Measurement of Corneal Elasticity With Acoustic Radiation Force Elastic Microscope

Abstract

Purpose: : Corneal biomechanics plays an important role in determining the eye’s structural integrity, optical power and the overall quality of vision. A critical limitation to increasing our understanding of how corneal biomechanics controls corneal stability and refraction is the lack of non-invasive technologies that microscopically measure local biomechanical properties, such as corneal elasticity within the 3D space. We demonstrate that by measuring the movement of a femtosecond laser generated cavitation bubble as it interacts with an acoustic radiation force, we can determine local values for an individual cornea’s Young’s modulus, without altering its structure.

Methods: : A porcine cadaver cornea was placed into a layer of gelatin within a water tank filled with de-ionized water.Femtosecond laser pulses with 5 mJ energy induced optical breakdown within the stroma and produced a cavitation bubble for radiation force measurements. A 20 MHz two-element confocal ultrasonic transducer applied 6.5 ms acoustic radiation force-chirp bursts to the bubble with the outer element (1.5 MHz) while the bubble position was monitored within the cornea using pulse-echoes with the center element (20 MHz). A cross-correlation method was used to measure bubble displacements and determine exponential time constants of the temporal responses. Maximum bubble displacements are inversely proportional to the local Young’s modulus.

Results: : The bubble response to acoustic radiation force shows a maximum displacement of approximately 6 mm. Young’s modulus estimates are made by accounting for differences in bubble size according to the square root of integrated backscatter and comparing with reference displacements in gelatin phantoms with known mechanical properties. Measurements yielded a Young’s modulus of E20MH= 16 +/- 2.5 kPa in the direction perpendicular to the corneal surface and were in reasonable agreement with elasticity values obtained with traditional microelastometer (E2M =13.7 +/- 9.8 kPa). Histology studies indicated no detectable damage at the location of the acoustic radiation force microscopy (ARFEM) measurements.

Conclusions: : Our results indicate that local elasticity of the cornea can be measured with ARFEM with spatial resolution in the micron range. Since ARFEM did not introduce permanent damage to the tissue it is candidate for possible future in vivo corneal elasticity studies.

Keywords: cornea: stroma and keratocytes • cornea: basic science • refractive surgery