Axial Length Estimates May Correct OCT Magnification Errors

Estimated axial length measurements obtained from keratometry and refractive measurements can correct OCT magnification errors in the absence of actual axial length measurements.

Using actual axial length (ALact) measurements to correct optical coherence tomography (OCT) magnification errors is more accurate than using estimated axial length (ALest) obtained from spherical equivalent (SE) and keratometry (K) measurements, according to research published in Eye and Vision. However, using ALest to correct for magnification errors more accurately measures fundus parameters than uncorrected images, according to the report. 

Researchers conducted a retrospective, cross-sectional analysis of 1301 participants from the Raine Study Gen2-20 year follow-up (mean age, 20 years; 673 men). Patients underwent comprehensive ophthalmic exams with cycloplegic refraction and were divided into 2 datasets; 1 to create (n=650) and 1 to validate (n=651) an association between AL, SE, and K. Investigators developed a formula, applied it to 46 participants with available AL, SE, and K data, and compared the ability of ALest and ALact to correct transverse magnification error in OCT images measuring foveal avascular zone area (FAZA). 

In the absence of ALact, clinicians should avoid relying on uncorrected measurements, and instead should use ALest to correct for magnification error as this provides for a more accurate measurement of fundus parameters.

An equation was created using multiple linear regression: ALest = 2.102K−0.4125SE+7.268; R2=0.794. Overall, good agreement between ALest and ALact was noted among the cohorts. The mean difference 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. The mean difference between uncorrected FAZA and FAZA measurements corrected with ALact was − 0.005±0.030 mm2 (95% LoA, −0.064−0.054 mm2).

“Our data suggest that ALact and ALest are comparable, especially when post-cycloplegic SE is used,” according to the researchers. “However, when applying AL in magnification error correction, ALact is more accurate than ALest and should be used preferentially in the clinical setting. In the absence of ALact, clinicians should avoid relying on uncorrected measurements, and instead should use ALest to correct for magnification error as this provides for a more accurate measurement of fundus parameters.”

Study limitations include the use of different instrumentation in determining K and SE within the cohorts, and an overrepresentation of participants who were of White ethnicity.

This article originally appeared on Optometry Advisor

References:

Dutt DDCS, Yazar S, Charng J, Mackey DA, Chen FK, Sampson DM. Correcting magnification error in foveal avascular zone area measurements of optical coherence tomography angiography images with estimated axial length. Eye and Vision. Published online August 1, 2022. doi:10.1186/s40662-022-00299-x