Abstract
Laser speckle-based techniques are frequently used to assess microcirculatory blood flow. Perfusion estimates are calculated either by analyzing the speckle fluctuations over time as in laser Doppler flowmetry (LDF), or by analyzing the speckle contrast as in laser speckle contrast imaging (LSCI). The perfusion estimates depend on the amount of blood and its speed distribution. However, the perfusion estimates are commonly given in arbitrary units as they are nonlinear and depend on the magnitude and the spatial distribution of the optical properties in the tissue under investigation. We describe how the spatial confinement of blood to vessels, called the vessel packaging effect, can be modeled in LDF and LSCI, which affect the Doppler power spectra and speckle contrast, and the underlying bio-optical mechanisms for these effects. As an example, the perfusion estimate is reduced by 25% for LDF and often more than 50% for LSCI when blood is located in vessels with an average diameter of 40 μm, instead of being homogeneously distributed within the tissue. This significant effect can be compensated for only with knowledge of the average diameter of the vessels in the tissue.
Highlights
Tissue vitality is strongly dependent on a functional microcirculation
We have shown how the speckle contrast can be calculated from the Doppler power spectrum,[17] a fact that facilitates the investigation of the vessel packaging effect for laser speckle contrast imaging (LSCI)
The aim of this paper is to present a model for the vessel packaging effect in laser Doppler flowmetry (LDF) and LSCI
Summary
Tissue vitality is strongly dependent on a functional microcirculation Deficiencies in this circulation can result in ischemic conditions with an increased risk for tissue necrosis. With optical techniques, such as laser Doppler flowmetry (LDF)[1] and laser speckle contrast imaging (LSCI),[2] it is possible to objectively assess both spatial and temporal variations in the microcirculatory perfusion. Common for most of these techniques is the challenge to make quantitative estimations of the microcirculatory parameters in the presence of other confounding components in the tissue. This includes the tissue composition of scattering and absorbing compounds and the spatial distribution of tissue optical properties. Simulating photon propagation in layered tissue using the Monte Carlo technique clearly shows that a nonhomogeneous layer distribution of the optical properties affects the amount of backscattered photons.[5,6,7] when confining blood to vessels rather than assuming a homogeneous distribution, a significant effect on photon propagation and the amount of backscattered light can be noted.[7,8,9,10]
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