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X. He, C. Roberts, X. Pan, J. Liu; Repeatability and Validation of an Ultrasonic System for Non-Invasive Measurements of Corneal Biomechanical Properties. Invest. Ophthalmol. Vis. Sci. 2007;48(13):1858.
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Non-invasive measurements of corneal biomechanical properties may provide important information to help elucidate the mechanisms of several ocular disorders. We previously verified the accuracy and sensitivity of using an ultrasonic system we developed in our lab in measuring corneal phantoms. The objective of this study is to examine the repeatability and accuracy of the ultrasonic system in measuring corneal thickness in fresh, enucleated porcine eyes.
Six enucleated porcine eyes were obtained and immersed in a Dexol-equivalent medium. The eyes were measured using both the ultrasonic system we developed and a common clinical pachymeter every 30 minutes over a 2h period. For ultrasonic measurements, a broadband transducer (10 MHz, XMS, Panametrics-NDT) was used to excite the corneal tissue. The reflected ultrasonic signals were recorded by a 500 MHz/8-bit digitizer (Acqiris, DP105), and displayed and processed by a computer. Each ultrasonic measurement was repeated three times. The ultrasonic signals were analyzed to obtain corneal thickness by using a time-of-flight algorithm that assumes a constant speed of sound in corneas (i.e., 1640 m/s, as used by the DGH pachymeters). For validation, corneal thickness was directly measured by a pachymeter (DGH 550 Pachette2, DGH Technology) at the same time point of each ultrasonic measurement. The pachymeter measurements were repeated five times. Other corneal properties were not directly verified in this study.
Based on the repeated measurements of the six eyes at five different postmortem time points, the pachymeter had a pooled standard deviation of 5.33 micron, and the ultrasonic system had a pooled standard deviation of 2.22 micron. The overall correlation of the averaged measurements between pachymeter and our ultrasound system of the six eyes at five different postmortem time points was 0.962, which was independent of the eye.
The results indicated that the ultrasonic system we developed was highly repeatable in measuring enucleated porcine eyes, with a standard deviation smaller than that of the pachymeter. A high correlation coefficient (96.2%) with the readings of a standard pachymeter indicates the reliability of the approach, with valid measurements of corneal thickness. The ultrasonic measurements of other corneal properties need to be verified in the future.
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