The Effects of Correcting for Ocular Magnification on Retinal Nerve Fiber Layer and Retinal Ganglion Cell Layer Thicknesses in Macular fdOCT Scans

Abstract

 

Purpose

 

To evaluate the effect of optical corrections on retinal nerve fiber layer (RNFL) and retinal ganglion cell plus inner plexiform layer (RGC+) thicknesses obtained from frequency-domain OCT (fdOCT) macular scans.

 

Methods

 

Horizontal line scans across the macula were obtained using fdOCT (Spectralis, Heidelberg, Inc.) from one eye of 253 healthy controls (31.5±29.9 yrs; range 3 to 65 yrs) with biometric data. Eyes were organized into three groups: all 253 eyes (G1), 123 eyes with similar age and refractive errors to those often used in clinical studies (G2; >30 yrs and refractive error ±5 D), and 18 eyes with refractive error <−5 D (G3). RNFL and RGC+ thicknesses were measured over a 3-mm region nasal from the foveal center and at two local regions (see Fig.) using an automated segmentation algorithm with manual corrections.[1] To correct for ocular magnification, the scans were scaled horizontally (corrected) by a factor dependent on corneal curvature and scan focus (in diopters) using the manufacturer's software. This is effectively the same as a correction based on axial length.[2]

 

Results

 

The average absolute % differences between uncorrected and corrected scans are shown in the table for the mean and local thicknesses of each group. For all conditions, G3 (high myopes) scans showed the greatest % difference in RNFL and RGC+ thicknesses, while G2 (clinical) scans were affected the least. In addition, RNFL thicknesses were more affected than RGC+ thicknesses in all conditions. For local measurements, the effect of correction depended on the distance from the scan center with both layers more affected at 2.8 than 1.4 mm.

 

Conclusions

 

The effect of optical corrections depended on the population characteristics and the nature of the measure; it was relatively small (<5%) for the group with ages and refractive errors similar to those used in clinical studies (G2). While it is thought that optical corrections are necessary for OCT data, in practice, the need for correction will depend on the population, the layer and location of interest, and the purpose of the study. [1] Raza et al. AO, 2011; [2] Bennett et al. Graefe's, 1994.

 

 

An fdOCT scan with the RNFL and RGC+ segmented.

 

 

Keywords: 552 imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound) • 610 nerve fiber layer • 531 ganglion cells