deletions | additions       diff --git a/When_a_laser_beam_of__1.tex b/When_a_laser_beam_of__1.tex  index 0d57042..301739b 100644  --- a/When_a_laser_beam_of__1.tex  +++ b/When_a_laser_beam_of__1.tex 
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In a medical context, induction of compression 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 and thus, to map optical absorption of the tissues \cite{22442475}. As the optical absorption coefficient of the tissue depends on the optical wavelength, different structures can be observed by tuning properly the laser wavelength. For example, oxygenated and de-oxygenated haemoglobin can be discriminated in this 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. At this frequency, shear waves are quickly attenuated, typically over a few microns in soft tissues, so only compression waves can propagate over a few centimeters.  We hypothesized in this study that a laser beam in a soft tissue can 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 techniques \cite{muthupillai1995magnetic}, \cite{sandrin2002shear}. As 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 waves can be induced by an external vibrator \cite{muthupillai1995magnetic}, a focused acoustic beam \ref{10385964}, \ref{sarvazyan1998shear},  \cite{11937286}, the Lorentz force \ref{25083665}, or natural body displacements \ref{21693392}. 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 show that shear waves can be induced by a laser beam, with a model of the underlying physical phenomenon. We also applied the technique in a biological tissue to evaluate its potential application in shear wave elastography. 
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