Effect of Corneal Hydration on Ultrasound Velocity and Backscatter

The cornea's acoustic properties (speed-of-sound, backscatter, attenuation) are related to its state of hydration. Our aim was to determine these properties as a function of corneal hydration using high-frequency ultrasound. Bovine corneas were suspended in a Dexsol-equivalent corneal preservation medium at 33 °C and then immersed successively in 75%, 50% and 25% medium and distilled water. Using a 38-MHz focused ultrasound transducer, we measured speed-of-sound and corneal thickness (n = 8) and stromal backscatter (n = 6) after 45-min immersion in each medium. Corneal speed-of-sound was modeled as a function of corneal thickness. We found the mean speed-of-sound to be 1605.4 ± 2.9 m/s in normotensive medium. The maximum observed speed-of-sound was 1616 m/s. As we decreased medium tonicity, the cornea swelled and the speed-of-sound decreased, reaching 1563.0 ± 2.2 m/s in water. Average corneal thickness increased from 969 ± 93 μm in 100% medium to 1579 ± 104 μm in water. Going from 100% medium to water, stromal backscatter (midband-fit) increased from -60.0 ± 0.8 dBr to -52.5 ± 3.5 dBr, spectral slope increased from -0.119 ± 0.021 to -0.005 ± 0.030 dB/MHz and attenuation coefficient decreased from 0.927 ± 0.434 to 0.010 ± 0.581 dB/cm-MHz. The observed correlation between acoustic backscatter and attenuation with the speed-of-sound offers a potential means for more accurate determination of speed-of-sound and, hence, thickness in edematous corneas. (E-mail: ros2012@med.cornell.edu)