Corneal collagen crosslinking (CXL) is a surgical procedure to treat corneal ectasia. Ultraviolet (UV) light and riboflavin are combined to modify corneal microstructure by forming additional chemical bounds between stromal collagen fibers. Because of limited in-depth penetration of riboflavin and UV-light attenuation, a demarcation between treated and untreated regions can be observed, suggesting a two-layer structure after treatment. Here, we present a method of elastic moduli reconstruction in both cornea layers using OCE-tracked guided waves and an analytical model of a 2-layer nearly incompressible transversely isotropic (NITI) medium. An example of reconstruction is demonstrated in an ex vivo human cornea.
Evaluating biomechanical properties is crucial to manage skin care and help guide skin cosmetic and reconstruction procedures. Wave-based OCE tracks mechanical waves propagating along the skin surface, where wave speed is used to reconstruct skin elasticity. The first obstacle to quantifying skin elasticity is anisotropy, i.e. 3 shear moduli must be determined. Second, the surface wave group velocity, usually measured experimentally, is very different from the phase velocity needed for elasticity reconstruction. Here we propose methods to overcome these challenges, test the methods using numerical simulations and use them to reconstruct all shear moduli in human skin in vivo.
Dynamic optical coherence elastography (OCE) tracks mechanical wave propagation in the subsurface region of tissue to map its shear modulus. For bulk shear waves, the lateral resolution of the reconstructed modulus map (i.e., elastographic resolution) can approach that for OCT, typically a few tens of microns. However, skin, cornea and many other tissues are layered or bounded leading to the formation of guided mechanical waves. We performed numerical simulations and acoustic micro-tapping experiments to show that in bounded media, the elastographic resolution cannot reach the OCT structural resolution and is mainly defined by the thickness of the bounded tissue layer.
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