Multiple forward scattering reduces the measured scattering coefficient of blood in visible-light optical coherence tomography

Abstract

AbstractOptical properties of blood encode oxygen-dependent information. Noninvasive optical detection of these properties is increasingly desirable to extract biomarkers for tissue health. Recently, visible-light optical coherence tomography (vis-OCT) demonstrated retinal oxygen saturation (sO2) measurements using the depth-resolved spectrum of blood. Such measurements rely on differences between the absorption and scattering coefficients of oxygenated and deoxygenated blood. However, there is still broad disagreement, both theoretically and experimentally, on how vis-OCT measures blood’s scattering coefficient. Incorrect assumptions of blood’s optical properties can add additional uncertainties or biases into vis-OCT’s sO2 model. Using Monte Carlo simulation of a retinal vessel, we determined that vis-OCT almost exclusively detects multiple-scattered photons in blood. Meanwhile, photons mostly forward scatter in blood within the visible spectral range, allowing photons to maintain ballistic paths and penetrate deeply, leading to a reduction in the measured scattering coefficient. We defined a scattering scaling factor (SSF) to account for such a reduction and found that SSF varied with measurement conditions, such as numerical aperture, depth resolution, and depth selection. We further experimentally validated SSF in ex vivo blood phantoms pre-set sO2 levels and in the human retina, both of which agreed well with our simulation.