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A. Barmettler, R. H. Silverman, H. O. Lloyd, J. A. Ketterling, Y. Yonekawa, D. J. Coleman; Effect of Acoustic Radiation Force on the Cornea in vivo. Invest. Ophthalmol. Vis. Sci. 2010;51(13):5774. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
The application of force to the cornea has long been used for measurement of intraocular pressure and more recently to probe the elastic properties of the cornea itself. Current modalities exert force onto the anterior surface of the cornea. Our aim was to ascertain if acoustic radiation force, which would be absorbed across the full corneal thickness, could be safely used to generate corneal surface displacements.
We tested two configurations. In the first, we used a dual-element probe having a central 40 MHz element and an outer 20 MHz ring, both with a common focus. The second probe was a single-element focused 35 MHz transducer. We used an arbitrary waveform generator in combination with a broadband power amplifier to produce tonebursts to excite the radiation-force emitting element (which in the first configuration was the 20 MHz ring only). We measured the radiation force produced at a series of voltages and determined the maximum duration of the toneburst for each voltage that would be permissible under FDA 510k standards for ophthalmic diagnostic ultrasound. We then acquired pulse/echo data 1000 times per second along one line-of-sight in the central cornea for 0.5 second, during the course of which a toneburst of 1-5 msec was emitted. We subsequently examined the digitized echo data to determine the time-course and magnitude of displacements of the epithelial surface, Bowman’s membrane and the posterior surface of the cornea. Data was acquired on ex vivo pig eyes at a series of intraocular pressure and on normal human subjects.
In ex vivo eyes, the magnitude of displacement decreased as intraocular pressure increased, but measuring about 40 µm in a normotensive eye. We also observed a transient (<4 msec) increase in corneal thickness immediately following the toneburst. In vivo, we observed smaller displacements, typically on the order of 10 µm, with recovery time of under 4 msec.
Transient displacements of the corneal surfaces were observed in ex vivo pig cornea and in vivo human corneas. The displacement magnitude is clearly affected by intraocular pressure. The relatively larger displacements observed in ex vivo compared to in vivo corneas suggests that edematous changes in the ex vivo cornea may have altered its elastic properties. Our findings demonstrate not only that this technique can be safely applied clinically but that it may provide information related to corneal elasticity.
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