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Abstract This paper describes a new method for estimating anisotropic mechanical properties of fibrous soft tissue by imaging shear waves induced by focused ultrasound (FUS) and analyzing their direction-dependent speeds. Fibrous materials with a single, dominant fiber direction may exhibit anisotropy in both shear and tensile moduli, reflecting differences in the response of the material when loads are applied in different directions. The speeds of shear waves in such materials depend on the propagation and polarization directions of the waves relative to the dominant fiber direction. In this study, shear waves were induced in muscle tissue (chicken breast) ex vivo by harmonically oscillating the amplitude of an ultrasound beam focused in a cylindrical tissue sample. The orientation of the fiber direction relative to the excitation direction was varied by rotating the sample. Magnetic resonance elastography (MRE) was used to visualize and measure the full 3D displacement field due to the ultrasound-induced shear waves. The phase gradient (PG) of radially propagating “slow” and “fast” shear waves provided local estimates of their respective wave speeds and directions. The equations for the speeds of these waves in an incompressible, transversely isotropic (TI), linear elastic material were fitted to measurements to estimate the shear and tensile moduli of the material. The combination of focused ultrasound and MR imaging allows noninvasive, but comprehensive, characterization of anisotropic soft tissue.
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IMAGING OF FOCUSED ULTRASOUND-INDUCED SHEAR WAVES TO PROBE MECHANICAL ANISOTROPY OF TISSUE
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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- Award ID(s):
- 1727412
- PAR ID:
- 10301383
- Date Published:
- Journal Name:
- Proceedings of the 2021 Design of Medical Devices Conference
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Conference Paper:
- https://doi.org/10.1115/DMD2021-1030
