Arc-scanning very high-frequency digital ultrasound for 3D pachymetric mapping of the corneal epithelium and stroma in laser in situ keratomileusis

PURPOSE: To test and demonstrate measurement precision, imaging resolution, 3D thickness mapping, and clinical utility of a new prototype 3D very high -frequency (VHF) (50 MHz) digital ultrasound scanning system for corneal ep ithelium, flap, and residual stromal thickness after laser in situ keratomi leusis (LASIK). METHODS: VHF ultrasonic 3D data was acquired by are-motion, meridional scan ning within a 10-mm zone. Digital signal processing techniques provided hig h-resolution B-scan imaging, and I-scan traces for high-precision pachymetr y in 4 eyes. Thickness maps of individual corneal layers were constructed. Reproducibility of epithelial, flap, and full corneal pachymetry was assess ed for single-point and 3D thickness mapping by repeated measures. Thicknes s mapping of the epithelium, stroma, flap, and full cornea were determined before and after LASIK. Preoperative to postoperative difference maps for e pithelium, flap, and stroma were produced to demonstrate anatomical changes in the thickness profile of each layer. RESULTS: Surface localization precision was 0.87 mu m. Central reproducibil ity for single-point pachymetry of epithelium was 0.61 mu m; flap, 1.14 mu m; and full cornea, 0.74 mu m. Reproducibility for central pachymetry on 3D thickness mapping was 0.5 mu m for epithelium and 1.5-mu m for full cornea . B-scans and 3D thickness maps after LASIK demonstrated resolution of epit helial, stromal component of the flap, and residual stromal layers. Large e pithelial profile changes were demonstrated after LASIK. Topographic variab ility of flap thickness and residual stromal thickness were significant. CONCLUSIONS: VHF digital ultrasound arc-B scanning provides high-resolution imaging and high-precision three-dimensional thickness mapping of corneal layers, enabling accurate anatomical evaluation of the changes induced in t he cornea by LASIK.