EPITHELIAL AND CORNEAL THICKNESS MEASUREMENTS BY HIGH-FREQUENCY ULTRASOUND DIGITAL SIGNAL-PROCESSING

Purpose: The authors determine the mean central corneal and epithelial thickness in a group of normal human subjects using a new high-freque ncy ultrasound technique, incorporating digital signal processing. Met hod: Both eyes of ten volunteers (age range, 23-44 years) were scanned through a normal saline standoff. Digitized ultrasonic echo data were mathematically transformed to produce a plot, the I-scan, which optim ally localizes acoustic interfaces to provide improved measurement pre cision. System precision was determined by analysis of variance of rep eated measures. Central epithelial thickness was obtained by averaging multiple measurements. Central corneal thickness was determined by fi tting measurements of apparent corneal thickness in consecutive parall el B-scans to a mathematically modeled cornea. A speed of sound of 164 0 m/second was used. Results: Epithelial pachymetric precision using A -scan and I-scan was 4.8 and 2.0 mu m (standard deviation), respective ly. The mean epithelial thicknesses for the right and left eyes were 5 0.7 +/- 3.7 mu m and 50.3 +/- 3.4 mu m, respectively. The mean corneal thicknesses in the right and left eyes were 514.6 +/- 38.4 mu m and 5 16.2 +/- 37.8 mu m, respectively. The root mean-square differences in epithelial and corneal thickness between the left and right eyes of ea ch subject were 1.3 and 7.7 mu m, respectively (neither was statistica lly significant). Conclusion: This system provides a pachymetric preci sion superior to current optical and ultrasound methods. Epithelial an d corneal pachymetry is obtained noninvasively by a method that is not limited to optically clear media.