Towards nondestructive assessment of corneal viscoelasticity by imaging lamb wave propagation with OCT

Abstract

 

Purpose

 

The concept of elastic wave imaging has been widely applied with different imaging modalities (e.g. ultrasonic imaging) for assessing the tissue mechanical properties. In this study, we focus on the development of a noncontact phase-based optical method that enables depth-resolved imaging and visualization of small-amplitude lamb wave propagation inside the cornea at ultra-high frame rate.

 

Methods

 

A focused air-puff system was used to induce micron-level corneal deformation with low-pressure short-duration air stream. Phase-resolved optical coherence tomography (OCT) was utilized to detect the deformation with nano-scale sensitivity over a 1-D scan line on the cornea. At each scanning position, M-mode OCT imaging (A-scan over time) was performed and precisely synchronized with the air-puff excitation. A 3-D (transverse-depthwise-temporal) data matrix containing the optical phase from the interferometry was recorded and processed to provide the displacement information through the cornea. By viewing the data matrix from the transverse-depthwise plane, the 2-D depth-resolved mapping of the corneal displacement over time was achieved as the visualization of the lamb wave propagation inside the cornea, which has the frame rate equivalent to the OCT A-line acquisition speed. The group velocity of the lamb wave was calculated based on the measurement of the time delay formed at the different spatial locations along the propagation. To demonstrate the feasibility, our pilot experiments were conducted on agar phantoms and ex vivo eyeballs from young and mature rabbits.

 

Results

 

We obtained the first high-resolution (micron scale) 2-D depth-resolved visualization of the lamb wave propagation in cornea at 25 kHz frame rate, from which the information of the wave velocity and the amplitude damping can be visualized and reveals the corneal viscoelastic properties. The group velocities of the lamb waves in young and mature corneas were quantified as 1.14±0.08 m/s and 1.30±0.10 m/s, respectively, indicating the age-related stiffness change of the cornea.

 

Conclusions

 

Our method provides a direct and explicit indication of the corneal viscoelasticity, and is potentially useful for high-resolution quantitative corneal elastography.

 

 

(a) OCT structure images of agar phantoms; (b) and (c) Wave propagation status at different temporal points.

 

 

Lamb wave propagation in mature rabbit cornea.

 

Keywords: 480 cornea: basic science • 552 imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound)