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Topography and volume measurements of the optic nerve using en-face optical coherence tomography

John A. Rogers, Adrian Gh. Podoleanu, George M. Dobre, David A. Jackson, and Fred W. Fitzke

Open Access Open Access

Abstract

A special imaging instrument was developed which can acquire optical coherence tomography (OCT) en-face images from the eye fundus, and simultaneously a confocal image. Using this instrument we illustrate for the first time the application of en-face OCT imaging to produce topography and perform area and volume measurements of the optic nerve. The procedure is compared with the topography, area and volume measurements using a confocal scanning laser ophthalmoscope.

©2001 Optical Society of America

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Figures (10)
Fig. 1.
Fig. 1. Relative orientation of the axial scan (A-scan), longitudinal slice (B-scan) and en-face or transversal slice (C-scan).

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Figure 2.
Figure 2. General set-up of the combined OCT/Confocal system. MX, MY: galvanometer mirrors of the XY scanning head. The detailed configuration is presented in references15,16.

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Fig. 3.
Fig. 3. Pair of images from the optic nerve acquired with the standalone OCT/confocal system in longitudinal regime at y=0. Top image: OCT; Bottom: confocal; Each image has 300×300 pixels. Horizontal: Δx~3 mm in both images; Δz (only the OCT image) ~2 mm depth (vertical axis, measured in air). RNFL (bright): retinal nerve fiber layer; PL (dark): photoreceptor layer; RPE (bright): retinal pigment epithelium; CC (bright): choriocapillaris.

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Fig. 4.
Fig. 4. (1 MB) Movie showing the pair of images from the optic nerve acquired with the standalone OCT/confocal system in transversal regime. Top image: OCT; Bottom: confocal. Each image has 300×300 pixels. Horizontal: Δx~3 mm, Vertical: Δy~3 mm in both images. The volume is explored from the retinal nerve fiber layer to the retinal pigment epithelium, along the optic axis. The OCT image displayed is at the depth shown by the double arrow in Figure 3 top.

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Fig. 5.
Fig. 5. Example of a software inferred A’-scan from the set in Figure 4, pixel 180×180, in a grid of pixels counted from the corner top left, up to 210×210 along X and Y axes. The continuous profile represents an interpolation over 60 points, one point for each OCT frame in the set, collected at a certain depth. Horizontal scale: Image number. Vertical scale: arbitrary units for the magnitude of the OCT signal. Interpolation was used to cover for the 4 frames removed from the collection.

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Fig. 6.
Fig. 6. Topography of the first surface, 186×186 pixels (1.9 mm×1.9 mm). Depth map (values in color bar, the frame number, depth could be inferred by multiplying the frame number by 20 µm). (a):View from the top, along the z direction; (b): 3D view; (c): as seen from direction A in b; (d) as seen from direction B in b. The vertical arrow in (c) denotes the transversal position of the A’-scan in Fig. 5.

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Fig. 7.
Fig. 7. Topography of the deepest surface, 186×186 pixels transversal (1.9 mm×1.9 mm). Depth map (values in color bar, the frame number, depth could be inferred by multiplying the frame number by 20 µm; the color map is different than that in Fig. 6.); (a): as seen from the top; (b): 3D view; (c): first and the deepest surfaces seen from the direction A in (d); (d): 3D views of the first and the deepest surfaces (Fig. 6(b) and 7(b) superposed).

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Fig. 8.
Fig. 8. The first image in the confocal set showing the contour line in red where A’ scans are calculated (300×3000 pixels).

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Fig. 9.
Fig. 9. Depth of the first surface, calculated from the depth position of the first peak in each A’-scan (such as that in Figure 5) originating on points situated on the contour drawn on the confocal image from the first frame in Fig. 8.

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Fig. 10.
Fig. 10. Colored map topography of the first surface. The colored areas correspond to: green, the area above the surface, blue, the area between the surface and the reference plane and red the area below the reference plane. Values for areas are given in the Table 1. Left: data were processed using the OCT depth resolution; Right: data from the fabricated set of 300 µm depth resolution.

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Tables (1)

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Table 1. Comparative results for the areas and volumes in the optic nerve measured with en-face OCT and with HRT.

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