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. 2022 Aug 1;9(1):29.
doi: 10.1186/s40662-022-00299-x.

Correcting magnification error in foveal avascular zone area measurements of optical coherence tomography angiography images with estimated axial length

Deepaysh D C S Dutt  1 , Seyhan Yazar  1   2 , Jason Charng  1 , David A Mackey  1 , Fred K Chen  3   4   5   6 , Danuta M Sampson  1   7   8
Affiliations

Correcting magnification error in foveal avascular zone area measurements of optical coherence tomography angiography images with estimated axial length

Deepaysh D C S Dutt et al. Eye Vis (Lond). .

Abstract

Background: To generate and validate a method to estimate axial length estimated (ALest) from spherical equivalent (SE) and corneal curvature [keratometry (K)], and to determine if this ALest can replace actual axial length (ALact) for correcting transverse magnification error in optical coherence tomography angiography (OCTA) images using the Littmann-Bennett formula.

Methods: Data from 1301 participants of the Raine Study Gen2-20 year follow-up were divided into two datasets to generate (n = 650) and validate (n = 651) a relationship between AL, SE, and K. The developed formula was then applied to a separate dataset of 46 participants with AL, SE, and K measurements and OCTA images to estimate and compare the performance of ALest against ALact in correcting transverse magnification error in OCTA images when measuring the foveal avascular zone area (FAZA).

Results: The formula for ALest yielded the equation: ALest = 2.102K - 0.4125SE + 7.268, R2 = 0.794. There was good agreement between ALest and ALact for both study cohorts. The mean difference [standard deviation (SD)] between FAZA corrected with ALest and ALact was 0.002 (0.015) mm2 with the 95% limits of agreement (LoA) of - 0.027 to 0.031 mm2. In comparison, mean difference (SD) between FAZA uncorrected and corrected with ALact was - 0.005 (0.030) mm2, with 95% LoA of - 0.064 to 0.054 mm2.

Conclusions: ALact is more accurate than ALest and hence should be used preferentially in magnification error correction in the clinical setting. FAZA corrected with ALest is comparable to FAZA corrected with ALact, while FAZA measurements using images corrected with ALest have a greater accuracy than measurements on uncorrected images. Hence, in the absence of ALact, clinicians should use ALest to correct for magnification error as this provides for more accurate measurements of fundus parameters than uncorrected images.

Keywords: Axial length; Keratometry; Littmann-Bennett formula; OCTA; Spherical equivalent.

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Conflict of interest statement

The authors declare that they have no competing interests.

Figures

Fig. 1
Fig. 1
Bland-Altman plots illustrating the agreement between the ALest and ALact for (a) validation dataset (post-cycloplegic SE), (b) validation dataset (non-cycloplegic SE), and (c) OCTA dataset (non-cycloplegic SE). The yellow regions represent the 95% CI. ALact, actual axial length; ALest, estimated axial length; CI, confidence interval; SE, spherical equivalent; SD, standard deviation
Fig. 2
Fig. 2
Bland-Altman plots illustrating the agreement between (a) FAZA corrected with ALest vs. FAZA corrected with ALact and (b) FAZA before correction vs. FAZA after correction with ALact. Plots of relative change between (c) FAZA after correction with ALest vs. FAZA after correction with ALact, and (d) FAZA before correction vs. FAZA after correction with ALact. The linear dashed lines in (c) and (d) are linear fits to the data. The horizontal dashed lines in c and d indicate a 5% relative change in the FAZA. The yellow regions in a and b represent the 95% CI. ALact, actual axial length; ALest, estimated axial length; CI, confidence interval; FAZA, foveal avascular zone area; SD, standard deviation
See this image and copyright information in PMC

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