Analysis of HRT images: comparison of reference planes.

PURPOSE The values of Heidelberg Retinal Tomograph (HRT; Heidelberg Engineering, Heidelberg, Germany) stereometric parameters depend on the reference plane (RP), the instability of which results in parameter variability. Identification of change depends on RP stability. This study was undertaken to evaluate the influence of various RPs on rim areas (RAs) in a longitudinal image series. METHODS A longitudinal image series of 31 subjects with ocular hypertension who had reproducible visual field loss and 19 normal subjects was analyzed using five different RPs: the standard RP (HRT software version 3.1.2.0), two 320-mum RPs (software ver. 3.1.2.0 and 1.7.0), a previously described experimental RP, and a new Moorfields RP. The Moorfields RP takes the standard RP at baseline and then is fixed relative to the reference ring for subsequent images. Linear regression of RA over time was performed, and the slope and residual SD (RSD) were calculated for each RP. Comparisons between RPs were made by paired t-tests. RESULTS In eyes with progressing disease, application of the standard RP resulted in significantly greater variability (as measured by the RSD) compared with other RPs (mean 0.057 mm(2) vs. 0.035-0.038 mm(2)). There was a trend toward faster RA change/time (mean, -0.0123 mm(2)/y) for the standard RP and slower (-0.0095 mm(2)/y) 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 mm(2)/y and mean global RSD 0.037mm(2). CONCLUSIONS Compared to the standard RP, the Moorfields RP has significantly lower variability and probably provides a greater facility for discriminating RA change from measurement variability in a longitudinal HRT image series.