The influence of contour line size and location on the reproducibility of topographic measurement with the Heidelberg Retina Tomograph

Background: the recommended contour line (CL) location with the Heidelberg Retina Tomograph (HRT) is on the inner edge of Elschnig's scleral ring. Thi s study investigated HRT parameter reproducibility when: (i) the CL size is altered relative to Elschnig's ring; (ii) the CL is either redrawn or impo rted between images. Methods: using the HRT, seven 10 degrees images were acquired for 10 normal volunteers and 10 primary open angle glaucoma (POAG) subjects. A CL was dr awn on one image for each subject using Elschnig's scleral ring for referen ce and imported into subsequent images. The CL diameter was then (a) increa sed by 50 mum; (b) increased by 100 mum; and (c) decreased by 50 mum. To in vestigate the effect of the method of contour line transfer between images a CL was: (1) defined for one image and imported to 6 subsequent images; (2 ) drawn separately for each image. Results: parameter variability improved as the size of the CL increased for the normal group relative to Elschnig's ring but was unchanged in the POAG group. The export/import function (method 1) resulted in better parameter reproducibility than the redrawing method for both groups. Conclusions: the exporting and importing function resulted in better parame ter variability for both subject groups and should be used for transferring CLs across images for the same subject. Increasing the overall CL size rel ative to Elschnig's scleral ring improved the reproducibility of the measur ed parameters in the normal group. No significant difference in parameter v ariability was observed for the POAG group. This suggests that the reproduc ibility of HRT images are affected more by the variation in topography betw een images than change in CL definition. (C) 2001 The College of Optometris ts. Published by Elsevier Science Ltd. All rights reserved.