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Editors Selection IGR 16-3
Heidelberg Retina Tomograph: Topographic change analysis
Comment by Rupert Bourne on:
28025 Agreement between Heidelberg Retina Tomograph-I and-II in detecting glaucomatous changes using topographic change analysis, Balasubramanian M; Bowd C; Weinreb RN et al., Eye, 2011; 25: 31-42
The use of imaging devices to detected structural change of the optic nerve head and nerve fibre layer is plagued by the issue of backwards compatability. In a recent paper by Balasubramanian et al. (163), this issue is addressed with the Heidelberg Retina Tomograph (HRT), a device in which images from an older version of the instrument (HRT-I, introduced in 1991) can be incorporated into an image series that includes those of a later version (HRT-II, introduced in 1999). The authors had recognized that optic discs with image series with a baseline image taken by the HRT-I appeared to show more deterioration on Topographic Change Analysis (TCA) than those with HRT-II. The TCA software corrects differences in parabolic distortion between baseline and follow-up examinations (parabolic error). Because of reduced field of view in HRT-I measurements, PEC is not currently applied if an image series contains HRT-I topographies. Following 66 eyes over five years, the authors compared the TCA results of two databases they constructed, one with HRT-II images only (HRT-II series), and one with the baseline HRT-II replaced with an HRT-I which had been performed on the same day (HRT-I-mixed series). Applying PEC to the HRT-I-mixed series resulted in no significant differences in TCA change between the HRT-I-mixed and HRT-II-only series. The authors did attempt to explain why they found a small number of eyes in which agreement remained poor despite PEC.
For those wishing to understand the issues associated with normalization of images in a progression series, this is a very useful and eloquently-written paper. Important take-home messages include obtaining images from both old and new instruments on the same day if 'upgrading' devices, and the need for good image quality of baseline measurements. Hopefully the PEC correction can be made available in the software for such mixed series. This would allow us to more accurately detect change over time, in some cases, over twenty years- a remarkable feat in an era of rapid technological advancement.
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