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Ravi S. Jonnal, Omer P. Kocaoglu, Qiang Wang, Zhuolin Liu, Donald T. Miller; Imaging The Living Human Cone Inner Segment. Invest. Ophthalmol. Vis. Sci. 2012;53(14):5601.
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© ARVO (1962-2015); The Authors (2016-present)
Combined with adaptive optics (AO), ultra-high resolution optical coherence tomography (UHR-OCT) can image the living human retina with a 3D resolution of 3 μm. Using this technique--combined with tools for registering, segmenting, and averaging volumetric images--we sought to characterize the external limiting membrane (ELM) and inner segment outer segment junction (IS/OS). Such a characterization may aid in interpretation of the OCT image, by facilitating comparison with histological, electron microscopic, and other well-accepted images of the inner segment.
We used the Indiana AO-UHR-OCT system  to acquire volumetric images of the retina, at a retinal eccentricity of 1.5 deg., in three subjects. Software was developed to automatically register and average volumes, and segment the ELM, IS/OS, and OS posterior tip (PT) layers, permitting comparisons between them. To characterize transverse structure within and between the ELM, IS/OS, and PT reflections, we used autocorrelation and cross correlation, respectively. To characterize their axial structure, we extracted A-lines from individual cones and computed the full width half height (FWHH) of the axial profile of each reflection and the RMS phase differences between them.
Averaging improved en face contrast by factors of 35% and 40% at the ELM and IS/OS layers, respectively. Autocorrelation revealed the ELM layer is dominated by bright spots with diameter similar to IS/OS and PT, but with less uniform intensity and packing, and lower contrast. Cross correlation revealed the IS/OS is significantly positively correlated with PT and negatively correlated with ELM--i.e. ELM is dark where IS/OS and PT are bright, and vice versa. The reflections at ELM and IS/OS were axially thin (3.6 μm FWHH), slightly larger than the axial point spread function of the OCT. RMS phase difference between ELM and IS/OS was less than 1.5 radians, which suggests that the thickness of those reflectors is less than 75 nm each.
The IS of individual cones is bounded by two thin reflectors. The IS/OS reflectance pattern aligns directly to that of the PT, while the ELM pattern, by contrast, does not. This difference suggests that the cone’s footprint in the ELM either: 1) reflects less light than that of neighboring rods and/or Muller cells; or 2) is laterally displaced relative to the IS/OS and PT portions of the same cone.
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