Analysis of HRT images: comparison of reference planes
Alessandro Poli 1*,
Nicholas Gabriel Strouthidis 2,
Tuan Ho 2,
and
David F. Garway-Heath 3
1 Eye Clinic, University of Verona, Piazzale Stefani 1, Verona, 37124, Italy; Glaucoma Research Unit, Moorfields Eye Hospital, London, United Kingdom
2 Glaucoma Research Unit, Moorfields Eye Hospital, London, United Kingdom
3 Moorfields Eye Hospital - Glaucoma Research Unit, London, United Kingdom
* To whom correspondence should be addressed. E-mail: aledott{at}tiscali.it.
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Abstract |
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Background: HRT stereometric parameter values depend on the position of a reference plane (RP), the instability of which results in parameter variability. The identification of change depends on the stability of the RP position. The purpose of the study was to evaluate the influence of various RPs on rim area (RA) values in longitudinal image series.
Methods: Longitudinal HRT image series of 31 ocular hypertensive subjects who developed reproducible visual field loss and 19 normal subjects were analyzed using 5 different RPs; the Standard RP (Software Version 3.1.2.0), two 320 µm RPs (Software Versions 3.1.2.0 and 1.7.0), a previously-described Experimental RP and a new 'Moorfields' RP. The Moorfields RP is set at the level of the Standard RP at baseline, and is then fixed relative to the reference ring for subsequent images. Linear regression of RA over time was performed and the standard error (residual standard deviation; RSD) of the rim change slope was calculated for each RP. Comparisons between RPs were made by paired T- test analyses.
Results: In progressing eyes, application of the Standard RP resulted in significantly greater variability (as measured by the RSD) compared to other RPs (mean 0.057 mm2 versus 0.035 to 0.038 mm2). There was a trend to faster RA change / time ( mean -0.0123 mm2 / year) for the Standard RP and slower (-0.0095 mm2 / year) for the Experimental RP. There was a trend for the Moorfields RP to result in the best signal-to-noise ratio (speed of RA change/variability) - mean RA slope -0.0118 mm2 / year; mean global RSD 0.037 mm2.
Conclusions: Compared to the Standard RP, the Moorfields RP has significantly lower variability and probably provides a greater facility to discriminate RA change from measurement variability in the analysis of longitudinal HRT image series.
Key Words:
optic nerve head, glaucoma, reference plane, HRT, reproducibility
