title: Magnetic resonance elastography of slow and fast shear waves illuminates differences in shear and tensile moduli in anisotropic tissue.
abstract: Mechanical anisotropy is an important property of fibrous tissues; for example, the anisotropic mechanical properties of brain white matter may play a key role in the mechanics of traumatic brain injury (TBI). The simplest anisotropic material model for small deformations of soft tissue is a nearly incompressible, transversely isotropic (ITI) material characterized by three parameters: minimum shear modulus (µ), shear anisotropy (ϕ=µ1µ-1) and tensile anisotropy (ζ=E1E2-1). These parameters can be determined using magnetic resonance elastography (MRE) to visualize shear waves, if the angle between the shear-wave propagation direction and fiber direction is known. Most MRE studies assume isotropic material models with a single shear (µ) or tensile (E) modulus. In this study, two types of shear waves, "fast" and "slow", were analyzed for a given propagation direction to estimate anisotropic parameters µ, ϕ, and ζ in two fibrous soft materials: turkey breast ex vivo and aligned fibrin gels. As expected, the speed of slow shear waves depended on the angle between fiber direction and propagation direction. Fast shear waves were observed when the deformations due to wave motion induced stretch in the fiber direction. Finally, MRE estimates of anisotropic mechanical properties in turkey breast were compared to estimates from direct mechanical tests.
authors.affiliation: Mechanical Engineering and Materials Science, Washington University in St. Louis, United States.
authors.forename: J L
authors.initials: JL
authors.lastname: Schmidt
authors.affiliation: Mechanical Engineering and Materials Science, Washington University in St. Louis, United States.
authors.forename: D J
authors.initials: DJ
authors.lastname: Tweten
authors.affiliation: Biomedical Engineering, Washington University in St. Louis, United States.
authors.forename: A N
authors.initials: AN
authors.lastname: Benegal
authors.affiliation: Biomedical Engineering, Washington University in St. Louis, United States.
authors.forename: C H
authors.initials: CH
authors.lastname: Walker
authors.affiliation: Electrical Engineering, Massachusetts Institute of Technology, United States.
authors.forename: T E
authors.initials: TE
authors.lastname: Portnoi
authors.affiliation: Mechanical Engineering and Materials Science, Washington University in St. Louis, United States.
authors.forename: R J
authors.initials: RJ
authors.lastname: Okamoto
authors.affiliation: Biomedical Magnetic Resonance Laboratory, Radiology, Washington University in St. Louis, United States.
authors.forename: J R
authors.initials: JR
authors.lastname: Garbow
authors.affiliation: Mechanical Engineering and Materials Science, Washington University in St. Louis, United States; Biomedical Engineering, Washington University in St. Louis, United States. Electronic address: pvb@wustl.edu.
authors.forename: P V
authors.initials: PV
authors.lastname: Bayly
mesh_terms: D000818:Animals; D016880:Anisotropy; D004548:Elasticity; D054459:Elasticity Imaging Techniques; D005260:Female; D005337:Fibrin / Q000502:physiology; D005782:Gels; D008321:Mammary Glands, Animal / Q000502:physiology; D055595:Mechanical Phenomena; D008962:Models, Theoretical; D014422:Turkeys
chemical_list: D005782:Gels; D005337:Fibrin
keywords: Anisotropy; MR Elastography; Shear waves; Transversely isotropic material