Abstract
Purpose: :
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.
Methods: :
We used the Indiana AO-UHR-OCT system [1] 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.
Results: :
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.
Conclusions: :
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.
Keywords: retina • image processing • imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound)