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_(est))from spherical equivalent(SE)and corneal curvature[keratometry(K)],and to determine if this AL_(est)can replace actual axial length(AL_(act))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_(est)against AL_(act)in correcting transverse magnification error in OCTA images when measuring the foveal avascular zone area(FAZA).Results:The formula for AL_(est)yielded the equation:AL_(est)=2.102K−0.4125SE+7.268,R^(2)=0.794.There was good agreement between AL_(est)and AL_(act)for both study cohorts.The mean difference[standard deviation(SD)]between FAZA corrected with AL_(est)and AL_(act)was 0.002(0.015)mm^(2)with the 95%limits of agreement(LoA)of−0.027 to 0.031 mm^(2).In comparison,mean difference(SD)between FAZA uncorrected and corrected with AL_(act)was−0.005(0.030)mm^(2),with 95%LoA of−0.064 to 0.054 mm^(2).Conclusions:AL_(act)is more accurate than AL_(est)and hence should be used preferentially in magnification error correction in the clinical setting.FAZA corrected with AL_(est)is comparable to FAZA corrected with AL_(act),while FAZA measurements using images corrected with AL_(est)have a greater accuracy than measurements on uncorrected images.Hence,in the absence of AL_(act),clinicians should use AL_(est)to correct for magnification error as this provides for more accurate measurements of fundus parameters than uncorrected images.