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1. IMAGING OF FOCUSED ULTRASOUND-INDUCED SHEAR WAVES TO PROBE MECHANICAL ANISOTROPY OF TISSUE

   https://doi.org/10.1115/DMD2021-1030  

   C.A. Guertler, R.J. Okamoto (April 2021, Proceedings of the 2021 Design of Medical Devices Conference)

   null (Ed.)

   It is important to understand mechanical anisotropy in fibrous soft tissues because of the relationship of anisotropy to tissue function, and because anisotropy may change due to injury and disease. We have developed a method to noninvasively investigate anisotropy, based on MR imaging of harmonic ultrasound-induced motion (MR-HUM), using focused ultrasound (FUS) and magnetic resonance elastography (MRE). MR-HUM produces symmetric, radial waves inside a tissue, which enables a simple assessment of anisotropy using features of the resulting shear wave fields. This method was applied to characterize ex vivo muscle tissue, which is known to exhibit mechanical anisotropy. Finite element (FE) simulations of the experiment were performed to illustrate and validate the approach. Anisotropy was characterized by ratios of apparent shear moduli and strain components in different directions. 

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2. Shear Wave Propagation and Estimation of Material Parameters in a Nonlinear, Fibrous Material

   https://doi.org/10.1115/1.4044504  

   Hou, Zuoxian; Okamoto, Ruth J.; Bayly, Philip V. (May 2020, Journal of Biomechanical Engineering)

   Abstract This paper describes the propagation of shear waves in a Holzapfel–Gasser–Ogden (HGO) material and investigates the potential of magnetic resonance elastography (MRE) for estimating parameters of the HGO material model from experimental data. In most MRE studies the behavior of the material is assumed to be governed by linear, isotropic elasticity or viscoelasticity. In contrast, biological tissue is often nonlinear and anisotropic with a fibrous structure. In such materials, application of a quasi-static deformation (predeformation) plays an important role in shear wave propagation. Closed form expressions for shear wave speeds in an HGO material with a single family of fibers were found in a reference (undeformed) configuration and after imposed predeformations. These analytical expressions show that shear wave speeds are affected by the parameters (μ0, k1, k2, κ) of the HGO model and by the direction and amplitude of the predeformations. Simulations of corresponding finite element (FE) models confirm the predicted influence of HGO model parameters on speeds of shear waves with specific polarization and propagation directions. Importantly, the dependence of wave speeds on the parameters of the HGO model and imposed deformations could ultimately allow the noninvasive estimation of material parameters in vivo from experimental shear wave image data. 

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3. Using compact fiber optic Sagnac interferometer for biological-based soft tissue elasticity characterization

   https://doi.org/10.1117/12.3051833  

   Xia, Jinjun; Xu, Suxuan (May 2025, SPIE)

   Rizzo, Piervincenzo; Su, Zhongqing; Ricci, Fabrizio; Peters, Kara J (Ed.)

   he further signal processing for wave signal extraction as in displacement-based detection systems. However, due to both interfering lights coming from sample surface, the collected light in a fiber-optic-based Sagnac interferometer system is very weak when applied to biological tissue, where the refractive index of tissue and air are close. The objective of this paper is to study the feasibility using a compact fiber-optic Sagnac interferometer to detect vibrational waves on a biological tissue surface. An actuator made with a 10mm x 10mm x 3mm piezoelectric chip loaded on a 3D-printed polymer-made prism-shaped wedge (1cm x1cm x1cm) was used for ultrasound surface wave excitation. A bulk copolymer-in-oil phantom (100mm diameter with 27mm height) was used to mimic biological tissues. A compact fiber-optic-based interferometer was used to detect the propagation of surface waves in the tissue mimicking phantom and the wave propagation speeds were determined based on the wave detection. Young’s modulus was calculated based on the measured wave speed on the phantom surface. A tensile testing machine was used to measure the Young’s modulus in a compression mode as a comparison. The results were compared. 

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4. A theoretical framework for predicting the heterogeneous stiffness map of brain white matter tissue

   https://doi.org/10.1088/1478-3975/ad88e4  

   Chavoshnejad, Poorya; Li, Guangfa; Solhtalab, Akbar; Liu, Dehao; Razavi, Mir_Jalil (October 2024, Physical Biology)

   Abstract Finding the stiffness map of biological tissues is of great importance in evaluating their healthy or pathological conditions. However, due to the heterogeneity and anisotropy of biological fibrous tissues, this task presents challenges and significant uncertainty when characterized only by single-mode loading experiments. In this study, we propose a new theoretical framework to map the stiffness landscape of fibrous tissues, specifically focusing on brain white matter tissue. Initially, a finite element (FE) model of the fibrous tissue was subjected to six loading cases, and their corresponding stress–strain curves were characterized. By employing multiobjective optimization, the material constants of an equivalent anisotropic material model were inversely extracted to best fit all six loading modes simultaneously. Subsequently, large-scale FE simulations were conducted, incorporating various fiber volume fractions and orientations, to train a convolutional neural network capable of predicting the equivalent anisotropic material properties solely based on the fibrous architecture of any given tissue. The proposed method, leveraging brain fiber tractography, was applied to a localized volume of white matter, demonstrating its effectiveness in precisely mapping the anisotropic behavior of fibrous tissue. In the long-term, the proposed method may find applications in traumatic brain injury, brain folding studies, and neurodegenerative diseases, where accurately capturing the material behavior of the tissue is crucial for simulations and experiments. 

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5. Enhanced Instantaneous Elastography in Tissues and Hard Materials Using Bulk Modulus and Density Determined without Externally Applied Material Deformation

   https://doi.org/10.1109/TUFFC.2019.2950343  

   Jin, Yuqi; Walker, Ezekiel; Krokhin, Arkadii; Heo, Hyeonu; Choi, Tae-Youl; Neogi, Arup (October 2019, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control)

   —Ultrasound is a continually developing technology that is broadly used for fast, non-destructive mechanical property detection of hard and soft materials in applications ranging from manufacturing to biomedical. In this study, a novel monostatic longitudinal ultrasonic pulsing elastography imaging method is introduced. Existing elastography methods require an acoustic radiational or dynamic compressive externally applied force to determine the effective bulk modulus or density. This new, passive M-mode imaging technique does not require an external stress, and can be effectively utilized for both soft and hard materials. Strain map imaging and shear wave elastography are two current categories of M-mode imaging that show both relative and absolute elasticity information. The new technique is applied to hard materials and soft material tissue phantoms for demonstrating effective bulk modulus and effective density mapping. As compared to standard techniques, the effective parameters fall within 10% of standard characterization methods for both hard and soft materials. As neither the standard A-mode imaging technique nor the presented technique require an external applied force, the techniques are applied to composite heterostructures and the findings presented for comparison. The presented passive M-mode technique is found to have enhanced resolution over standard A-mode modalities. 

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