The speed at which shear waves propagate in tissue can be used to quantify the shear modulus of the tissue. As many groups have shown, shear waves can be generated within tissues using focused, impulsive, acoustic radiation force excitations, and the resulting displacement response can be ultrasonically tracked through time. The goals of the work herein are two-fold: first, to develop and validate an algorithm to quantify shear wave speed from radiation force-induced, ultrasonicallydetected displacement data that is robust in the presence of poor displacement signal-to-noise ratio (SNR), and second, to apply this algorithm to in vivo datasets acquired in human volunteers in order to demonstrate the clinical feasibility of using this method to quantify the shear modulus of liver tissue in longitudinal studies. The ultimate clinical application of this work is non-invasive quantification of liver stiffness in the setting of fibrosis and steatosis.In the proposed algorithm, time to peak (TTP) displacement data in response to impulsive acoustic radiation force outside the region of excitation (ROE) are used to characterize the shear wave speed of a material, which is used to reconstruct the material's shear modulus. The algorithm is developed and validated using finite element method (FEM) simulations. Using this algorithm on simulated displacement fields, reconstructions for materials with shear moduli (μ) ranging from 1.3-5 kPa are accurate to within 0.3 kPa, while stiffer shear moduli ranging from 10-16 kPa are accurate to within 1.0 kPa. Ultrasonically tracking the displacement data, which introduces jitter in the displacement estimates, does not impede the use of this algorithm to reconstruct accurate shear moduli.Using in vivo data acquired intercostally in 20 volunteers with body mass indices (BMI) ranging from normal to obese, liver shear moduli have been reconstructed between 0.9 and 3.0 kPa, with an average precision of ±0.4 kPa. These reconstructed liver moduli are consistent with those reported in the literature (μ = 0.75-2.5 kPa) with a similar precision (±0.3 kPa). Repeated intercostal liver shear modulus reconstructions were performed on 9 different days in 2 volunteers over a 105 day period, yielding an average shear modulus of 1.9 ± 0.50 kPa (1.3-2.5 kPa) in the first volunteer, and 1.8 ± 0.4 kPa (1.1-3.0 kPa) in the second volunteer. The simulation and in vivo data to date demonstrate that this method is capable of generating accurate and repeatable liver stiffness measurements and appears promising as a clinical tool for quantifying liver stiffness.

Conventional ultrasound images are formed by delay-and-sum beamforming of the backscattered echoes received by individual elements of the transducer aperture. Although the delay-and-sum beamformer is well suited for ultrasound image formation, it is corrupted by speckle noise and challenged by acoustic clutter and phase aberration. We propose an alternative method of imaging utilizing the short-lag spatial coherence of the backscattered echoes. Compared to matched B-mode images, short-lag spatial coherence (SLSC) images demonstrate superior SNR and CNR in simulated and experimental speckle-generating phantom targets, but are shown to be challenged by limited point target conspicuity. Matched B-mode and SLSC images of a human thyroid are presented. The challenges and opportunities of real-time implementation of SLSC imaging are discussed.

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A full-wave equation that describes nonlinear propagation in a heterogeneous attenuating medium is solved numerically with finite differences in the time domain (FDTD). Three-dimensional solutions of the equation are verified with water tank measurements of a commercial diagnostic ultrasound transducer and are shown to be in excellent agreement in terms of the fundamental and harmonic acoustic fields and the power spectrum at the focus. The linear and nonlinear components of the algorithm are also verified independently. In the linear nonattenuating regime solutions match results from Field II, a well established software package used in transducer modeling, to within 0.3 dB. Nonlinear plane wave propagation is shown to closely match results from the Galerkin method up to 4 times the fundamental frequency. In addition to thermoviscous attenuation we present a numerical solution of the relaxation attenuation laws that allows modeling of arbitrary frequency dependent attenuation, such as that observed in tissue. A perfectly matched layer (PML) is implemented at the boundaries with a numerical implementation that allows the PML to be used with high-order discretizations. A −78 dB reduction in the reflected amplitude is demonstrated. The numerical algorithm is used to simulate a diagnostic ultrasound pulse propagating through a histologically measured representation of human abdominal wall with spatial variation in the speed of sound, attenuation, nonlinearity, and density. An ultrasound image is created in silico using the same physical and algorithmic process used in an ultrasound scanner: a series of pulses are transmitted through heterogeneous scattering tissue and the received echoes are used in a delay-and-sum beam-forming algorithm to generate a images. The resulting harmonic image exhibits characteristic improvement in lesion boundary definition and contrast when compared with the fundamental image. We demonstrate a mechanism of harmonic image quality improvement by showing that the harmonic point spread function is less sensitive to reverberation clutter.

Objectives Our goals were to explore the spatial variability within the cervix and the sensitivity of shear wave speeds (SWS) to assess softness/stiffness differences in ripened (softened) versus unripened tissue. Methods We obtained SWS estimates from hysterectomy specimens (n=22), a subset of which were ripened (n = 13). Multiple measurements were made longitudinally along the cervical canal on both the anterior and posterior sides of the cervix. Statistical tests of differences in the proximal vs. distal, anterior vs. posterior, and ripened vs. unripened cervix were performed with individual two-sample t-tests and a linear mixed model. Results We discovered that SWS estimates monotonically increase from distal to proximal longitudinally along the cervix, that they also vary in the anterior compared to the posterior cervix, and that they are significantly different in ripened vs. unripened cervical tissue. Specifically, the mid position SWS estimates for the unripened group were 3.45±0.95 m/s (anterior) and 3.56±0.92 m/s (posterior), and 2.11±0.45 m/s (anterior) and 2.68±0.57 m/s (posterior) for the ripened (p<0.001). Conclusions We propose that shear wave speed estimation may be a valuable research and, ultimately, diagnostic tool for objective quantification of cervical stiffness/softness.