Cellular morphometry of the outer retina using adaptive optics OCT

Abstract

 

Purpose

 

Diseases and disorders which impact the outer retina, such as AMD, diabetic retinopathy, macular hole, and retinal detachment are routinely evaluated using optical coherence tomography (OCT). As such, correct interpretation of clinical OCT images is of great importance. Interpretation of these images, though, is clouded by optical properties of the eye, the complexity of light-tissue interactions, and image post-processing. We have designed and built a custom OCT system equipped with adaptive optics that provides greatly improved 3D resolution and permits precise instrument calibration and characterization. We investigated the OCT outer retinal band 3, thought to originate from either the cone outer segment tips or zone of interdigitation between cone outer segments (OS) and apical processes of the RPE.

 

Methods

 

Five subjects were imaged between 1.5 and 4.25°. Cones were segmented and aligned by their band 3 peaks. The resulting aligned B-scan was averaged and the thickness of band 3 was measured. Next, after bulk motion correction, the phase of the band 3 reflection was extracted and variance of this phase, which is a measure of the surface's roughness, was computed.

 

Results

 

After subtracting measured axial blur, average thickness of band 3, measured in single cones, was 2.1 μm. Typical variance of bulk-motion-corrected phase measurements was 0.09 rad, which corresponds to a surface roughness of 4 nm RMS.

 

Conclusions

 

Cellular measurements of band 3 suggest strongly that the reflection comes from a thin, optically smooth origin, such as a reflective surface, and not from an axially extended zone of scattering material. This surface must be located distal to the cone OS lumen and proximal to the RPE body. The most likely origin is the distal plasma membrane of the cone outer segment.  

 

AO-OCT B-scan taken at 1.5° temporal to the fovea. In (a), band 3 reflections from single cones are marked. Axial shifts of reflections are evident. In (b), the cones are shown without axial alignment. In (c), the cones are aligned by band 3.

 

 

(a) Bulk-corrected phase of band 3 of the cones in Fig. 1. Average longitudinal reflectance profiles of the (b) unaligned cones and (c) the band 3-aligned cones. Unaligned band 3 FWHM is 10.8 μm and aligned FWHM is 4.1 μm, which includes the 2.8 μm (measured) axial PSF. This suggests that the bulk of band 3's thickness in commercial OCT images is due to axial displacements of the reflections.