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O. Gal, M.S. Patel, A.A. Deobhakta, A. Sarup, K. Sarup, R.H. Silverman, D.J. Coleman; Speed of Sound Measurement in Bovine Cornea at 35 MHz . Invest. Ophthalmol. Vis. Sci. 2006;47(13):1364.
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© ARVO (1962-2015); The Authors (2016-present)
The speed of sound of the cornea is of crucial importance in ultrasound determination of corneal thickness. A velocity of 1638–1640 M/sec is the accepted standard, although the literature shows much variation in actual experimental findings. In this experiment we measured the speed of sound and thickness of bovine corneas as a function of hydration using a 35 MHz lithium niobate transducer.
Corneas were excised from freshly extracted bovine eyes, retaining approximately 1 mm of circumferential scleral tissue. The corneas were positioned in a temperature regulated Dexsol equivalent preservation media over an optically flat quartz plate. With the plate near the focal plane, scans were performed with and without the cornea intervening. Based on the previously measured speed of sound of the medium, the acoustically determined corneal thickness (in microseconds) and the range to the glass plate with cornea intervening and absent, the speed of sound of the cornea was computed. The corneas were scanned while being placed into decreasing concentrations of media starting from 100%, to 75%, 50%, 25%, water, and back into 100% media at 33ºC. Three measurements were made of each of eleven corneas.
The average speed of sound of the cornea was 1592±5 (SE) M/sec in 100% medium, decreasing incrementally with medium concentration to 1552±2 (SE) M/sec in water. Mean corneal thickness measurements performed in 100% medium and water were 1041±97 (SD) microns and 1886±135 (SD) microns, respectively. (The within cornea standard deviation was 6.5 microns.)
The speed of sound in pure medium at 33ºC, representing physiological conditions, was 1592 M/sec. This is approximately 2% below the accepted standard, but above many experimental findings reported in the literature. Our results reveal that as the cornea becomes more edematous and thickened, the speed of sound decreases. We intend to determine if acoustic backscatter changes associated with edema might be used to approximate speed of sound non–invasively, allowing determination of true cornea thickness.
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