Recent advances in dynamic OCE have resulted in tools that can generate/track sub-mm wavelength mechanical waves in tissue. However, reconstructing material elasticity from measured wavefields needs an appropriate model accounting for tissue anisotropy, structure and geometry. We assume that tissues consisting of collagen fibers can be locally described with a model of a nearly incompressible transverse isotropic (NITI) medium using three elastic parameters to describe shear and tensile behavior. Examples of NITI media are discussed and the problem of inversion of moduli from bulk shear, Rayleigh and guided waves is considered.
Reconstructive skin surgeries drive the clinical need for non-contact objective measurements of skin elasticity. Here we demonstrate that all three of skin’s elastic constants (in-plane and out-of-plane shear moduli and an additional modulus defining skin’s tensile anisotropy) and the orientation of collagen fibers in dermis can be determined from Rayleigh wave anisotropy in-plane with acoustic micro-tapping (AuT) OCE. A nearly-incompressible transverse isotropic (NITI) model was used to reconstruct skin’s moduli from OCE measurements in human forearm in vivo for five healthy volunteers. Co-registered polarization-sensitive (PS-) OCT shows that optical and mechanical axes are co-aligned at measured sites.
Accurate estimates of corneal mechanical properties may improve diagnosis and treatment of many ophthalmic conditions. Recently, we introduced a nearly incompressible transversely isotropic (NITI) model based on two independent shear moduli determining tensile and out-of-plane shear behavior. Here, we directly compare acoustic micro-tapping OCE (AμT-OCE) modulus estimates with those obtained from mechanical testing on ex-vivo porcine corneas. Both OCE and mechanical testing show tensile behavior governed by a Young’s modulus on the order of MPa and out-of-plane shear behavior by a modulus on the order of tens of kPa, suggesting strong anisotropy in the cornea.
Using numerical and analytical models of wave propagation in mechanically anisotropic materials, we highlight the complications associated with quantitative estimates of mechanical moduli in human skin. To obtain reliable, quantitative measurements of moduli in human skin, multiple aspects of mechanical wave propagation in structures typical of skin must be considered. Using a nearly incompressible transverse isotropic (NITI) model, preliminary measurements of both shear moduli (G and μ) in healthy in vivo human skin are presented.
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