Interplay of temperature, thermal‐stresses and strains in laser‐assisted modification of collagenous tissues: Speckle‐contrast and OCT‐based studies

Abstract

Moderate heating of collagenous tissues such as cartilage and cornea by infrared laser irradiation can produce biologically nondestructive structural rearrangements and relaxation of internal stresses resulting in the tissue reshaping. The reshaping results and eventual changes in optical and biological properties of the tissue strongly depend on the laser‐irradiation regime. Here, a speckle‐contrast technique based on monochromatic illumination of the tissue in combination with strain mapping by means of optical coherence elastography (OCE) is applied to reveal the interplay between the temperature and thermal stress fields producing tissue modifications. The speckle‐based technique ensured en face visualization of cross correlation and contrast of speckle images, with evolving proportions between contributions of temperature increase and thermal‐stresses determined by temperature gradients. The speckle‐technique findings are corroborated by quantitative OCE‐based depth‐resolved imaging of irradiation‐induced strain‐evolution. The revealed relationships can be used for real‐time control of the reshaping procedures (e.g., for laser shaping of cartilaginous implants in otolaryngology and maxillofacial surgery) and optimization of the laser‐irradiation regimes to ensure the desired reshaping using lower and biologically safer temperatures. The figure of waterfall OCE‐image demonstrates how the strain‐rate maximum arising in the heating‐beam center gradually splits and drifts towards the zones of maximal thermal stresses located at the temperature‐profile slopes.

Strain and elasticity imaging in compression optical coherence elastography: The two‐decade perspective and recent advances

“…It can be emphasized that although mapping of strains and stiffness are closely related in the compressional OCE, the OCT‐based strain mapping itself is a valuable technique that can be used not only as an auxiliary means for mapping stiffness, but applied to strain imaging of any origin, both “rapid” (e.g., thermally‐induced 37, 48–51 ) and “slow”, e.g. related to osmotic processes in tissues 52 or slow deformations produced by residual internal stresses) 53 .…”

“…D, is a structural image of the cornea‐silicone sandwich. Snapshots A, B and C show different stages of the strain filed evolution in the cornea itself and the supporting silicone layers, whereas E, shows a “waterfall” image of the time evolution of interframe strain profile along the white dashed line in panel (c) (more detailed discussion and other examples can be found in 48–51 ). It can be emphasized that because of aperiodic character of the thermally‐produced strain utilization of temporal averaging for improvement of SNR was impossible.…”

“…Examples of visualization of laser‐induced aperiodic fairly fast‐varying strains are given in Figure 7 above to illustrate the possibility to visualize both “instantaneous” (interframe) strains during the heating pulse, as well as cumulative (partially irreversible) strains produced by a series of laser pulses. The OCT‐based visualization enabled deeper insight in the complex spatio‐temporal dynamics of these laser‐produced strains, 48–51 which is important for optimization of irradiation regimes and can be used in control systems for future practical applications in ophthalmology.…”

“…Namely, OCE‐visualization of the residual cumulative strain (dilatation) of the tissue in combination with subsequent evaluation of the Young's modulus reduction in the dilated region made it possible to evaluate averaged parameters of the sub‐resolution pores induced in the tissue by laser irradiation 50 . Furthermore, by analogy with methods used in geophysics, 72 the analysis of nonlinear stress–strain curves in corneal samples made it possible to evaluate the distribution of interstitial gaps in the collagenous matrix over the aspect ratio and demonstrate how this distribution is modified by the laser‐produced heating 71 (more information and numerous examples of OCE‐visualizations can be found in) 48–51 …”

Study of Laser Radiation Effect on the Cornea of the Eye by Speckle Interferometry

“…In addition, research is being carried out in the direction of developing a control system that allows one to determine the threshold for the onset of stress relaxation in the corneal tissues during laser irradiation. The studies are conducted in two different directions, one of which is based on optical coherent elastography [12][13][14][15][16][17], and the second direction is based on the speckle interferometry method, that the present work is devoted to.…”