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Raksha Urs, Harriet O. Lloyd, Jeffrey A. Ketterling, Ronald H. Silverman; Acoustic Radiation Force to Measure Corneal Elasticity. Invest. Ophthalmol. Vis. Sci. 2011;52(14):4199. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
To determine corneal elasticity using acoustic radiation force.
Acoustic radiation force (ARF) was directed at the rabbit cornea in vivo, using a single-element Lithium niobate transducer (38 MHz central frequency, 6 mm aperture and 12 mm focal length). Ten pushing pulses were applied at 75% duty cycle, at 5 kHz PRF, with imaging impulses interleaved between push pulses to allow radiofrequency (RF) data acquisition during the push mode. After the push sequence, the cornea was imaged for another 80 ms. RF data were sampled at 400 MHz (12 bits/sample). A 40 µm diameter needle hydrophone calibrated up to 60 MHz was used to characterize the acoustic field. Allowing for the beam characteristics and the attenuation coefficient of the cornea, the stress applied to the cornea was 450 Pa. Continuous displacement of the front and back surfaces of the cornea was computed with a spline-based algorithm (Viola et al, 2005), and change in corneal thickness was calculated.
Corneal thickness decreased exponentially by 4.92 µm (from 366.2 µm) during the 2ms push mode. Thickness then relaxed exponentially to its original size during the next 80 ms. Push data were fit to a first order exponential curve, yielding an asymptote (maximum change in thickness) of -4.85 µm and a time constant of 0.64 ms for the push data. Using the asymptote from the push data, the strain was calculated to be 0.013 and the resulting elastic modulus was 34 kPa.
The spline-based algorithm allowed sub-sample displacement detection of the corneal surfaces, permitting more accurate determination of corneal thickness. Initial studies have shown that ARF-induced change in corneal thickness is dependent on the intraocular pressure. Further investigation is required to establish the relationship between the two. The method described in this abstract could be developed to detect diseases like keratoconus where corneal elasticity is believed to be altered.
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