Recently we used ultrasound from an air-coupled transducer for non-contact excitation of broadband mechanical waves in soft tissue such as cornea. The transient displacement, generated by “Acoustic Micro-Tapping” (AuT), was then measured using phase-sensitive spectral domain OCT (SD-OCT). In addition traditional surface wave speed measurement, we investigated complementary methods to characterize the mechanical properties of the target material. We note that the maximum frequency, as well as the group velocity, of the surface wave is related to both the phase velocity of the material and the spatial width of the acoustic pulse. If the spatial and temporal profile of the excitation is well defined, it may be possible to infer elastic modulus from the frequency profile of a propagating mechanical wave. To assess the effect of the spatial profiles of the AuT excitation on frequency profiles of resulting mechanical waves, acoustic pulses with different spatial width (from 0.1 to 1 mm) were applied to agar phantoms with different shear modulus (from 1 to 100 kPa) to generate mechanical waves, and a SD-OCT system with a functional frame rate of 47 kHz was used to track wave propagation. For validation, simulations with the same acoustic and mechanical properties were performed using a finite element method (FEM) to analyze induced wave propagation. The phantom experiment and simulation exhibited similar increase in the maximum frequency with decreasing excitation width. Both estimates also agreed well with previous theoretical results.
Liang Gao, Mitchell Kirby, Shaozhen Song, David Li, Soon Joon Yoon, Tueng Shen, Ivan Pelivanov, Ruikang Wang, and Matthew O'Donnell, "Relationship between wave frequency and elasticity for non-contact acoustic micro-tapping (Conference Presentation)," Proc. SPIE 10496, Optical Elastography and Tissue Biomechanics V, 104960J (Presented at SPIE BiOS: January 27, 2018; Published: 14 March 2018); https://doi.org/10.1117/12.2290412.5751465947001.
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