Guddati, Murthy N.

Increased arterial stiffness is an early indicator of various cardiovascular diseases (CVD). Arterial Dispersion Ultrasound Vibrometry (ADUV) employs acoustic radiation force to induce arterial wall motion, enabling estimation of arterial wave velocity (i.e., phase velocity). This study investigates the correlation between ADUV-derived wave velocity and traditional arterial stiffness metrics, including distensibility, compliance, and stiffness index in the right and left common carotid arteries (CCAs). In a cohort of 59 subjects, categorized as (a) confirmed CVD (n = 27), (b) individuals with cardiovascular risk factors (n = 20), and (c) healthy controls (n = 12), we assessed arterial wave velocity across the cardiac cycle, split into 10 stages, to capture variations in mechanical properties. Univariate analysis demonstrated a statistically significant inverse linear relationship between ADUV-derivedwave velocity and distensibility for the right CCA during the systolic phase (cardiac stages 3–5) and diastolic phase (cardiac stages 7–9) (−0.35 < ρ < −0.50, p < 0.02). The wave velocity showed associations with compliance in some of these phases. No significant correlations were observed between wave velocity and the stiffness index. Our findings provide insights into the clinical relevance of ADUV-derived wave velocity as a biomarker for arterial stiffness and its potential to complement or enhance traditional stiffness assessments.

Arterial stiffness predicts cardiovascular disease, the leading cause of death worldwide. Ultrasound elastography methods that measure wave velocity have promise for evaluating the stiffness of blood vessels in vivo. However, vessel geometry modulates wave velocity by inducing geometric dispersion. Here, we compare Young's modulus E estimates obtained using in vivo wave velocity measurements and techniques that either do or do not consider geometry to ex vivo reference E values from mechanical testing . An acoustic radiation force was applied to generate propagating waves in the right common carotid artery (N = 9) and external jugular vein (N = 10) of anesthetized pigs. Vessels were imaged with ultrafast ultrasound to measure wave motion. Group velocity cg was estimated from wave motion using a Radon transform. E was estimated from cg using a bulk media assumption which neglects geometry, as well as the Moens–Korteweg equation and a semi-analytical finite element model, which consider geometry. Pigs were euthanized and vessels were excised. Ring-shaped samples were extracted and tested in a uniaxial tension setup to obtain reference E values for comparison with E values obtained from wave-based techniques. Techniques considering geometry exhibited higher correlation with reference E values, underscoring the need to incorporate geometry for accurate vascular elastography assessments.

Shear Wave Elastography (SWE) involves estimating mechanical properties through inversion, i.e., matching measured and simulated propagation characteristics of shear waves in the tissue. The accuracy of the estimated properties depends significantly on the specific characteristics/responses that are being matched. These could range from simple group velocity to dispersion curves and to full-wave response (particle velocity measurements). Using specific applications of arterial, liver, and tumor elstography, we illustrate that effective SWE is performed by resorting to an inversion approach, or combination of inversion approaches, guided by the underlying physics. To this end, we present inversion approaches ranging from matching dispersion characteristics to matching full waveform responses and provide rationale for choosing the appropriate technique(s) depending on the problem at hand.

Arterial stiffness is a predictor of cardiovascular diseases, the leading causes of death worldwide. Ultrasound-based methods that measure arterial wave propagation have promise for evaluating local stiffness in vivo. However, geometric properties of arteries cause dispersion, invalidating typical assumptions underlying the relationship between shear modulus G and group velocity cg, which clinical implementations of ultrasound shear wave elastography do not consider. Here, we examine the dependence of these estimates on geometry and evaluate alternative approaches. Wave motion in the proximal wall of an artery after application of focused acoustic radiation force is simulated with a semi-analytical finite element (SAFE) model, and cg is estimated using a time-to-peak algorithm to determine G with several methods. First, the value of G a clinical scanner would report is calculated assuming a bulk medium. Second, a pulse wave velocity (PWV)-based G estimate is calculated by taking cg = PWV and applying the Moens–Korteweg equation. Third, we develop an interpolation-based method to provide a corrected G estimate using data generated by the SAFE model. Simulation results show severe geometry-dependent bias with the bulk method, which is partially ameliorated with the PWV method and substantially improved with the interpolation approach. Results are validated using arterial phantoms.

Arterial stiffness is recognized as a highly clinically relevant and independent prognostic biomarker of cardiovascular disease. Arterial dispersion ultrasound vibrometry (ADUV) uses acoustic radiation force to generate propagating waves in the arterial wall and measures the wave motion to estimate arterial mechanical properties from fitting phase velocity dispersion curves to a waveguide model. ADUV measurements were made with a Verasonics V1 system with a L7-4 linear array transducer in 63 human subjects (27 male/36 female) from ages 50–88 years old. We made measurements at 10 time points within the cardiac cycle, termed cardiac stages. We estimated the phase velocity (median of cp from 400-600 Hz) and G determined from matching the dispersion curves with those from a waveguide model incorporating the artery diameter and wall thickness. Within the same study visit, we measured the intima-media thickness and diameter changes through the cardiac cycle with a General Electric Logiq E9 or E10 scanner. We also tonometry to determine the carotid-femoral pulse wave velocity. We report here comparisons of median phase velocity, cp, and inverted shear modulus, G, at different stages of the cardiac cycle with clinical metrics of arterial stiffness.

