Spatial resolution in optical coherence elastography of bounded media

TL;DR

This paper investigates the spatial resolution limits of optical coherence elastography in bounded media, revealing that tissue boundaries constrain resolution more than wave type or excitation method, with implications for imaging thin tissues like the cornea.

Contribution

The study demonstrates through simulations and experiments that bounded tissue layers limit elastographic resolution, contrasting with previous assumptions of OCT-level resolution in bulk shear wave imaging.

Findings

Bounded media restrict elastographic resolution to tissue thickness.

Broadband excitation improves modulus reconstruction with fewer artifacts.

Harmonic waves cause phase instabilities and artifacts in bounded tissues.

Abstract

Dynamic optical coherence elastography (OCE) tracks mechanical wave propagation in the subsurface region of tissue to image its shear modulus. For bulk shear waves, the lateral resolution of the reconstructed modulus map (i.e., elastographic resolution) can approach optical coherence tomography (OCT) capabilities, typically a few tens of microns. Here we perform comprehensive numerical simulations and acoustic micro-tapping OCE experiments to show that for the typical situation of guided wave propagation in bounded media, such as cornea, the elastographic resolution cannot reach the OCT resolution and is mainly defined by the thickness of the bounded tissue layer. We considered the excitation of both broadband and quasi-harmonic guided waves in a bounded, isotropic medium. Leveraging the properties of broadband pulses, a robust method for modulus reconstruction with minimum artifacts at interfaces is demonstrated. In contrast, tissue bounding creates large instabilities in the phase of harmonic waves, leading to serious artifacts in modulus reconstructions.