An analytic model is described for application in ultrasonic tissue characterization. The model is applicable to clinical broadband pulse echo systems. It treats spectra derived from received echo signals and relates them to physical tissue properties. The model can be applied to deterministic tissue structures (e.g., retinal detachments, larger blood vessels, and surface layers of the kidney) and to stochastic tissue structures (e.g., various tumors). The beam patterns included in the model are those generated by focused transducers typically used in high-resolution clinical ultrasound. Appropriate calibration procedures are also treated; these are needed for interpretation of absolute spectral parameters. The results obtained with the analytic model have been used to design a digital processing system and the associated techniques which are now being applied during examinations of the eye and abdominal organs. The results have proven useful in interpreting data from various types of tissues. To illustrate the application of these results, representative clinical data, obtained from the digital system, are presented for two types of tissue architectures. The first case is a detached retina representing a deterministic structure characterized by well-defined thickness and reflection coefficients. The second case is asteroid hyalosis and represents a stochastic entity in which the positions of small scattering particles are best described in statistical terms, and characterization is accompanied by means of normalized power spectra.

PURPOSE-To characterize the in vivo epithelial thickness profile in a population of normal eyes. METHODS-An epithelial thickness profile was measured by Artemis 1 (Ultralink LLC) very highfrequency (VHF) digital ultrasound scanning across the central 10-mm diameter of the cornea of 110 eyes of 56 patients who presented for refractive surgery assessment. The average, standard deviation, minimum, maximum, and range of epithelial thickness were calculated for each point in the 10×10mm Cartesian matrix and plotted. Differences between the epithelial thickness at the corneal vertex and peripheral locations at the 3-mm radius were calculated. The location of the thinnest epithelium was found for each eye and averaged. Correlations of corneal vertex epithelial thickness with age, spherical equivalent refraction, and average keratometry were calculated. RESULTS-The mean epithelial thickness at the corneal vertex was 53.4±4.6 μm, with no statistically significant difference between right and left eyes, and no significant differences in age, spherical equivalent refraction, or keratometry. The average epithelial thickness map showed that the corneal epithelium was thicker inferiorly than superiorly (5.9 μm at the 3-mm radius, P<.001) and thicker nasally than temporally (1.3 μm at the 3-mm radius, P< 001). The location of the thinnest epithelium was displaced on average 0.33 mm temporally and 0.90 mm superiorly with reference to the corneal vertex. CONCLUSIONS-Three-dimensional thickness mapping of the corneal epithelium demonstrated that the epithelial thickness is not evenly distributed across the cornea; the epithelium was significantly thicker inferiorly than superiorly and significantly thicker nasally than temporally with a larger inferosuperior difference than nasotemporal difference. The human corneal epithelium has five to seven cell layers and an accepted central thickness of approximately 50 to 52 μm. 1 Bowman's layer, a dense collagenous layer approximately 8 to 10 μm thickness lies between the epithelium and stroma. The anterior margin of Bowman's

PURPOSE To characterize the epithelial, stromal, and total corneal thickness profile in a population of eyes with keratoconus. METHODS Epithelial, stromal, and total corneal thickness profiles were measured in vivo by Artemis very high-frequency (VHF) digital ultrasound scanning (ArcScan) across the central 6- to 10-mm diameter of the cornea on 54 keratoconic eyes. Maps of the average, standard deviation, minimum, maximum, and range of epithelial, stromal, and total corneal thickness were plotted. The average location of the thinnest epithelium, stroma, and total cornea were found. The cross-sectional semi-meridional stromal and total corneal thickness profiles were calculated using annular averaging. The absolute stromal and total corneal thickness progressions relative to the thinnest point were calculated using annular averaging as well as for 8 semi-meridians individually. RESULTS The mean corneal vertex epithelial, stromal, and total corneal thicknesses were 45.7 ± 5.9 µm, 426.4 ± 38.5 µm and 472.2 ± 41.4 µm respectively. The average epithelial thickness profile showed an epithelial doughnut pattern characterized by localized central thinning surrounded by an annulus of thick epithelium. The thinnest epithelium, stroma, and total cornea were displaced on average by 0.48 ± 0.66 mm temporally and 0.32 ± 0.67 mm inferiorly, 0.31 ± 0.45 mm temporally and 0.54 ± 0.37 mm inferiorly, and 0.31 ± 0.43 mm temporally and 0.50 ± 0.35 mm inferiorly, respectively, with reference to the corneal vertex. The increase in semi-meridional absolute stromal and total corneal thickness progressions was greatest inferiorly and lowest temporally. CONCLUSIONS Three-dimensional thickness mapping of the epithelial, stromal, and total corneal thickness profiles characterized thickness changes associated with keratoconus and may help in early diagnosis of keratoconus.

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