Yufeng Deng

Dependence of shear wave spectral content on acoustic radiation force excitation duration and spatial beamwidth

Shear Wave Elasticity Imaging (SWEI) has become increasingly popular to non-invasively characterize liver fibrosis. Generating adequate shear wave displacements in human liver in vivo can be challenging, especially in the high Body Mass Index (BMI) population being evaluated for Non-Alcoholic Fatty Liver Disease (NAFLD). Technical approaches to improve ARF-induced displacements include (1) using more aggressive focal configurations to generate higher peak force amplitudes, and (2) increasing the duration of the acoustic radiation force excitation. Using finite element method (FEM) models of Gaussian ARF excitations of varying spatial extent and temporal duration, we have demonstrated that shear wave velocity spectra are affected by both the spatial distribution and temporal duration of acoustic radiation force shear wave excitation sources. Shear wave spectra can be affected by the ARF spatial extent in the orthogonal dimension, which can be important when lateral:elevation excitation beamwidth anisotropies exist as a function of depth. Shear wave spectral content differences in different stiffness media are minimized for tightly-focused ARF excitations when using longer excitation durations, decreasing from ~60% for 100 μs excitations to ~10% for 1.5 ms excitations. These absolute and relative differences are significantly reduced for broader excitations. We have demonstrated similar trends experimentally using tissue-mimicking phantoms.

RSNA QIBA ultrasound shear wave speed Phase II phantom study in viscoelastic media

Using ultrasonic shear wave speed (SWS) estimates has become popular to noninvasively evaluate liver fibrosis, but significant inter-system variability in liver SWS measurements can preclude meaningful comparison of measurements performed with different systems. The RSNA Quantitative Imaging Biomarker Alliance (QIBA) ultrasound SWS committee has been developing elastic and viscoelastic (VE) phantoms to evaluate system dependencies of SWS estimates. The objective of this study is to compare SWS measurements between commercially-available systems using phantoms that have viscoelastic properties similar to those observed in normal and fibrotic liver. CIRS, Inc. fabricated three phantoms using a proprietary oil-water emulsion infused in a Zerdine® hydrogel that were matched in viscoelastic behavior to healthy and fibrotic human liver data. Phantoms were measured at academic, clinical, government and vendor sites using different systems with curvilinear arrays at multiple focal depths (3.0, 4.5 & 7.0 cm). The results of this study show that current-generation ultrasound SWS measurement systems are able to differentiate viscoelastic materials that span healthy to fibrotic liver. The deepest focal depth (7.0 cm) yielded the greatest inter-system variability for each phantom (maximum of 17.7%) as evaluated by IQR. Inter-system variability was consistent across all 3 phantoms and was not a function of stiffness. Median SWS estimates for the greatest outlier system for each phantom/focal depth combination ranged from 12.7-17.6%. Future efforts will include performing more robust statistical analyses of these data, comparing these phantom data trends with viscoelastic digital phantom data, providing vendors with study site data to refine their systems to have more consistent measurements, and integrating these data into the QIBA ultrasound shear wave speed measurement profile.

Analyzing the Impact of Increasing Mechanical Index and Energy Deposition on Shear Wave Speed Reconstruction in Human Liver

Shear wave elasticity imaging (SWEI) has found success in liver fibrosis staging. However, technical failure and unreliable shear wave speed (SWS) estimation have been reported to increase both with elevated patient body mass index (BMI) and in the presence of significant hepatic fibrosis. Elevated BMI results in a significant amount of subcutaneous fat which attenuates acoustic radiation force (ARF) and abberates tracking beams. Advanced fibrosis results in small displacement amplitudes in stiff livers. This work evaluates hepatic SWEI measurement success as a function of push pulse energy using 2 Mechanical Index (MI) values (1.6 and 2.2) over a range of pulse durations. The rate of successful SWS estimation for 8 repeated measurements is linearly proportional to the push energy level. As expected, elevated push energy in SWEI measurements results in higher displacement signal-to-noise ratio (SNR). SWEI measurements with elevated push energy are successful in patients for whom standard push energy levels failed. Deep liver capsule is shown to be an indicator for lower yield of SWS estimation. Patients with deep liver capsules are likely to benefit from elevated push energies. We conclude that there is clinical benefit to using elevated acoustic output for hepatic SWS measurement in “difficult to image” patients.

