19 February 2018 Comparison between thermoelastic and ablative induced elastic waves in soft media using ultra-fast line-field low coherent holography
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Abstract
Laser induced elastic waves in soft media have great potential to characterize tissue biomechanical properties. The instantaneous increase in local temperature caused by absorption of laser energy leads to a mechanical perturbation in the sample, which can then propagate as a pressure (or an elastic) wave. The generation of the elastic wave can be via thermoelastic or ablative processes depending on the absorption coefficient of the sample and incident laser fluence. It is critical to differentiate between these regimes because only the thermoelastic regime is useful for nondestructive analysis of tissues. To investigate the transition point between these two different regimes, we induced elastic waves in tissue mimicking agar phantoms mixed with different concentrations of graphite powder. The elastic waves were excited by a 532nm pulsed laser with a pulse duration of 6 ns. The fluence of the pulsed laser was tuned from 0.08 J/cm2 to 3.19 J/cm2 , and the elastic wave was captured by ultra-fast line-field low coherent holography system capable of single-shot elastic wave imaging with nanometer-scale displacement sensitivity. Different concentrations of graphite powder enabled excitation in sample with controlled and variable attenuation coefficient, enabling measurement of the transition between the thermoelastic and ablative regimes. The results show that the transition from thermoelastic to ablative generated waves was 0.75 J/cm2 and 1.84 J/cm2 for phantoms with optical attenuation coefficients of 6.64±0.32 mm-1 and 26.19±1.70 mm-1, respectively. Our results show promise for all optical biomechanical characterization of tissues.
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Susobhan Das, Chih-Hao Liu, Alexander Schill, Kirill V. Larin, "Comparison between thermoelastic and ablative induced elastic waves in soft media using ultra-fast line-field low coherent holography", Proc. SPIE 10496, Optical Elastography and Tissue Biomechanics V, 104960G (19 February 2018); doi: 10.1117/12.2290384; https://doi.org/10.1117/12.2290384
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