Abstract Objective. Measuring waves induced with acoustic radiation force (ARF) in arteries has been studied over the last decade. To date, it remains a challenge to quantitatively assess the local arterial biomechanical properties. The cylindrical shape and waveguide behavior of waves propagating in the arterial wall pose complexities to determining the mechanical properties of the artery. Approach . In this paper, an artery-mimicking tube in water is examined utilizing three-dimensional measurements. The cross-section of the tube is measured while a transducer is translated over 41 different positions along the length of the tube. Motion in the radial direction is calculated using two components of motion which are measured from the two orthogonal views of the cross-section. This enables more accurate estimation of motion along the circumference of tube. Main results . The results provide more information to categorize the motion in tube wall into two types of responses: a transient response and a steady state response. The transient response is caused by ARF application and the waves travel along the length of the tube for a relatively short period of time. This corresponds to the axial and circumferential propagating waves. The two circumferential waves travel along the circumference of tube in CW (clockwise) and CCW (counter-clockwise) direction and result in a standing wave. By using a directional filter, the two waves were successfully separated, and their propagation was more clearly visualized. As a steady state response, a circumferential mode is generated showing a symmetric motion (i.e. the proximal and distal walls move in the opposite direction) following the transient response. Significance. This study provides a more comprehensive understanding of the waves produced in an artery-mimicking tube with ARF application, which will provide opportunities for improving measurement of arterial mechanical properties.

Abstract Objective . Arterial dispersion ultrasound vibrometry (ADUV) relies on the use of guided waves in arterial geometries for shear wave elastography measurements. Both the generation of waves through the use of acoustic radiation force (ARF) and the techniques employed to infer the speed of the resulting wave motion affect the spectral content and accuracy of the measurement. In particular, the effects of the shape and location of the ARF beam in ADUV have not been widely studied. In this work, we investigated how such variations of the ARF beam affect the induced motion and the measurements in the dispersive modes that are excited. Approach. The study includes an experimental evaluation on an arterial phantom and an in vivo validation of the observed trends, observing the two walls of the waveguide, simultaneously, when subjected to variations in the ARF beam extension ( F / N ) and focus location. Main results. Relying on the theory of guided waves in cylindrical shells, the shape of the beam controls the selection and nature of the induced modes, while the location affects the measured dispersion curves (i.e. variation of phase velocity with frequency or wavenumber, multiple modes) across the waveguide walls. Significance. This investigation is important to understand the spectral content variations in ADUV measurements and to maximize inversion accuracy by tuning the ARF beam settings in clinical applications.

Abstract Arterial stiffness is an important biomarker for many cardiovascular diseases. Shear wave elastography is a recent technique aimed at estimating local arterial stiffness using guided wave inversion (GWI), i.e. matching the computed and measured wave dispersion. This paper develops and validates a new GWI approach by synthesizing various recent observations and algorithms: (a) refinements to signal processing to obtain more accurate experimental dispersion curves; (b) an efficient forward model to compute theoretical dispersion curves for immersed, incompressible cylindrical waveguides; (c) an optimization framework based on the recent observation that the measured dispersion curve is multimodal, i.e. it matches for not one but two different wave modes in two different frequency ranges. The resulting inversion approach is validated using extensive experimental data from rubber tube phantoms, not only for modulus estimation but also to simultaneously estimate modulus and wall thickness. The observations indicate that the modulus estimates are best performed with the information on wall thickness. The approach, which takes less than half a minute to run, is shown to be accurate, with the modulus estimated with less than 4% error for 70% of the experiments.

Shear Wave Elastography (SWE) has been used to estimate arterial stiffness, an important biomarker for early cardiovascular diseases. In arterial SWE, the carotid artery is excited with acoustic radiation force and the observed wave propagation characteristics are used to estimate the arterial stiffness. We present an efficient guided wave inversion procedure, where the elastic modulus of the arterial wall is estimated by matching wave dispersion from experiments and simulation. Central to the proposed approach is an efficient forward model that computes the dispersion relation for the incompressible, immersed cylindrical tube; the model achieves high efficiency by combining semi-analytical finite element methods, selective reduced integration, and perfectly matched discrete layers. We utilize this forward model in an optimization framework to develop a streamlined methodology to estimate the modulus of the arterial wall. In this talk, we present the formulation of the method as well as validation using phantom experiments on 10 rubber tubes (VytaFlex-10), followed by preliminary work on inverting for the viscoelastic modulus.