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When a laser beam of sufficient energy is incident on a medium, the absorption of the electromagnetic radiation leads to an increase of the local temperature.
There is consequently a local dilatation, and Due to thermal effects, displacements occur in the
resulting displacement medium which can propagate as elastic waves. Elastic waves can be separated in two components in a bulk: compression waves, corresponding to a curl-free propagation; and shear waves, corresponding to a divergence-free propagation \cite{aki2002quantitative}. This phenomenon has been notably observed in metals.
Measuring Measures of the compression and shear waves
can be is notably used as a method of inspection to reveal potential cracks in the solid.
In a medical context, induction of compression
wave waves by laser has been studied for the last ten years, with the development of photoacoustic imaging \cite{Xu_2006}. In this technique, a laser beam is absorbed by the tissue, which induces by thermal expansion compression waves, which are themselves acquired by acoustic transducers. Time of flight measurements allows then to find the source of the
waves. The waves and thus, to map optical absorption of the tissues \cite{22442475}. As the optical absorption coefficient of the tissue depends on the optical wavelength,
so different structures can be observed by tuning properly the laser wavelength. For example, oxygenated and de-oxygenated haemoglobin can be discriminated in this
way. way Noninvasive imaging of hemoglobin concentration and oxygenation in the rat brain using high-resolution photoacoustic tomography\cite{16674205}. The frequency of the elastic waves used in photoacoustic imaging are typically of a few
megahertz \cite{Xu_2006}. megahertz. At this frequency,
only compression waves can propagate over a few centimeters, as shear waves are quickly attenuated, typically over a few microns in soft
tissues. tissues, so only compression waves can propagate over a few centimeters.
We hypothesized in this study that
applying a laser beam in a soft tissue can
also nevertheless induce shear
waves in addition to compression waves. Shear waves have drawn an increasing interest in medical imaging, with the development for the last two decades of shear wave elastography methods \cite{muthupillai1995magnetic}, \cite{10385964}, \cite{sandrin2002shear}. As
it its names indicates, this term covers the techniques used to measure or map the elastic properties of biological tissues using shear wave propagation. The shear modulus, directly proportional to Young's modulus in soft tissues, varies indeed of several orders of magnitude in human body and potentially offers an excellent contrast. As a shear wave propagates in an organ at a speed proportional to the square root of the shear modulus, measuring its speed throughout the organ allows to compute the shear modulus of the tissue \cite{10385964}. Shear wave elastography techniques have been successfully applied for the detection of various pathologies in organs such as the liver \cite{sandrin2003transient}, the breast \cite{goddi2012breast}, \cite{sinkus2005viscoelastic}, the prostate \cite{cochlin2002elastography}, \cite{souchon2003visualisation}, the bladder \cite{25574440} and the eye cornea \cite{tanter2009high}, \cite{22627517}.
In this study, we
wanted to show that shear waves can be induced by a laser
beam and to characterize beam, with a model of the underlying physical phenomenon.
Finally, we We also applied the technique in a biological tissue to evaluate its
potential application in shear wave elastography.
The Z axis is defined here as the laser beam axis, and the ultrasound probe is in the XZ plane, as illustrated by Figure \ref{Figure1}.
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