Layer-based, depth-resolved computation of attenuation coefficients and backscattering fractions in tissue using optical coherence tomography

Abstract

Structural optical coherence tomography (OCT) images of tissue stand to benefit from greater functionalization and quantitative interpretation. The OCT attenuation coefficient µ, an analogue of the imaged sample’s scattering coefficient, offers potential functional contrast based on the relationship of µ to sub-resolution physical properties of the sample. Attenuation coefficients are computed either by fitting a representative µ over several depth-wise pixels of a sample’s intensity decay, or by using previously-developed depth-resolved attenuation algorithms by Girard et al. [Invest. Ophthalmol. Vis. Sci. 52, 7738 (2011). 10.1167/iovs.10-6925] and Vermeer et al. [Biomed. Opt. Express 5, 322 (2014). 10.1364/BOE.5.000322]. However, the former method sacrifices axial information in the tomogram, while the latter relies on the stringent assumption that the sample’s backscattering fraction, another optical property, does not vary along depth. This assumption may be violated by layered tissues commonly observed in clinical imaging applications. Our approach preserves the full depth resolution of the attenuation map but removes its dependence on backscattering fraction by performing signal analysis inside individual discrete layers over which the scattering properties (e.g., attenuation and backscattering fraction) vary minimally. Although this approach necessitates the detection of these layers, it removes the constant-backscattering-fraction assumption that has constrained quantitative attenuation coefficient analysis in the past, and additionally yields a layer-resolved backscattering fraction, providing complementary scattering information to the attenuation coefficient. We validate our approach using automated layer detection in layered phantoms, for which the measured optical properties were in good agreement with theoretical values calculated with Mie theory, and show preliminary results in tissue alongside corresponding histological analysis. Together, accurate backscattering fraction and attenuation coefficient measurements enable the estimation of both particle density and size, which is not possible from attenuation measurements alone. We hope that this improvement to depth-resolved attenuation coefficient measurement, augmented by a layer-resolved backscattering fraction, will increase the diagnostic power of quantitative OCT imaging.

Depth-dependent attenuation and backscattering characterization of optical coherence tomography by stationary iterative method

“…Therefore, their analytical solutions cannot be acquired directly without knowledge of other characteristics. To address this issue, the recent studies often measured them individually while assuming the other variables to be constant 15 , 22 . These methods typically rely on accurate layer segmentation, which may fail to perform the task when there are gradual changes in tissue structure or extremely thin tissue layers, e.g., the retinal tissue.…”

“…However, all these computational approaches assumed a constant backscattering of the attenuated light, denoted as the backscattered fraction R(z). In theory, R(z) depends strongly on the particle size, relative refractive index, 22 and the numerical aperture (NA) of the OCT system 23 . Previous studies proved that there is a significant difference in the backscattered fraction measured from intravascular OCT images 24 .…”

“…Therefore, the assumption of fixed R(z) can result in considerable under- or overestimation of tissue attenuation. More recently, Cannon et al 22 . proposed an improved model that can measure the depth-resolved attenuation coefficients and the layer-resolved backscattering fractions simultaneously, and thus able to reduce measurement errors for heterogeneous samples.…”

Accuracy and precision of depth-resolved estimation of attenuation coefficients in optical coherence tomography

“…Thus, the adoption of μOCT allows us to describe tissue attenuation as measured by OCT as an effective parameter that does not require an estimate of the relative weight of single and multiple scattering contributions. When applying the DRE method to multiple layers with varying optical properties, the DRE method generally fails to extract the correct optical properties, unless for each layer μOCT∝μs only (e.g., no absorption) and pNA is a constant throughout the sample 21 . Whether or not these conditions are met in practice should ultimately be verified by experiments while the level of inaccuracy and imprecision may be estimated using simulations.…”

“…Fiske et al 18 showed that the coefficient of variation δ D ≪ 1 already, even without preaveraging so we neglect that term in Eq. (21). See also Appendix D for further justification.…”