Theory Of Diffusing-wave Spectroscopy Research Articles

Theoretical model of blood flow measurement by diffuse correlation spectroscopy.

Diffuse correlation spectroscopy (DCS) is a noninvasive method to quantify tissue perfusion from measurements of the intensity temporal autocorrelation function of diffusely scattered light. However, DCS autocorrelation function measurements in tissue better match theoretical predictions based on the diffusive motion of the scatterers than those based on a model where the advective nature of blood flow dominates the stochastic properties of the scattered light. We have recently shown using Monte Carlo (MC) simulations and assuming a simplistic vascular geometry and laminar flow profile that the diffusive nature of the DCS autocorrelation function decay is likely a result of the shear-induced diffusion of the red blood cells. Here, we provide theoretical derivations supporting and generalizing the previous MC results. Based on the theory of diffusing-wave spectroscopy, we derive an expression for the autocorrelation function along the photon path through a vessel that takes into account both diffusive and advective scatterer motion, and we provide the solution for the DCS autocorrelation function in a semi-infinite geometry. We also derive the correlation diffusion and correlation transfer equation, which can be applied for an arbitrary sample geometry. Further, we propose a method to take into account realistic vascular morphology and flow profile.

Analyzing the relationship between decorrelation time and tissue thickness in acute rat brain slices using multispeckle diffusing wave spectroscopy.

Novel techniques in the field of wavefront shaping have enabled light to be focused deep inside or through scattering media such as biological tissue. However, most of these demonstrations have been limited to thin, static samples since these techniques are very sensitive to changes in the arrangement of the scatterers within. As the samples of interest get thicker, the influence of the dynamic nature of the sample becomes even more pronounced and the window of time in which the wavefront solutions remain valid shrinks further. In this paper, we examine the time scales upon which this decorrelation happens in acute rat brain slices via multispeckle diffusing wave spectroscopy and investigate the relationship between this decorrelation time and the thickness of the sample using diffusing wave spectroscopy theory and Monte Carlo photon transport simulation.

On line diffusing wave spectroscopy during rheological measurements: A new instrumental setup to measure colloidal instability and structure formation in situ

A novel setup was employed to investigate the destabilization of colloids, using a rheometer in line with diffusing wave spectroscopy (DWS) in backscattering mode. The newly designed system consists of a concentric cylinder couette geometry fitted with a window to perform simultaneous light scattering measurements. To test the ability of this Rheo-DWS setup to follow the dynamics of destabilization of model systems such as skim milk and emulsions, parallel experiments were also carried out using a separate transmission DWS equipment. For skim milk, containing 10% of volume of protein particles (casein micelles) of about 200nm in diameter, the physical constraints imposed by the rheological setup did not allow calculation of the correct value of the diffusion coefficient, as the diffusion approximation could not be fulfilled. On the other hand, for concentrated milk (4× the original volume fraction) and for oil in water emulsions, the characteristic decay constant, τ, yielded correct size values calculated using known DWS theory. The ability to follow the dynamics of early gelation using Rheo-DWS was tested by measuring the changes occurring during gelation of skim milk, concentrated milk and emulsions. There were no statistically significant differences between the gelation times measured in situ using backscattering DWS and transmission DWS. The data obtained by light scattering were in full agreement with rheology experiments. The results demonstrate the potential of such setup to follow the dynamics of colloidal destabilization and structure formation in situ while subjecting them to simultaneous rheological measurements.