Evaluating the effect of the new alignment algorithm for longitudinal series of Heidelberg retina tomography images

To evaluate the impact of a new image-alignment algorithm on the repeatability of longitudinal measurements obtained from Heidelberg retina tomograph (HRT) images.

HRTI and HRTII image series from 124 patients with glaucoma or ocular hypertension were made available from previously reported studies and were reprocessed with the old and new image-alignment algorithms. Improvements afforded by the new alignment algorithm were examined by considering statistically significant improvement in repeatability of specific stereometric parameters (SP), namely rim area (RA), rim volume (RV), cup volume (CV) and cup shape measure (CSM). A further comparison was made by examining reduction in the variability of pixel-by-pixel height measures within image series.

In some HRT image series, the new algorithm automatically corrected obvious misalignment events that occurred with the previous algorithm. However, average improvement in repeatability of the SP in HRTI image series was not statistically significant (P = 0.13) and there was no statistically significant reduction in pixel-by-pixel height measurement variability (P =0.73). In HRTII image series, there was evidence of improvement, on average, in the repeatability of some parameters (RA, P = 0.01; RV, P = 0.02; CSM, P = 0.05), but not in CV (P = 0.22). There was a large reduction in pixel-by-pixel variability in HRTII image series (P < 0.001).

There was no evidence to show that the new algorithm improved repeatability, on average, in HRTI images. However, the application of the new algorithm to HRTII image series marginally improved repeatability in stereometric measures and yielded a significant reduction in pixel-by-pixel variability.