Optical coherence tomography B-scans (1:1 pixel in scale) were exported as image files after setting the baseline as reference. The ALCSD and ONHSD were measured with reference to the axial and transverse scaling factors (μm/pixel) of individual B-scans as reported in the Spectralis OCT operating system. Images were analyzed in a computer using a customized program developed in Matlab (R2010a; The MathWorks, Inc., Natick, MA, USA). Two anatomical landmarks (1) the Bruch's membrane opening (BMO) and (2) the choroid-sclera interface (CSI) were referenced for measurements of ALCSD and ONHSD (
Fig. 1). ALCSD
BMO and ONHSD
BMO were measured after the BMO reference line (a line joining the BMO in a B-scan), the internal limiting membrane, and the anterior LC and the ONH surfaces were manually located (assisted by the piecewise cubic Hermite interpolation polynomial). ALCSD
CSI and ONHSD
CSI were measured with reference to the CSI reference line. The CSI reference line was a line joined by the CSI located between 1700 and 1800 μm from the BMO central axis point.
14 The CSI positions were marked along the 1700- to 1800-μm distance from the BMO central axis point to minimize errors associated with focal fluctuations of the CSI.
14 The ALCSD
BMO and ALCSD
CSI were defined as the perpendicular distances from the BMO and CSI reference lines, respectively, to the detectable anterior LC surface, which was visualized as the interface bordering the prelaminar tissue (moderate-intensity signal below the ONH surface) and the LC (high-intensity vertical striations;
Fig. 1C).
5,7 ONHSD
BMO and ONHSD
CSI were defined as the perpendicular distances from the BMO and CSI reference lines, respectively, to the ONH surface (
Fig. 1D). The ALCSD and ONHSD of an eye were measured as the averages of the six radial B-scans. To account for the fact that the radial scan pattern under-represented the peripheral compared with the central ONH in the measurement of ALCSD and ONHSD (the OCT sampling density was higher near the optic disc center than the peripheral), each of the delineated distances at the anterior LC and ONH surfaces was weighted in proportion to its distance from the center point between the pair of BMO points using the formulas:
where
ȳ represents the mean ALCSD/ONSHD of an eye, which is given by averaging the mean values of each of the 6 radial B-scans
ȳi (for
i = 1,2,…,6).
ȳi represents the weighted average of the distance measure
yij, which is the distance at point
j in B-scan
i with a weighting given by its distance from the BMO center point,
dij.