Nonlinear characterization of elasticity using quantitative optical coherence elastography

Abstract

Optical coherence elastography (OCE) has been used to perform mechanical characterization on biological tissue at the microscopic scale. In this work, we used quantitative optical coherence elastography (qOCE), a novel technology we recently developed, to study the nonlinear elastic behavior of biological tissue. The qOCE system had a fiber-optic probe to exert a compressive force to deform tissue under the tip of the probe. Using the space-division multiplexed optical coherence tomography (OCT) signal detected by a spectral domain OCT engine, we were able to simultaneously quantify the probe deformation that was proportional to the force applied, and to quantify the tissue deformation. In other words, our qOCE system allowed us to establish the relationship between mechanical stimulus and tissue response to characterize the stiffness of biological tissue. Most biological tissues have nonlinear elastic behavior, and the apparent stress-strain relationship characterized by our qOCE system was nonlinear an extended range of strain, for a tissuemimicking phantom as well as biological tissues. Our experimental results suggested that the quantification of force in OCE was critical for accurate characterization of tissue mechanical properties and the qOCE technique was capable of differentiating biological tissues based on the elasticity of tissue that is generally nonlinear.

In vivo volumetric quantitative micro-elastography of human skin

“…The accuracy of elasticity estimation using quantitative micro-elastography, was calculated to be within 8% of the actual elasticity on phantoms with known elasticity [25]. In the case of skin, stiff regions beyond the imaging range of the system, e.g., bone, can affect the level of preload on the skin within our field-of-view and, due to the non-linear stress-strain response of skin [26,39], different elasticity values may be measured in regions that may have the same intrinsic mechanical properties. However, since the compliant layer provides an estimate of preload stress, the presence of non-uniform preloading can be identified, as we have recently demonstrated [38].…”

“…Further, they may operate at a different preload applied to the skin, which is usually not reported, making accurate comparison impossible. In future, non-linear elasticity could be measured with OCE [26], and enable a more direct comparison with other techniques.…”

“…Under the assumption that the stress at each lateral position is uniform with depth and is uniaxial [28], dividing the stress at the skin surface by the local, axial strain in the tissue volume provides an estimate of the elasticity of skin. Specifically, this provides a tangent modulus, which is equivalent to Young's modulus, under the assumption that the tissue being scanned is linear elastic, which is often the case in soft tissues at strains up to 20% [26]. The strain in the compliant layer brought about by the preload, which we refer to as the bulk strain, ( , ) bulk x y ε , is measured by dividing the change in layer thickness by the initial thickness at each lateral (x,y) position, 0 ( , ) ( , ) ( , ) bulk x y l x y l x y ε = Δ , in the field of view.…”

“…We have demonstrated optical palpation for in vivo imaging of human skin lesions (scars and a nevus) [23]. Qiu et al have also recently reported a similar technique and demonstrated it to show the stress-strain relationship of skin in vivo [26].…”

“…Soft tissue is often linear elastic up to strains up to 20% [26], but tends to be hyperelastic in general (increasing elasticity with strain).…”