The Effect of Retinal Melanin on Optical Coherence Tomography Images

Abstract

PurposeWe assessed the effect of melanin on the appearance of hyperreflective outer retinal bands in optical coherence tomography (OCT) images.MethodsA total of 23 normal subjects and 51 patients with albinism were imaged using the Bioptigen high-resolution spectral-domain OCT. In addition, three wild type, three albino (slc45a2b4/b4), and eight tyrosinase mosaic zebrafish were imaged with the hand-held Bioptigen Envisu R2200 OCT. To identify pigmented versus nonpigmented regions in the tyrosinase mosaic zebrafish, en face summed volume projections of the retinal pigment epithelium (RPE) were created from volume scans. Longitudinal reflectivity profiles were generated from B-scans to assess the width and maximum intensity of the RPE band in fish, or the presence of one or two RPE/Bruch's membrane (BrM) bands in humans.ResultsThe foveal RPE/BrM appeared as two bands in 71% of locations in patients with albinism and 45% of locations in normal subjects (P = 0.0003). Pigmented zebrafish retinas had significantly greater RPE reflectance, and pigmented regions of mosaic zebrafish also had significantly broader RPE bands than all other groups.ConclusionsThe hyperreflective outer retinal bands in OCT images are highly variable in appearance. We showed that melanin is a major contributor to the intensity and width of the RPE band on OCT. One should use caution in extrapolating findings from OCT images of one or even a few individuals to define the absolute anatomic correlates of the hyperreflective outer retinal bands in OCT images.Translational RelevanceMelanin affects the appearance of the outer retinal bands in OCT images. Use of animal models may help dissect the anatomic correlates of the complex reflective signals in OCT retinal images.

Directional optical coherence tomography reveals melanin concentration-dependent scattering properties of retinal pigment epithelium

“…The dominant scatterers in the pigmented RPE have been identified as the melanin granules (melanosomes). 18,43 The scattering nature of a melanin granule depends on its shape and effective size relative to the wavelength of the light used for imaging. The average size of the melanin granules varies from 600 nm to 1 μm in diameter, resulting in a size (diffraction) parameter x ¼ 2πr∕λ in the four to six range (for λ ¼ 1.05 μm), clearly placing melanosome scattering in the Mie scattering regime range.…”

“…Owing to high melanin scattering, the RPE appears as a thick bright band that can obscure adjacent layers. 18 Thus, RPE melanin scattering limits the visibility of Bruch's membrane (BrM) and rod outer segment tips, as well as cone outer segment tips in the human fovea, in OCT images. The relative levels of melanin in different individuals could be potentially derived from the changes in the reflectivity of the RPE layer or from the visibility of adjacent layers.…”

“…However, the accuracy of this approach is often limited by several factors, including system axial and lateral resolution and position of the focal plane, OCT sensitivity roll-off, acquisition settings, and intersubject variability. 18 The ability of RPE melanin to depolarize the backscattered light enables polarization-sensitive OCT (PSOCT) to segment RPE layer and study melanin-related changes in RPE. 19,20 In addition to the investigation in animal models, this feature of PSOCT has also been also used in clinical studies of agerelated macular degeneration, RPE detachment, and pseudovitelliform dystrophies.…”

Imaging Melanin Distribution in the Zebrafish Retina Using Photothermal Optical Coherence Tomography

“… 6 , 8 Melanin is a major contributor to the appearance of the OCT RPE band and by extension the surrounding bands corresponding to the photoreceptors and Bruch's membrane. 9 , 10 An independent, validated method to image the melanin would allow researchers to quantify changes in pigmentation in the RPE and better understand how melanin affects OCT images, which could lead to a better understanding of diseases for researchers and better diagnosis and monitoring tools for clinicians.…”

Interspecies Variation of Outer Retina and Choriocapillaris Imaged With Optical Coherence Tomography

“…Melanin granules (melanosomes) were found to be a primary source of reflectivity on OCT 76 and presence of melanin affects ORB appearance and reflectivity. 77 All OCT B-scans evaluated in this study were acquired with a Spectralis HRAþOCT system, which is important to note because the design and processing algorithms of the OCT system have a large impact on final OCT B-scan quality. 78 The current study's results and conclusions are thus valid only for data generated with a Spectralis HRAþOCT system, which is still useful on a broad scale because the Spectralis HRAþOCT system is one of the most widely used OCT units on the market.…”