Technique for detecting serial topographic changes in the optic disc and peripapillary retina using scanning laser tomography

Purpose. To describe and evaluate a new statistical technique for detecting topographic changes in the optic disc and peripapillary retina measured wi th confocal scanning laser tomography. Methods. The 256 x 256-pixel array of topographic height values obtained wi th each image from the Heidelberg Retina Tomograph (Heidelberg Engineering, Heidelberg, Germany) was divided into an array of 64 x 64 superpixels, whe re each superpixel contained 16 (i.e., 4 x 4) pixels. An analysis of varian ce technique was developed to analyze each superpixel with three baseline a nd three follow-up images. The performance of the technique was tested with and without adjustment for spatial correlation of topographic values using computer simulations and with real data from a normal control subject and a patient with progressive glaucomatous disc change. Results. Computer simulation with fixed population means and variance, and varying spatial correlation showed a monotonically increasing number of sup erpixels with significant test results (false positives), with 20% false-po sitives when the spatial correlation was 0.8 (the approximate median value in real patient data). The number of false-positive results was similar (17 %) in serial images of a normal subject. When corrected for spatial correla tion, the number of false-positives was independent of the level of spatial correlation and remained at the expected value of less than 5% in both sim ulations and real data. Although the number of significant test results in the patient with progressive glaucoma decreased after correction for spatia l correlation, the change was readily apparent. Statistical power to detect mean differences in topographic values ranging from 0.5 to 4.0 SDs in comp uter simulation showed low power for changes of 1 SD or less, but increased dramatically with larger changes. Conclusions. This technique has a high level of sensitivity to detect chang es in the optic disc while maintaining a high level of specificity.