Forward multiple scattering dominates speckle decorrelation in whole-blood flowmetry using optical coherence tomography

Abstract

Quantitative blood flow measurements using optical coherence tomography (OCT) have a wide potential range of medical research and clinical applications. Flowmetry based on the temporal dynamics of the OCT signal may have the ability to measure three-dimensional flow profiles regardless of the flow direction. State-of-the-art models describing the OCT signal temporal statistics are based on dynamic light scattering (DLS), a model which is inherently limited to single scattering regimes. DLS methods continue to be applied to OCT despite the knowledge that red blood cells produce strong forward multiple scattering. Here, we postulate that forward multiple scattering is the primary mechanism causing the rate of speckle-decorrelation derived from data acquired in vivo to deviate from the rate of decorrelation determined in phantom experiments. We also postulate that multiple scattering contributions to decorrelation are only present when the sample exhibits velocity field inhomogeneities larger than the scale of a resolution volume and are thus absent in rigid bulk motion. To test these hypotheses, we performed a systematic study of the effects of forward multiple scattering on OCT signal decorrelation with phantom experiments under physiologically relevant flow conditions and relative bulk motion. Our experimental results confirm that the amount of forward multiple scattering affects the proportionality between lateral flow and decorrelation. We propose that multiply scattered light carries information from different locations in the sample and each location imprints scattering dynamics on the scattered light causing increased decorrelation rates. Our analysis confirms that the detection of forward scattered light inside the vessel lumen causes an increase in the rate of decorrelation which results in an overestimation of blood flow velocities at depths as shallow as 40 µm into whole blood for OCT systems with typical numerical apertures used in retinal imaging.

Using the Dynamic Forward Scattering Signal for Optical Coherence Tomography based Blood Flow Quantification

“…2(a) that the DBS signal is affected by multiple-scattering as has been described Ref. [14]. Although we referred to these signals as DBS, they would be more accurately labelled DBS+DFS.…”

Signal-carrying speckle in optical coherence tomography: a methodological review on biomedical applications

“…Ferris et al. 49 used phantom data to study the effects of multiple scattering on the speckle decorrelation. Their conclusions confirm that speckle decorrelation is dependent on parameters such as the concentration and size of particles and velocity field inhomogeneities.…”

“…In most of the reviewed works, the authors aimed to further understand the physical meaning of the light speckle and how to model it mathematically. 3 , 16 , 17 , 21 , 39 , 40 , 49 , 53 , 55 Although its interpretability remains a challenging task, a number of works have shown the feasibility of speckle-derived quantifications for biomedical imaging related tasks, including classification (e.g., healthy/pathological), 1 , 2 , 4 , 8 , 12 – 14 , 23 – 26 , 32 – 34 , 37 , 43 – 46 segmentation (e.g., vessels), 15 motion quantification (when dynamic data is provided), 6 , 48 , 49 , 51 – 53 and image fidelity. 54 Nevertheless, some aspects and limitations of those techniques should be discussed to improve future works.…”