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

Abstract

Significance: Shear wave optical coherence elastography is an emerging technique for characterizing tissue biomechanics that relies on the generation of elastic waves to obtain the mechanical contrast. Various techniques, such as contact, acoustic, and pneumatic methods, have been used to induce elastic waves. However, the lack of higher-frequency components within the elastic wave restricts their use in thin samples. The methods also require moving parts and/or tubing, which therefore limits the extent to which they can be miniaturized. Aim: To overcome these limitations, we propose an all-optical approach using photothermal excitation. Depending on the absorption coefficient of the sample and the laser pulse energy, elastic waves are generated either through a thermoelastic or an ablative process. Our study aimed to experimentally determine the boundary between the thermoelastic and the ablative regimes for safe all-optical elastography applications. Approach: Tissue-mimicking graphite-doped phantoms and chicken liver samples were used to investigate the boundary between thermoelastic and ablative regimes. A pulsed laser at 532 nm was used to induce elastic waves in the samples. Laser-induced elastic waves were detected using a line field low coherence holography instrument. The shape of the elastic wave amplitude was analyzed and used to determine the transition point between thermoelastic and ablative regimes. Results: The transition from the thermoelastic to the ablative regime is accompanied by the nonlinear increase in surface wave amplitude as well as the transformation of the wave shape. Correlation between the absorption coefficient and the transition point energy was experimentally determined using graphite-doped phantoms and applied to biological samples ex vivo. Conclusions: Our study described a methodology for determining the boundary region between thermoelastic and ablative regimes of elastic wave generation. These can be used for the development of a safe method for completely noncontact, all-optical microscale assessment of tissue biomechanics using laser-induced elastic waves.

Wave-based optical coherence elastography: the 10-year perspective

“…In the ablative regime, partial vaporization of the medium (damage) is produced, which, in consequence, produces local displacements with a larger amplitude. Therefore, for a selected laser energy fluence, the motion will be produced in the thermoelastic or ablative regime depending on the optical absorption of the sample, as explored using OCE in tissue-mimicking phantoms and tissues [111]. Laser-pulse stimulation has been applied in the elastography of cornea [112], skin [113], and liver [111] for the non-contact generation of large bandwidth SAWs and Lamb waves.…”

“…Therefore, for a selected laser energy fluence, the motion will be produced in the thermoelastic or ablative regime depending on the optical absorption of the sample, as explored using OCE in tissue-mimicking phantoms and tissues [111]. Laser-pulse stimulation has been applied in the elastography of cornea [112], skin [113], and liver [111] for the non-contact generation of large bandwidth SAWs and Lamb waves. Moreover, laser stimulation in conjunction with photo-absorbers [114], and dye-loaded perfluorocarbon nanodroplets (also named as 'nanobombs') [79][80][81][82] inside tissues allows the localized generation of shear and LSWs of great importance in the transverse elastic characterization of tissues.…”

“…It demonstrated the possibility of generating transient Lamb wave propagation in the cornea using a solid-state Nd:YAG laser with 530 nm central wavelength producing a 6 ns pulsed excitation. Then, this technology was further explored in comparison with ultrasonic excitation techniques [114] and analyzed in thermoelastic and ablative regimes depending on the photothermal absorption of the sample and laser pulse energy for its safe application in the elastography of tissues [111]. Recently, laser excitation interaction with dye-loaded perfluorocarbon nanodroplets allowed the generation of high-frequency LSWs propagating towards depth in phantoms [79,80,82].…”

Existing and Potential Applications of Elastography for Measuring the Viscoelasticity of Biological Tissues In Vivo

“…The oscillation amplitude is also directly related to the complex shear modulus of the sample from which further extraction of viscoelastic parameters can be achieved depending on the choice of a suitable rheological model. In pulse laser OCE [93], wave generation is achieved by focusing pulsed laser irradiation toward the sample. By absorption of the light and localized thermal expansion within the sample, the irradiation is converted into compressional and shear waves that further propagate within the sample.…”

Longitudinal shear wave elasticity measurements of millimeter-sized biomaterials using a single-element transducer platform

“…Most importantly, ARF with coupling medium could rely on no physical contract with the sample; Therefore, when probing fragile 3D hydrogels or engineering biomaterials containing living cells for nondestructive longitudinal studies, ARF is the frequently chosen approach. Several other methods generate the sample perturbations, including pneumatic [ 22 , 23 ] and optical excitations [ 24 ]. The former does not require direct contact, while the tubing and mechanics to deliver the air pulse limit its miniaturization for small samples.…”