topography topics

Making Scaling Work for You

BY KENNETH A. LEBOW, OD, FAAO
AUGUST 1999

Scaling is a basic concept to remember when viewing CT maps. In order to view these maps correctly, you must understand the general shape of the curvature patterns and how it relates to scaling. The same map can appear differently, depending on the scale used to determine the curvature. Thus, determining which scale to use in a particular situation is key to success.

The Normalized Scale

The autosize, or normalized scale, automatically adjusts and subdivides a specific cornea into several dioptric intervals based on the actual dioptric range of the cornea measured, usually over an approximate 6.00D curvature range. Consequently, based on color representation, maps may appear to represent similar dioptric curvatures when using this scale, but depending upon a patient's specific corneal curvatures, these intervals may vary in range and dioptric value and may artificially give the appearance that all maps represent equal curvatures. Although normalized maps should not be visually compared without specifically referring to the actual dioptric scale, they do provide very detailed information about the eye being examined.

The Standard Scale

The standard, or absolute scale, assigns the same color to a specific dioptric interval of curvature and forces the data to fit within that range. Although the range varies by instrument, each individual instrument's range is consistent. Therefore, for each map viewed, a direct comparison of images from different eyes or from significant curvature changes for one eye (e.g. pre-operative and post-operative refractive surgery patients) can be made quickly and accurately.

Since the standard scale significantly expands the dioptric range used, clinically significant irregularities may be masked when comparing eyes with widely disparate curvature readings. When the range is standardized, the steepest curvatures appear red (upper left map of standard scale), while the flattest curvatures appear blue (upper right map). General patterns remain the same, but intricate curvature changes disappear because the interval of measurement also increases from 0.25D to 0.50D.

Remember to use normalized maps when viewing one eye at a time and standard maps when comparing different eyes or the same eye that has undergone significant curvature change.

Dr. Lebow is a member of the AOA and a Fellow of the AAO. He is in private practice in Virginia Beach, Virg.

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