Greenleaf, James F.

Applications, such as shear wave elastography and Doppler imaging, require high frame rates. Plane wave compounding (PWC) is widely used for these applications. Comb detection was proposed to combine the high frame rate of PWC and the high signal quality of focused beam scanning. Comb detection transmits multiple focused beams simultaneously to increase the frame rate. These comb beams are scanned laterally to cover the whole region-of-interest. The simultaneous multiple transmission of focused beams causes crosstalk between neighboring beams. Transmit crosstalk is related to the pressure field of transmit beams, and receive crosstalk is determined by the beam profile of receive beamforming. In this study, we varied f-number (F/N) and apodization window and measured their effects on crosstalk based on the pressure field and an arterial wall simulation using Field II. For transmit design, transmit apodization with a Hamming window significantly reduced crosstalk compared to a rectangular window and transmit F/N had little impact on the crosstalk. Regarding receive beamforming, a Hamming window led to lower crosstalk than the rectangular window and F/N from 1 to 5 suppressed crosstalk to below 30 dB. The effects of crosstalk in shear wave motion data from phantom experiments were also analyzed. Comb detection showed less motion artifact than PWC in phantom experiments.

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) is an established ultrasonic imaging technique, that provides quantitative measurements of tissue mechanical properties. This technique relies on the combination of acoustic radiation force push beams to generate propagating shear waves and ultrafast imaging, which allows high two-dimensional imaging frame rates to capture the induced motion. Utilizing coherent plane wave compounding (PWC) can improve signal-to-noise ratio (SNR) of acquired data. While the image quality improves, the pulse repetition frequency (PRF) is reduced, reaching limiting bandwidths to perform SWE. To address these challenges, a method for making SWE measurements with high PRF and SNR is presented. The idea follows the Time Aligned Sequential Tracking method, proposed to allow SWE with enhanced SNR in conventional clinical ultrasound scanners. The time alignment process is applied here to the full field-of-view, acquired by using plane waves transmitted at different angles. In this way, the full PRF and SNR gains are obtained. Results are shown from experiments in tissue-mimicking phantoms, where motion and extracted shear wave velocity are compared for the traditional and proposed data processing schemes. Preliminary results on arterial phantoms are also discussed, to emphasize the beneficial effects of the method in dispersive geometries.

A lung ultrasound surface wave elastography (LUSWE) technique is developed for assessing interstitial lung disease (ILD). In LUSWE, a 0.1 second harmonic vibration is generated on the chest wall of a subject using a handheld vibrator. An ultrasound probe is aligned with the vibration indenter in the same intercostal region to measure the generated surface wave speed of the lung. A human subject is examined in a sitting position. The lung is tested at the total lung volume. The upper anterior lungs are tested through the second intercostal space in the mid-clavicular line. The lower lateral lungs are tested in the mid-axillary line and the lower posterior lungs are tested in the mid-scapular line. In a prospective clinical study, we measure both lungs through the six intercostal regions for patients and healthy controls. The surface wave speed is measured at 100 Hz, 150 Hz, and 200 Hz. Significant differences of wave speed between patients and controls were found in all lung regions at all frequencies. We also found positive correlation between LUSWE and CT analyses. LUSWE is a safe and noninvasive technique for generating and measuring surface wave propagation on the lung. LUSWE may be useful for assessing ILD.

Arterial stiffness is a well-known biomarker of early cardiovascular disease. Shear wave dispersion ultrasound vibrometry (SDUV) has emerged as a promising technique to estimate local arterial stiffness from the observed dispersion of guided waves. With the ultimate goal of developing real-time inversion for arterial stiffness from SDUV measurements, we develop and validate highly efficient and accurate computational algorithms that compute the wave dispersion in multi-layered immersed tubes. The proposed approach carefully combines Fourier transformation and one-dimensional finite-element discretization to accurately capture fully three-dimensional wave propagation. The method is several orders of magnitude more efficient than three-dimensional finite element simulation, and eliminates other complexities such as the need for absorbing boundary conditions. The method is validated using SDUV experiments on tissue-mimicking phantoms. The validation exercise uncovered an important detail that is often overlooked—the dispersion curve captured through SDUV experiments does not correspond to a single dispersion curve, but a combination of multiple dispersion curves. This implies that proper identification of the dispersion curves could be critical to estimating the arterial stiffness. In this talk, we present the details of the proposed method including formulation, computational complexity, verification and validation.

Abstract Diastolic dysfunction causes close to half of congestive heart failures and is associated with increased stiffness in left-ventricular myocardium. A clinical tool capable of measuring viscoelasticity of the myocardium could be beneficial in clinical settings. We used Lamb wave Dispersion Ultrasound Vibrometry (LDUV) for assessing the feasibility of making in vivo non-invasive measurements of myocardial elasticity and viscosity in pigs. In vivo open-chest measurements of myocardial elasticity and viscosity obtained using a Fourier space based analysis of Lamb wave dispersion are reported. The approach was used to perform ECG-gated transthoracic in vivo measurements of group velocity, elasticity and viscosity throughout a single heart cycle. Group velocity, elasticity and viscosity in the frequency range 50–500 Hz increased from diastole to systole, consistent with contraction and relaxation of the myocardium. Systolic group velocity, elasticity and viscosity were 5.0 m s −1 , 19.1 kPa, 6.8 Pa · s, respectively. In diastole, the measured group velocity, elasticity and viscosity were 1.5 m s −1 , 5.1 kPa and 3.2 Pa · s, respectively.