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

Background: To generate and validate a method to estimate axial length estimated (AL) from spherical equivalent (SE) and corneal curvature [keratometry (K)], and to determine if this AL can replace actual axial length (AL) 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 AL against AL in correcting transverse magnification error in OCTA images when measuring the foveal avascular zone area (FAZA).

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

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