Robust characterization of viscoelastic materials from measurements of group shear wave speeds

This study describes a new method to determine the stiffness μ₀ and viscosity η of viscoelastic materials by observing shear wave propagation following localized, impulsive excitations and measuring the group shear wave speeds V_disp and V_vel determined using the shear wave displacement and velocity signals, respectively. The stiffness and viscosity are found by analytically solving the equation of motion describing shear wave propagation and calculating lookup tables μ₀(V_disp, V_vel) and η(V_disp, V_vel) to give μ₀ and η from the measured shear wave speeds. Results are presented for three viscoelastic phantoms and one approximately elastic phantom. The method is robust because group shear wave speeds are easily measured experimentally, the lookup tables are relatively insensitive to the size and shape of the excitation, and the difference ΔV = V_vel - V_disp gives a first order measure of viscosity in the same way that the group shear wave speed gives a measure of the material stiffness.

On System-Dependent Sources of Uncertainty and Bias in Ultrasonic Quantitative Shear-Wave Imaging

Ultrasonic quantitative shear-wave imaging methods have been developed over the last decade to estimate tissue elasticity by measuring the speed of propagating shear waves following acoustic radiation force excitation. This work discusses eight sources of uncertainty and bias arising from ultrasound system-dependent parameters in ultrasound shear-wave speed (SWS) measurements. Each of the eight sources of error is discussed in the context of a linear, isotropic, elastic, homogeneous medium, combining previously reported analyses with Field II simulations, full-wave 2-D acoustic propagation simulations, and experimental studies. Errors arising from both spatial and temporal sources lead to errors in SWS measurements. Arrival time estimation noise, speckle bias, hardware fluctuations, and phase aberration cause uncertainties (variance) in SWS measurements, while pulse repetition frequency (PRF) and beamforming errors, as well as coupling medium sound speed mismatch, cause biases in SWS measurements (accuracy errors). Calibration of the sources of bias is an important step in the development of shear-wave imaging systems. In a well-calibrated system, where the sources of bias are minimized, and averaging over a region of interest (ROI) is employed to reduce the sources of uncertainty, an SWS error <; 3% can be expected.

Accounting for the Spatial Observation Window in the 2-D Fourier Transform Analysis of Shear Wave Attenuation

Recent measurements of shear wave propagation in viscoelastic materials have been analyzed by constructing the 2-D Fourier transform (2DFT) of the shear wave signal and measuring the phase velocity c(ω) and attenuation α(ω) from the peak location and full width at half-maximum (FWHM) of the 2DFT signal at discrete frequencies. However, when the shear wave is observed over a finite spatial range, the 2DFT signal is a convolution of the true signal and the observation window, and measurements using the FWHM can yield biased results. In this study, we describe a method to account for the size of the spatial observation window using a model of the 2DFT signal and a non-linear, least-squares fitting procedure to determine c(ω) and α(ω). Results from the analysis of finite-element simulation data agree with c(ω) and α(ω) calculated from the material parameters used in the simulation. Results obtained in a viscoelastic phantom indicate that the measured attenuation is independent of the observation window and agree with measurements of c(ω) and α(ω) obtained using the previously described progressive phase and exponential decay analysis.

