High-resolution quantitative imaging of cornea elasticity using supersonic shear imaging.

The noninvasive estimation of in vivo mechanical properties of cornea is envisioned to find several applications in ophthalmology. Such high-resolution measurements of local cornea stiffness could lead to a better anticipation and understanding of corneal pathologies such as Keratoconus. It could also provide a quantitative evaluation of corneal biomechanical response after corneal refractive surgeries and a tool for evaluating the efficacy of new cornea treatments such as cornea transplant using femtosecond laser or therapy based on Riboflavin/UltraViolet-A Corneal Cross Linking (UVA CXL). In the very important issue of glaucoma diagnosis and management, the fine tuning corneal elasticity measurement could also succeed to strongly correlate the applanation tonometry with the "true" intra-ocular pressure (IOP). This initial investigation evaluates the ability of ultrafast and high-resolution ultrasonic systems to provide a real-time and quantitative mapping of corneal viscoelasticity. Quantitative elasticity maps were acquired ex vivo on porcine cornea using the supersonic shear imaging (SSI) technique. A conventional 15 MHz linear probe was used to perform conventional ultrasonic imaging of the cornea. A dedicated ultrasonic sequence combines the generation of a remote palpation in the cornea and ultrafast (20,000 frames/s) ultrasonic imaging of the resulting corneal displacements that evolve into a shear wave propagation whose local speed was directly linked to local elasticity. A quantitative high-resolution map (150 microm resolution) of local corneal elasticity can be provided by this dedicated sequence of ultrasonic insonifications. Quantitative maps of corneal elasticity were obtained on ex vivo freshly enucleated porcine corneas. In the cornea, a quite homogenous stiffness map was found with a 190 kPa +/- 32 kPa mean elasticity. The influence of photodynamic Riboflavin/UVA induced CXL was measured. A significant Young's modulus increase was obtained with a mean 890 kPa +/- 250 kPa posttreatment Young's modulus (460% increase), located in the anterior part of the cornea. Simulations based on 3-D time domain finite differences simulation were also performed and found to be in good agreement with ex vivo experiments. The SSI technique can perform real-time, noninvasive, high-resolution, and quantitative maps of the whole corneal elasticity. This technique could be real time and straightforward adapted for a very wide field of in vivo investigations.