Optical coherence tomography scan circle location and mean retinal nerve fiber layer measurement variability

Abstract

Purpose: To investigate the effect on optical coherence tomography (OCT) retinal nerve fiber layer (RNFL) thickness measurements of varying the standard 3.4-mm-diameter circle location.

Methods: The optic nerve head (ONH) region of 17 eyes of 17 healthy subjects was imaged with high-speed, ultrahigh-resolution OCT (hsUHR-OCT; 501 x 180 axial scans covering a 6 x 6-mm area; scan time, 3.84 seconds) for a comprehensive sampling. This method allows for systematic simulation of the variable circle placement effect. RNFL thickness was measured on this three-dimensional dataset by using a custom-designed software program. RNFL thickness was resampled along a 3.4-mm-diameter circle centered on the ONH, then along 3.4-mm circles shifted horizontally (x-shift), vertically (y-shift) and diagonally up to +/-500 microm (at 100-microm intervals). Linear mixed-effects models were used to determine RNFL thickness as a function of the scan circle shift. A model for the distance between the two thickest measurements along the RNFL thickness circular profile (peak distance) was also calculated.

Results: RNFL thickness tended to decrease with both positive and negative x- and y-shifts. The range of shifts that caused a decrease greater than the variability inherent to the commercial device was greater in both nasal and temporal quadrants than in the superior and inferior ones. The model for peak distance demonstrated that as the scan moves nasally, the RNFL peak distance increases, and as the circle moves temporally, the distance decreases. Vertical shifts had a minimal effect on peak distance.

Conclusions: The location of the OCT scan circle affects RNFL thickness measurements. Accurate registration of OCT scans is essential for measurement reproducibility and longitudinal examination (ClinicalTrials.gov number, NCT00286637).

Figures

FIGURE 1

Plot of the linear mixed-effects model for mean RNFL thickness versus x- and y-shift. A plane transecting the model at a distance 3.98 µm from the maximum indicates our threshold level for mean RNFL thickness.

FIGURE 2

A summary of changes associated with horizontal and vertical scan circle displacements. When the scan circle was displaced horizontally, peak distance changes, but superior and inferior RNFL thicknesses remained stable. During vertical displacements, peak distance remained relatively stable, but the superior and inferior RNFL thicknesses changed.

FIGURE 3

Plot of the peak distance as a function of x-shift with y-shift set to 0 (a), and y-shift with x-shift set to 0 (b).

FIGURE 4

A plot of the maximum range of displacement of the center of the 3.4-mm scan circle for the mean and the quadrants, derived from the linear mixed-effects models and thresholds. The x-shifts (horizontal displacements of the center of the 3.4-mm circle from the center of the ONH) and y-shifts (vertical displacements) outside of these ellipses should be interpreted with caution.

FIGURE 5

(a) High-speed, ultrahigh-resolution OCT fundus image with the disc margin manually drawn, the geometric center of the disc (single white dot), and a 3.4-mm circle centered on the disc. (b) Inner ring: range of possible locations of the center of the circle; shaded area: the corresponding locations of the 3.4-mm circle that will not affect mean RNFL thickness measurements beyond the threshold level (3.98 µm).

FIGURE 6

An example of a StratusOCT scan showing circle placement errors and corresponding changes in peak distance. Left: A well-centered scan. Right: As the scan circle is displaced temporally, the peaks move closer together and superior quadrant and clock hours measurements change from borderline (yellow) to normal (green) while nasal quadrants and clock hours become thicker.