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Attenuation map or measurements based on local attenuation coefficient slope (ACS) in quantitative ultrasound (QUS) has shown potential for diagnosis of liver steatosis. In liver cancers, tissue abnormalities and tumors detected using ACS are also of interest to provide new image contrast to clinicians. Current phantom-based approaches have the limitation of assuming comparable speed of sound between the reference phantom and insonified tissues. Moreover, these methods present the inconvenience for operators to acquire data on phantoms as well as on patients. The main goal was to alleviate these drawbacks by proposing a methodology for constructing phantom-free regularized (PF-R) local ACS maps and investigate the performance in both homogeneous and heterogeneous media. The proposed method was tested on two tissue mimicking media with different ACS constructed as homogeneous phantoms, side-byside and top-to-bottom phantoms, and inclusion phantoms with different attenuations. Moreover, an in-vivo proof-of-concept was performed on healthy, steatotic and cancerous human liver datasets. Modifications brought to previous works include: a) a linear interpolation of the power spectrum in log-scale; b) the relaxation of the underlying hypothesis on the diffraction factor; c) a generalization to nonhomogeneous local ACS; and d) an adaptive restriction of frequencies to a more reliable range than the usable frequency range. Regularization was formulated as a generalized LASSO, and a variant of the Bayesian Information Criterion (BIC) was applied to estimate the Lagrangian multiplier on the LASSO constraint. In addition, we evaluated the proposed algorithm when applying median filtering before and after regularization. Tests conducted showed that the PF-R yielded robust results in all tested conditions, suggesting potential for additional validation as a diagnosis method.

Changes in biomechanical properties of biological soft tissues are often associated with physiological dysfunctions. Since biological soft tissues are hydrated, viscoelasticity is likely suitable to represent its solid-like behavior using elasticity and fluid-like behavior using viscosity. Shear wave elastography is a non-invasive imaging technology invented for clinical applications that has shown promise to characterize various tissue viscoelasticity. It is based on measuring and analyzing velocities and attenuations of propagated shear waves. In this review, principles and technical developments of shear wave elastography for viscoelasticity characterization from organ to cellular levels are presented, and different imaging modalities used to track shear wave propagation are described. At a macroscopic scale, techniques for inducing shear waves using an external mechanical vibration, an acoustic radiation pressure or a Lorentz force are reviewed along with imaging approaches proposed to track shear wave propagation, namely ultrasound, magnetic resonance, optical, and photoacoustic means. Then, approaches for theoretical modeling and tracking of shear waves are detailed. Following it, some examples of applications to characterize the viscoelasticity of various organs are given. At a microscopic scale, a novel cellular shear wave elastography method using an external vibration and optical microscopy is illustrated. Finally, current limitations and future directions in shear wave elastography are presented.

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Assessment of ultrasound shear wave attenuation (SWA) can increase the accuracy of liver steatosis grading. Recent developments allowed to propose the revisited frequency shift method for SWA imaging. This method was tested on force-fed ducks for foie gras production (n = 6), and a feasibility study on patients with non-alcoholic steatohepatitis (NASH n = 27 + 13 healthy volunteers) was conducted. Liver biopsy and magnetic resonance imaging proton density fat fraction (MRI-PDFF) were available as reference standards. For the human study, shear wave dispersion (SWD) was also computed. A subset of participants had repeated measurements (<1-month) to assess repeatability. The mean SWA (coefficient of variation within the attenuation map) for healthy duck livers were 0.77 (0.66), 1.18 (0.22), and 1.52 Np/m/Hz (0.14), and for fatty duck livers they were higher at 3.13 (0.55), 3.16 (0.24), and 4.84 Np/m/Hz (0.23). Receiver operating characteristic (ROC) curves using biopsy as reference were compared for steatosis grades >0 (>S0), >S1, and >S2. SWA was as good as MRI-PDFF, and both were better than SWD for steatosis grading (p < 0.05). The repeatability of SWA was very good (mean intraclass correlation coefficient of 0.97). In summary, SWA seems promising to become an ultrasound reference standard for steatosis grading in NASH.