Experimental classification of surface waves in optical coherence elastography

Various types of waves are produced when a harmonic force is applied to a semi-infinite half space elastic medium. In particular, surface waves are perturbations with transverse and longitudinal components of displacement that propagate in the boundary region at the surface of the elastic solid. Shear wave speed estimation is the standard for characterizing elastic properties of tissue in elastography; however, the penetration depth of Optical Coherence Tomography (OCT) is typically measured in millimeters constraining the measurement region of interest to be near the surface. Plane harmonic Rayleigh waves propagate in solid-vacuum interfaces while Scholte waves exist in solid-fluid interfaces. Theoretically, for an elastic solid with a Poisson’s ratio close to 0.5, the ratio of the Rayleigh to shear wave speed is 95%, and 84% for the Scholte to shear wave. Our study demonstrates the evidence of Rayleigh waves propagating in the solid-air boundary of tissue-mimicking elastic phantoms. Sinusoidal tone-bursts of 400Hz and 1000 Hz were excited over the phantom by using a piezoelectric actuator. The wave propagation was detected with a phase-sensitive OCT system, and its speed was measured by tracking the most prominent peak of the tone in time and space. Similarly, this same experiment was repeated with a water interface. In order to obtain the shear wave speed in the material, mechanical compression tests were conducted in samples of the same phantom. A 93.9% Rayleigh-shear and 82.4% Scholte-Shear speed ratio were measured during experiments which are in agreement with theoretical results.