Analyzing the impact of increasing Mechanical Index (MI) and energy deposition on shear wave speed (SWS) reconstruction in human liver

Shear wave elasticity imaging (SWEI) has found success in liver fibrosis staging. However, technical failure and unreliable shear wave speed (SWS) estimation have been reported to increase both with elevated patient body mass index (BMI) and in the presence of significant hepatic fibrosis. Elevated BMI results in a significant amount of subcutaneous fat which attenuates acoustic radiation force (ARF) and abberates tracking beams. Advanced fibrosis results in small displacement amplitudes in stiff livers. This work evaluates hepatic SWEI measurement success as a function of push pulse energy using 2 Mechanical Index (MI) values (1.6 and 2.2) over a range of pulse durations. The rate of successful SWS estimation for 8 repeated measurements is linearly proportional to the push energy level. As expected, elevated push energy in SWEI measurements results in higher displacement signal-to-noise ratio (SNR). SWEI measurements with elevated push energy are successful in patients for whom standard push energy levels failed. Deep liver capsule is shown to be an indicator for lower yield of SWS estimation. Patients with deep liver capsules are likely to benefit from elevated push energies. We conclude that there is clinical benefit to using elevated acoustic output for hepatic SWS measurement in “difficult to image” patients.

Evaluating the Benefit of Elevated Acoustic Output in Harmonic Motion Estimation in Ultrasonic Shear Wave Elasticity Imaging

Harmonic imaging techniques have been applied in ultrasonic elasticity imaging to obtain higher-quality tissue motion tracking data. However, harmonic tracking can be signal-to-noise ratio and penetration depth limited during clinical imaging, resulting in decreased yield of successful shear wave speed measurements. A logical approach is to increase the source pressure, but the in situ pressures used in diagnostic ultrasound have been subject to a de facto upper limit based on the Food and Drug Administration guideline for the mechanical index (MI <1.9). A recent American Institute of Ultrasound in Medicine report concluded that an in situ MI up to 4.0 could be warranted without concern for increased risk of cavitation in non-fetal tissues without gas bodies if there were a concurrent clinical benefit. This work evaluates the impact of using an elevated MI in harmonic motion tracking for hepatic shear wave elasticity imaging. The studies indicate that high-MI harmonic tracking increased shear wave speed estimation yield by 27% at a focal depth of 5 cm, with larger yield increase in more difficult-to-image patients. High-MI tracking improved harmonic tracking data quality by increasing the signal-to-noise ratio and decreasing jitter in the tissue motion data. We conclude that there is clinical benefit to use of elevated acoustic output in shear wave tracking, particularly in difficult-to-image patients.

Nonlinear Ultrasound Propagation in Homogeneous and Heterogeneous Media: Factors Affecting the in situ Mechanical Index (MI)

This study used 3D nonlinear ultrasound simulations to identify the sources of error when estimating the in situ Peak Rarefaction Pressure (PRP) as compared to linear derating in the context of the Mechanical Index (MI) measurement. We found that varying material nonlinearity within the range of soft tissue (5 < B/A < 10) does not affect PRP estimation. We also found that for large apertures (F/1.5), phase aberration is the dominant factor that causes the linear derating MI method to consistently report higher PRP than actually occurs in situ. For smaller apertures (F/5), both phase aberration and reverberation clutter cause the linear derating MI method to report lower PRP than actually occurs in situ.

Comparison of SWEI methods for measuring the frequency dependent phase velocity and attenuation in viscoelastic materials

Shear wave (SW) propagation in viscoelastic materials is characterized by a complex, frequency-dependent shear modulus μ(f). Measurements of μ(f) typically require Fourier transform techniques to decompose an observed SW signal into discrete frequency components and two independent measurements such as the phase velocity c(f) and attenuation α(f) to determine the real and imaginary parts of μ(f).This study compares three analysis methods to determine the frequency dependent phase velocity c(ω) and attenuation α(ω) from shear wave propagation in viscoelastic materials following acoustic radiation force impulse excitations. The analysis methods considered include shear wave spectroscopy, finite window 2D Fourier transform analysis, and analysis using group shear wave speeds (gSWS) calculated using fractional derivatives of the shear wave signals. For simulated shear wave data, results are compared to the true values of c(ω) and α(ω) calculated from the material parameters used in the simulations and show good agreement among the analysis methods, with somewhat lower agreement from the gSWS method in cases with low viscosity. Results from phantom measurements show good agreement among the three methods for measurements of c(ω). For α(ω), there is more scatter among repeated experimental measurements and less agreement among the analysis methods.