Pol Grasland-Mongrain edited Introduction.tex  over 8 years ago

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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. Due to thermal effects, displacements occur in the medium. These displacements can then propagate as elastic waves. Elastic waves are 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}. Measures of the compression and shear waves is notably used as a method of inspection to reveal potential cracks in a solid such as a metal. In a medical context, induction of compression waves by laser has been studiedfor 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 compression waves by thermal expansion. Using an iterative algorithm and a light propagation model, acoustic sources can be recovered with appropriate detection scheme to identify optical absorption in the medium \cite{Xu_2006},  \cite{22442475}. As the optical absorption coefficient of the tissue depends on the optical wavelength, different structures can be observed by tuning properly But  thelaser wavelength. For example, oxygenated and de-oxygenated haemoglobin can be discriminated in this way \cite{16674205}.  The  elastic waves used in photoacoustic imaging are typically of a few megahertz. At megahertz: at  this frequency, in a soft tissue,  shear waves are quickly attenuated, typically over a few microns in soft tissues, so only compression waves can propagate over a few centimeters. Shear waves have however  drawn an increasing interest in medical imaging, with the development of shear wave elastography techniques for the last two decades \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 of several orders of magnitude between different tissues in human body and potentially offers a good 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. Shear waves can be induced by an external vibrator \cite{muthupillai1995magnetic}, a focused acoustic beam \cite{sarvazyan1998shear}, \cite{11937286}, the Lorentz force \cite{16051039}, \cite{grasland2014elastoEMarticle}, or natural body displacements \cite{gallot2011passive}. 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{12878247}, the bladder \cite{25574440} and the eye cornea \cite{tanter2009high}, \cite{22627517}. \cite{tanter2009high}. Recently, Li et al. have used induce surface acoustic wave with a laser to assess bladder wall elasticity \cite{22627517} and cornea elasticity \cite{22442475}.  In this study, we show that shear waves can be induced in soft tissues by a laser beam. We also propose a model for the underlying physics. We finally applied the technique in a biological tissue to evaluate its potential application in shear wave elastography.