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Optical coherence elastography (OCE) holds great promise for quantitative characterization of corneal elasticity including robust measurements of the intraocular pressure (IOP) independent of corneal mechanical properties. To translate this method into a viable clinical tool, however, requires wideband, highly accurate mechanical wave measurements using mechanical stimulation requiring no physical contact with the cornea. We have developed a method of non-contact mechanical stimulation of soft media with precise spatial and temporal shaping. We call it acoustic micro-tapping (AuT) because it employs focused, air-coupled ultrasound (US) to induce significant mechanical displacement at the boundary of a soft material using reflection-based radiation force. Combining it with high-speed, four-dimensional (three space dimensions plus time) phase-sensitive optical coherence tomography (PhS-OCT) creates a non-contact tool for high-resolution and quantitative dynamic elastography of soft tissue at near real-time imaging rates. To demonstrate this approach, we present OCE results on a porcine cornea using a homemade, focused 1 MHz air-coupled piezoelectric transducer with a matching layer to launch an US wave through air onto the sample surface. To provide an acoustic line source approximating a 1-D excitation, the transducer was made from a cylindrical segment of a piezoelectric tube. A high-speed (1.6 MHz A-Scan rate) PhS-OCT system was utilized to measure acoustic wave propagation in the cornea at different intraocular pressures (IOPs). Results from this OCE study demonstrate that an air-coupled US wave reflected from an air/tissue interface provides significant radiation force to generate displacement for elasticity imaging for full mechanical characterization of the cornea.
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