Corneal thickness mapping by topographical ultrasonic pachometry

Ultrasonic measurements of corneal thickness (CT) more than 25 points from a 6-mm diameter area of the central cornea were used to create computerized CT maps. Two subjects were evaluated in different sessions by topographical ultrasonic pachometry. The thickness values were stored as a two-dimensional array z(x,y), whose indices refer to positional coordinates x (in the nasal–temporal direction) and y (in the superior–inferior direction). Pachometric maps were then constructed by plotting thickness values against horizontal and vertical (x,y) position coordinates. Assisted computer interpolation between data points provides color-coded continuous pachometric maps. Data were processed and analyzed separately and an average map was also obtained for each subject. Goodness of fit for the average maps measured by coefficient of determination (r²) was very high in both examples (r² = 0.98 and r² = 0.99). Fitting standard error was 5.42 μm and 4.17 μm, respectively. Also, points not measured in the ultrasonic evaluation could be automatically predicted from the fitted model. The technique described provides an affordable and reliable method for thickness mapping of full corneal tissue, enabling accurate anatomical evaluation of the changes induced in the cornea by refractive surgery procedures, contact lenses, or disease.