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Shear waves with a displacement amplitude of 0.5 $\mu$m have been observed in the chicken breast sample. Although Muthupillai et al. indicated that displacements as small as 100 nm should be sufficient to perform elastography measurements \cite{muthupillai1995magnetic}, displacements 5 to 20 $\mu$m are commonly used in practice, both in acoustic radiation force imaging \cite{nightingale2002acoustic} and magnetic resonance elastography \cite{sinkus2005imaging}.  This study used an ultrasound device to image the sample and track shear waves, due to its high temporal resolution, availability and ease of use. However, for a clinical implementation such as brain elasticity imaging, MRI is more suited for tracking shear waves, as acoustic waves used in ultrasound imaging for shear wave tracking are attenuated by the skull. A clinical MRI scanner has, moreover, a five to ten times higher magnetic field than the permanent magnet used in this study, which study. This  would increasethe shear wave amplitude  by a similar factor. factor the shear wave amplitude, reaching thus the range of clinical applications.  Regarding the safety of the method, strong magnetic fields in MRI systems are considered biologically harmless \cite{schenck2000safety}. Safety guidelines based on clinical reports have also been provided for the TMS technique \cite{rossi2009safety}, and it is now considered as harmless if some precautions are followed. Combination of TMS with strong magnetic fields can produce displacements in the range of a few tenths of micrometers or less in biological tissues, but no harmful effects have been reported so far with shear waves of this amplitude \cite{skurczynski2009evaluation}, \cite{ehman2008vibration}. Precautions linked to the proposed technique are consequently the same as those for TMS and MRI -- mainly the absence of ferromagnetic materials in the body. 
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