Hyperspectral optical coherence tomography for in vivo visualization of melanin in the retinal pigment epithelium

Abstract

Previous studies for melanin visualization in the retinal pigment epithelium (RPE) have exploited either its absorption properties (using photoacoustic tomography or photothermal optical coherence tomography [OCT]) or its depolarization properties (using polarization sensitive OCT). However, these methods are only suitable when the melanin concentration is sufficiently high. In this work, we present the concept of hyperspectral OCT for melanin visualization in the RPE when the concentration is low. Based on white light OCT, a hyperspectral stack of 27 wavelengths (440‐700 nm) was created in post‐processing for each depth‐resolved image. Owing to the size and shape of the melanin granules in the RPE, the variations in backscattering coefficient as a function of wavelength could be identified—a result which is to be expected from Mie theory. This effect was successfully identified both in eumelanin‐containing phantoms and in vivo in the low‐concentration Brown Norway rat RPE.

Visible-Light Optical Coherence Tomography Fibergraphy for Quantitative Imaging of Retinal Ganglion Cell Axon Bundles

“…Pixels with a higher DiFM indicate higher backscattering than surrounding tissues. 13 This process is summarized as…”

Melanin concentration and depolarization metrics measurement by polarization-sensitive optical coherence tomography

“…Nevertheless, it is of great interest how the depolarization metrics respond to the shape, size distribution, and absolute concentration of the melanin granules. These parameters of the melanin granules were measured by Baumann et al 8 and Harper et al 32 previously. Although the melanin granules were filtered by vacuum filtration to ensure the size of the melanin granules was less www.nature.com/scientificreports/ Figure 5.…”

“…Nevertheless, it is of great interest how the depolarization metrics respond to the shape, size distribution, and absolute concentration of the melanin granules. These parameters of the melanin granules were measured by Baumann et al 8 and Harper et al 32 previously. Although the melanin granules were filtered by vacuum filtration to ensure the size of the melanin granules was less than 5 m in our study as described in “ Methods ”, it was one of our limitations that we did not measure these parameters of the melanin granules because of limited access to electron microscopy, particle size analyzer, and freeze-drying machine.…”

“…A physical mechanism that the scattering from the melanin granule results in the spatial depolarization is still poorly understood. In the case of Mie scattering from spherical particles, it is known that the spectral response of the scattered light is modulated 31 , 32 and that the depolarization is also observed when a density of the Mie scatterers is high 8 . Melanin granules, such as the melanosome and the melanolipofuscin, are nonspherical particles whose size is from 1 to a few m 8 , 33 , 34 , which is in the order of the wavelength as well as the Mie scatterers.…”

Analysis of a SD-OCT-based hyperspectral system for spectral reflectance measurements

“…Nowadays, sOCT is nearly always based on the association of a SC light source and an Ultra-High Resolution-OCT (UHR-OCT) system. State of the art sOCT can reach spectral resolutions in the 10 nm range with a spatial resolutions below 10 µm [6,9].…”