Watch Your Placido!
BY LORETTA B. SZCZOTKA, OD, MS
Many of us frequently dismiss looking at the Placido displays of acquired topography images. Most corneal topography systems can display a Placido image as a single map at any time, and
users programming their favorite maps can usually include it as one of the default displays. The Placido image serves the map interpreter as an aide in determining whether data is accurate.
Topographers are not yet sophisticated enough to catch all artifacts of data acquisition or interpretation. The phrase "garbage in = garbage out" applies, and Placido image review helps determine which maps are valid prior to implementing diagnostic and treatment decisions.
Some ring digitization errors can be detected by simply viewing the Placido image.
Limbal Ring Digitization
Some topographers rely on a border detection algorithm to determine the location of the rings off the reflected image. This is typical of wide ring cone systems which search for sharp black-white borders typically detected at both edges of each illuminated ring (Figure 1). A potential artifact second to this detection scheme is that the border detection program may image the black-to-white corneal-scleral junction as one of the reflected Placido rings. If so, the resulting color coded map will show an artificially flat zone in the periphery which looks like a blue wing extending from the remainder of the map. The user could have detected and potentially removed this artifact if the system allows editing of data points when viewing the Placido image prior to processing.
Figure 1: Example of a thick ring Placido system.
Tear Meniscus Errors
Converse to an artificially flat zone interpreted by an instrument due to wide distances between assumed reflected rings, an erroneous steep zone can result if mires are artificially spaced very close together. For example, when acquiring an image from a photosensitive patient with excessive tear pooling at the inferior tear meniscus, the closely reflected mires at the inferior corneal position may be erroneously close together and possibly result in a "pseudo-keratoconus" pattern. Prevent this misdiagnosis by viewing the Placido image early.
Finally, in extremely steep, distorted, dry or irregular corneas, certain points on a ring may not satisfactorily reflect back to the digital video camera. When the computer begins to digitize the rings, it may skip a ring or the rings may cross when the system tries to find the next logical point in succession. This is common in keratoconus over the steepest area of the cone where scarring may also be present. It results in an artifactual map from the area over the irregular mires (and often in the entire wedge extending peripherally from the poorly imaged zone). Quantitative data from such an irregular area as defined by the Placido image is suspicious do not use it for specific diagnosis and management decisions.
Dr. Szczotka is an assistant professor at Case Western Reserve University Dept. of Ophthalmology and Director of the Contact Lens Service at University Hospitals of Cleveland.
Contact Lens Spectrum, Issue: July 2000