VARIABILITY OF CONTOUR LINE ALIGNMENT ON SEQUENTIAL IMAGES WITH THE HEIDELBERG RETINA TOMOGRAPH

Background: The influence of the contour line alignment software algor ithm on the variability of the Heidelberg Retina Tomograph (HRT) param eters remains unclear. Methods: Nine discrete topographic images were acquired with the HRT from the right eye in six healthy, emmetropic su bjects. The variability of topometric data obtained from the same topo graphic image, analyzed within different samples of images, was evalua ted. A total of four mean topographic images was computed for each sub ject from: all nine discrete images (A), the first six of those images (B), the last six of those nine images (C), and the first three combi ned with the last three images (D). A contour line was computed on the mean topographic image generated from the nine discrete topographic i mages (A). This contour line was then applied to the three other mean topographic images (B, C, and D), using the contour line alignment in the HRT software. Subsequently, the contour line on the mean topograph ic images was applied to each of the discrete members of the particula r images subsets used to compute the mean topographic image, and the t opometric data for these discrete topographic images was computed succ essively for each subset. Prior to processing each subset, the contour line on the discrete topographic images was deleted. This strategy pr ovided a total of three analyses on each discrete topographic image: a s a member of the nine images (mean topographic image A), and as a mem ber of two subsets of images(mean topographic image B, C, and/or D). T he coefficient of variation (100xSD/mean) of the topographic parameter s within those three analyses was calculated for each discrete topogra phic image in each subject (''interimage'' coefficient of variation). In addition, a coefficient of variation between the nine discrete topo graphic images (''interimage'' coefficient of variation) was calculate d. Results: The ''intraimage'' and ''interimage'' variability for the various topographic parameters ranged between 0.03% and 3.10% and betw een 0.03% and 24.07% respectively. The ''intraimage'' coefficients of variation and ''interimage'' coefficients of variation correlated sign ificant (r(2)=0.77; P<0.0001). Conclusion. A high ''intraimage'' varia bility, i.e. a high variability in contour line alignment between sequ ential images, might be an important source of test re-test variabilit y between sequential images.