Generation of shear waves by laser in soft media in the ablative and thermoelastic regimes

Abstract

This article describes the generation of elastic shear waves in a soft medium using a laser beam. Our experiments show two different regimes depending on laser energy. Physical modeling of the underlying phenomena reveals a thermoelastic regime caused by a local dilatation resulting from temperature increase, and an ablative regime caused by a partial vaporization of the medium by the laser. Computed theoretical displacements are close to experimental measurements. A numerical study based on the physical modeling gives propagation patterns comparable to those generated experimentally. These results provide a physical basis for the feasibility of a shear wave elastography technique (a technique which measures a soft solid stiffness from shear wave propagation) by using a laser beam.When a laser beam of sufficient energy is incident on a medium, the absorption of the electromagnetic radiation leads to an increase in the local temperature. Due to thermal effects, displacements occur in the medium, which can then propagate as elastic waves. Elastic waves within a bulk can be separated into two components: compression waves, corresponding to a curl-free propagation, and shear waves, corresponding to a divergencefree propagation 1 . Measures of the transmission characteristics of compression and shear waves are useful for inspecting solids, such as a metal, to reveal potential cracks or defects 21 . In biological tissues, induction of compression waves by laser has been studied with the development of photoacoustic imaging 23 . Elastic waves used in photoacoustic imaging are typically of a few megahertz; at this frequency, shear waves are quickly attenuated in soft tissues, typically over a few microns, and only compression waves can propagate over a few centimeters.

Viscoelasticity Imaging of Biological Tissues and Single Cells Using Shear Wave Propagation

“…It would also be interesting to detect the contrast in the PA signal due to elastic property variation separately from that of other parameters (including the optical absorption coefficient). Grasland-Mongrain et al generated shear waves in soft tissues in ablative and thermoelastic regimes with a 532 nm Nd:YAG laser [415]. However, it remained a challenge to keep the laser beam energy within safety limits for use in biomedical applications.…”

Laser-induced elastic wave classification: thermoelastic versus ablative regimes for all-optical elastography applications

“…All-optical excitation of elastic waves has many advantages compared to other methods. [28][29][30][31][32][33][34] Similar to photoacoustics, pulsed laser irradiation is converted into elastic waves by light absorption and localized heat expansion of the tissues. This transient mechanical perturbation generates compressional and shear waves that propagate through the sample.…”

“…Different techniques have been used to enhance the absorption of pulsed laser energy by the sample, such as the injection of nanoparticles, [35][36][37] droplets of perfluorocarbon, 38 and doping with graphite. 34 Naturally, the endogenous mechanism of the elastic wave generation would be preferred for many biological and clinical applications.…”

“…34 While thermoelastic and ablative mechanisms for compressional wave generation in metals have been intensively investigated in the past, [39][40][41][42][43][44][45] there are very few reports on generation of elastic waves in soft tissues. 34,46 The thermally induced disturbance on a sample by low laser irradiation is mainly a result of the thermal expansion of the sample material. But, at high laser irradiation, the excitation area experiences an ablation process that causes a nonlinear dependence between laser energy and the displacement amplitude, as well as damage of the sample.…”

Integrated optical coherence tomography and multielement ultrasound transducer probe for shear wave elasticity imaging of moving tissues

“…This makes it possible to detect tissue displacement well below 1 mm-detection of ∼10-nm displacements has been reported using phase-sensitive measurements. 16 Shear waves in tissues can be created using various methods, such as a laser pulse, 17 mechanical actuator, 18 air-pulse system, 19 ultrasound, 20 and the Lorentz force. 21 The speed of shear wave propagation can be calculated as a ratio of a distance between the excitation and observation points (i.e., travel distance) to the time of shear wave propogation.…”