Non-contact acoustic micro-tapping optical coherence elastography (AμT-OCE) in the cornea: Mechanical model, ex vivo and in vivo results, and direct comparison with mechanical tests

Abstract

The cornea is one of the primary determinants of visual performance. Its highly pressurized anisotropic structure of collagen fibrils embedded in a hydrated proteoglycan matrix forms the clear refractive primary lens of the eye. Despite recent success in corneal topography, there are no clinical tools to predict shape changes from vision-correction therapies such as LASIK and CXL due to the absence of non-contact methods that can map corneal elasticity in vivo. Here we review our newest results in quantification of corneal elasticity. Recently, we produced a technology combining non-contact excitation of sub-mm wavelength mechanical waves in the cornea using air-coupled acoustic micro-tapping (AμT), with 1.6 MHz scan-rate phase-sensitive OCT to track their propagation three-dimensionally in real time. However, corneal moduli reconstruction from measured wavefields requires an anisotropic elastic model and cornea’s bounded, non-flat geometry must be taken into account. Thus, a nearly incompressible transversally isotropic (NITI) model of corneal elasticity was developed. We have performed multiple ex vivo and in vivo studies in rabbit, porcine and human corneas and compared AμT-OCE results directly with conventional mechanical tests (tensile extensometry and shear rheometry). These studies demonstrate that in-plane Young’s and out-of-plane shear moduli can be accurately mapped with non-contact, non-invasive AμT-OCE.