Longitudinal shear waves (LSW) are waves with longitudinally polarized displacement that travel at the shear wave speed through depth when generated at the surface of tissues. In this study, we explore LSW generated by a circular glass plate in contact with the sample. Results demonstrated the potential of LSW in detecting an elasticity gradient along axial (resolution < 0.13 mm) and lateral (resolution < 0.78 mm) directions simultaneously. Finally, LSWs are used for the elastography of ex vivo mouse brain and demonstrated differentiated LSW speed values between the cerebral cortex (2.91 m/s) and cerebellum/midbrain (1.18 m/s) regions.
A number of approaches employ optical coherence tomography (OCT) to obtain the mechanical properties of biological tissue. These are generally referred to as optical coherence elastography (OCE), and have demonstrated promising applications with studies in cornea, breast, muscle, skin, and other soft tissues. A particular application of interest is the brain, in which changes in local and global elastic properties may correlate with the onset and progression of degenerative brain diseases. In this preliminary study, mice brains are studied ex vivo and in situ with preservation of the brain/skull anatomical architecture. A small 6 mm diameter portion of the skull is replaced with a glass cap to allow for OCT imaging. Various permutations of source placement for generating shear waves and modes of excitation are evaluated to optimize the experimental setup. The use of reverberant shear wave fields, which takes advantage of inevitable reflections from boundaries and tissue inhomogeneities, allow for estimation of the shear wave speed, which is directly related to the elastic modulus of soft tissues. Preliminary estimates for the shear wave speed in brains of recently deceased mice are obtained. This study demonstrates potential applications in brain OCE ex vivo and in vivo.
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