Repeatability of layered corneal pachymetry with the artemis very high-frequency digital ultrasound arc-scanner

Abstract

Purpose: To assess the three-dimensional repeatability of thickness measurements for epithelium, stroma, cornea, flap, and residual stromal bed using the Artemis very high-frequency (VHF) digital ultrasound arc-scanner (ArcScan Inc).

Methods: Five consecutive measurements were obtained for 10 eyes of 10 patients 1 year after LASIK using the Artemis VHF digital ultrasound arc-scanner across the central 10-mm diameter of the cornea. Repeatability analysis was performed for thickness measurements for each corneal layer-epithelium, stroma, cornea, flap, and residual stromal bed. The standard deviation of repeated measurements (point-repeatability) was calculated for each measurement location in 0.1-mm steps for the 10×10-mm matrix. The pooled standard deviation of the point-repeatability for each measurement location within the central 1-, 2-, and 3-mm radius was calculated (region-repeatability). The corneal thickness of the baseline scan set was compared to that of subsequent scan sets within the same session and plotted over time to assess any possible hydration effects of the immersion technique.

Results: The repeatability at the corneal vertex was 0.58 μm for epithelium, 1.78 μm for stroma, 1.68 μm for cornea, 1.68 μm for flap, and 2.27 μm for residual stromal bed. The region-repeatability within the central 1-mm radius was 1.01 μm for epithelium, 3.44 μm for stroma, 3.35 μm for cornea, 2.81 μm for flap, and 3.97 μm for residual stromal bed. The mean difference in corneal thickness from the baseline value was within 1.25 μm for each of the subsequent four scan sets over a 5-minute immersion period.

Conclusions: Layered pachymetry of the epithelium, stroma, cornea, flap, and residual stromal bed showed high repeatability with the Artemis VHF digital ultrasound arc-scanner. The high repeatability validates the use of the Artemis for in vivo layered pachymetry.

Figure 1

Non-geometrically corrected B-scan of a cornea 12 months after LASIK. The scan was performed in the visual axis in the horizontal plane. The B-scan is constructed of 128 pulse echo vectors stacked horizontally. There is a large z-axis zoom in scale (2 mm represented horizontally on the image) relative to the lateral distance (10 mm represented vertically on the image). The B-scan demonstrates interfaces detected ultrasonically for the epithelial surface (Ei), the surface of Bowman's layer (Bi), the lamellar interface (Ii), and the endothelial/posterior corneal surface (Pi). Superimposed on the figure is the I-scan derived by digital processing of the radio frequency ultrasonic B-scan data. The I-scan demonstrates sharp, distinct peaks representing the acoustic interfaces of saline-epitheium (E), epithelium-Bowman's (B), the lamellar interface (I), and the endothelial-aqueous interface (P).

Figure 2

Geometrically corrected B-scan of a cornea 12 months after LASIK. Digital signal processing is performed on the B-scan signal and layer thickness measurements are obtained by a computer algorithm on the I-scan (see Figure 1), resulting in the red line image of the interfaces.

Figure 3

The average point-repeatability plotted for the 10-mm diameter for the epithelium, stroma, cornea, flap, and residual stromal bed (RST) based on 5 consecutive measurements from 10 left eyes. The color scale represents the point-repeatability in microns. A Cartesian 1-mm grid is superimposed with the origin at the corneal vertex. Positive x values represent the temporal cornea and negative values represent the nasal cornea. Positive y values represent the superior cornea and negative values represent the inferior cornea. A white circle is superimposed on the maps to identify a central region with 3-mm radius.

Figure 4

Line plot showing the mean difference between the minimum corneal thickness of the first scan set and each of the four subsequent scan sets. The first scan set is shown as a reference point in red. The blue data points are the mean difference from the first scan set. The error bars represent one standard deviation for each of the subsequent scan sets.

Figure 5

Geometrically corrected (top) and non-geometrically corrected (bottom) B-scan of a cornea 9 years after LASIK with an Automated Corneal Shaper microkeratome (Bausch & Lomb). The flap interface can still be clearly identified along its entire length 9 years postoperatively. The bottom scan has been zoomed to emphasize the interfaces, so that the image width represents 10 mm and the image height represents 1.2 mm.