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Ruikang K Wang, Mitchell Kirby, Chenxi Li, Woo June Choi, Giovanni Gregori, Philip J Rosenfeld; An explanation for why choroidal blood vessels appear dark on clinical OCT images. Invest. Ophthalmol. Vis. Sci. 2017;58(8):4754.
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© ARVO (1962-2015); The Authors (2016-present)
To determine the physical reasons why choroidal blood vessels (CBV) appear relatively dark (void of signal) on clinical OCT images of normal eyes while they appear relatively bright (stronger signal) in eyes with geographic atrophy (GA) secondary to AMD.
According to the Raleigh and Mie theory of light-particle interaction, we developed a simple model to simulate light scattering and propagation within the RPE, choroid, sclera, and red blood cells (RBCs). Retinal ultrastructure from histology was used to estimate the effective sizes of scattering particles that composed each tissue complex. The parameters considered included particle size, scattering (us) and absorption co-efficient, and the anisotropic factor (g, tendency of light to go forward or backward when interacting with a particle). The larger the g value, the lower the probability for light to be backscattered by a particle, thus making it more difficult for OCT to detect. To confirm simulation results, we fabricated a three-layer scattering phantom to simulate the RPE, choroid, and sclera tissue complexes. Two flow tubes of a 50um diameter, one flowing with fresh blood and another with intralipid, were situated in the middle layer to simulate CBVs.
At the 850 nm OCT wavelength, it was found that the densely populated melanin (average effective size of ~0.2um) within the RPE complex would be strongly scattering particles with us ~150/mm and g value between 0.75 and 0.85. The tissue components within the stroma and sclera with an effective particle size of ~2um were relatively less scattering with us ~53/mm and g ~0.87. However, the RBCs, with an average size of ~ 7um, scatter the light at g = 0.99 with us ~75/mm. These parameters explain the low likelihood that photons would be backscattered from RBCs within the CBV lumens and detected by OCT. Microfluidic phantom experiments confirmed the theoretical predictions (See Figure).
The melanin particles located within the RPE are mainly responsible for the dark appearance of the CBV lumens on clinical OCT images. A progressive loss of RPE tissue components increases the OCT detection probability of photons being backscattered from the RBCs within the CBVs, thus making the inner portion of the choroidal vessels appear brighter in patients with GA. However, the outer portions of the larger vessels continue to appear dark due to high scattering and absorption of RBCs.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
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