision PWV measurements were obtained by tracking the 50% upstroke of the waveforms. Finally, the dicrotic notch, reflected wave, and several inflection points were qualitatively identified in the carotid and aortic anterior wall motion waveforms and shown in one representative subject

In-vivo pulse wave imaging for arterial stiffness measurement under normal and pathological conditions

Numerous studies have identified arterial stiffening as a strong indicator of cardiovascular pathologies such as hypertension and abdominal aortic aneurysm (AAA). Pulse Wave Imaging (PWI) is a novel, noninvasive ultrasound-based method to quantify regional arterial stiffness by measuring the velocity of the pulse wave that propagates along arterial walls after each left ventricular contraction. The PWI method employs 1D cross-correlation speckle tracking to compute axial incremental displacements, then tracks the position of the displacement wave in the anterior wall of the vessel to estimate pulse wave velocity (PWV). PWI has been validated on straight tube aortic phantoms and aortas of healthy humans as well as normal and AAA murine models. This paper presents and compares preliminary PWI results from normal, hypertensive, and AAA human subjects. PWV was computed in select cases from each subject category. The measured PWV values in hypertensive (N = 5) and AAA (N = 2) subjects were found to be significantly higher than in normal subjects (N = 8). In all subjects, the spatio-temporal profile and waveform morphologies of the pulse wave were generated from the displacement data for visualization and qualitative evaluation of the pulse wave propagation. While the waveforms were found to maintain roughly the same shape in normal subjects, those in the AAA and most hypertensive cases changed drastically along the imaged aortic segment, suggesting non-uniform wall mechanical properties.

Noninvasive evaluation of varying pulse pressures in vivo using brachial sphymomanometry, applanation tonometry, and Pulse Wave Ultrasound Manometry

The routine assessment and monitoring of hypertension may benefit from the evaluation of arterial pulse pressure (PP) at more central locations (e.g. the aorta) rather solely at the brachial artery. Pulse Wave Ultrasound Manometry (PWUM) was previously developed by our group to provide direct, noninvasive aortic PP measurements using ultrasound elasticity imaging. Using PWUM, radial applanation tonometry, and brachial sphygmomanometry, this study investigated the feasibility of noninvasively obtaining direct PP measurements at multiple arterial locations in normotensive, pre-hypertensive, and hypertensive human subjects. Two-way ANOVA indicated a significantly higher aortic PP in the hypertensive subjects, while radial and brachial PP were not significantly different among the subject groups. No strong correlation (r2 < 0.45) was observed between aortic and radial/brachial PP in normal and pre-hypertensive subjects, suggesting that increases in PP throughout the arterial tree may not be uniform in relatively compliant arteries. However, there was a relatively strong positive correlation between aortic PP and both radial and brachial PP in hypertensive subjects (r2 = 0.68 and 0.87, respectively). PWUM provides a low-cost, non-invasive, and direct means of measuring the pulse pressure in large central arteries such as the aorta. When used in conjunction with peripheral measurement devices, PWUM allows for the routine screening of hypertension and monitoring of BP-lowering drugs based on the PP from multiple arterial sites.

Pulse Wave Imaging (PWI) and arterial stiffness measurement of the human carotid artery: An in vivo feasibility study

Noninvasive quantification of regional arterial stiffness has been shown to be of high clinical importance. Pulse Wave Imaging (PWI) has been previously developed by our group to visualize the propagation of the pulse wave along the artery and to estimate the regional pulse wave velocity (PWV). The objectives of this paper are to 1) determine the feasibility of PWI in the human carotid artery in vivo and 2) assess the stiffness of the human carotid artery in vivo using applanation tonometry and ultrasound-based motion estimation. For PWI, the left common carotid arteries of eight healthy volunteers were scanned with a 10 MHz linear array transducer at a high frame rate of 1127 Hz. The RF signals were used to estimate the axial velocity of the arterial wall using a 1D cross-correlation based speckle tracking method. Regional PWV was estimated from the spatiotemporal variation of the axial wall velocities and was found equal to 4.5 ± 0.4 m/s in eight subjects, in agreement with findings reported in the literature. PWI was thus proven feasible in the human carotid artery. For stiffness identification, the pressure and regional wall displacement of the carotid artery in seven healthy subjects were estimated. The circumferential stress-strain relationship was then established assuming (i) a linear elastic two-parallel spring model and (ii) a two-dimensional, nonlinear, hyperelastic model. A slope change in the stress-strain curve was defined as a transition point. The average Youngs moduli of the elastic lamellae, elastin-collagen fibers, and collagen fibers were found to be equal to 0.15 ± 0.04, 0.89 ± 0.27 and 0.75 ± 0.29 MPa, respectively. The average incremental Youngs moduli before and after the transition point of the intact wall were found to be equal to 0.16 ± 0.04 MPa and 0.90 ± 0.25 MPa, respectively. The before and after transition point moduli of the tunica adventitia were found to be equal to 0.18 ± 0.05 MPa and 0.84 ± 0.22 MPa, respectively. The before and after transition point moduli of the tunica media were found to be equal to 0.19 ± 0.05 MPa and 0.90 ± 0.25 MPa, respectively. Thus, the feasibility of measuring the regional stress-strain relationship and stiffness of the normal human carotid artery in vivo noninvasively was demonstrated.

Noninvasive assessment of age-related arterial changes using the carotid stress-strain relationship in vivo: A pilot study

The feasibility of noninvasively measuring regional carotid artery stiffness by way of the stress-strain relationship was recently demonstrated in young normal adults in vivo. In this paper, similar methods were used to assess the stress-strain curve and derive the Youngs moduli in young and older subjects to evaluate the sensitivity of this approach in assessing age-related wall changes. Two types of recordings were performed on the common carotid arteries of 3 young (23-24 years) and 2 older (37 and 43 years) subjects: (a) RF signals acquired at 505-642 Hz, and (b) the pulse pressure signal using applanation tonometry. Subsequently, (a) arterial strain was calculated from the diameter waveform obtained using the radial displacements estimated by a 1D cross-correlation technique on the RF signals, and (b) arterial stress was estimated using the pressure (tonometry) signal, the diameter, and the wall thickness measured on the B-mode. The strain and stress signals were combined to produce the stress-strain curve. Using bilinear curve fitting, the Youngs modulus of the elastin-collagen fibers (E2), as well as the moduli for elastin (E1) and collagen (E3), separately, were estimated, under the assumption of a linear elastic two-parallel spring model. E1 and E2 were significantly larger in the older subjects, indicating stiffer tissues probably due to reduced elastin, which is in agreement with the related literature. No differences in E3 were noted between young and older subjects. The method holds promise for characterizing age-related arterial changes and can provide useful insight into the complex phenomena involved in arterial biomechanics.