Abstract
Laser Speckle Imaging (LSI) is fast, noninvasive technique to image particle dynamics in scattering media such as biological tissue. While LSI measurements are independent of the overall intensity of the laser source, we find that spatial variations in the laser source profile can impact measured flow rates. This occurs due to differences in average photon path length across the profile, and is of significant concern because all lasers have some degree of natural Gaussian profile in addition to artifacts potentially caused by projecting optics. Two in vivo measurement are performed to show that flow rates differ based on location with respect to the beam profile. A quantitative analysis is then done through a speckle contrast forward model generated within a coherent Spatial Frequency Domain Imaging (cSFDI) formalism. The model predicts remitted speckle contrast as a function of spatial frequency, optical properties, and scattering dynamics. Comparison with experimental speckle contrast images were done using liquid phantoms with known optical properties for three common beam shapes. cSFDI is found to accurately predict speckle contrast for all beam shapes to within 5% root mean square error. Suggestions for improving beam homogeneity are given, including a widening of the natural beam Gaussian, proper diffusing glass spreading, and flat top shaping using microlens arrays.
Highlights
Dynamic Light Scattering (DLS) is a method which utilizes the decorrelation of coherent light to estimate the motion of particles in scattering media
Our results show that the source intensity profile can have a significant effect on the speckle contrast and on the resulting speckle flow index
The close agreement between the forward model predicted by coherent Spatial Frequency Domain Imaging (cSFDI) and experimental results suggest this technique is an accurate method for predicting the effect of light structure on speckle contrast
Summary
Dynamic Light Scattering (DLS) is a method which utilizes the decorrelation of coherent light to estimate the motion of particles in scattering media. It has been shown that in highly scattering media, the electric-field temporal autocorrelation function G1(τ) = 〈E(0)E*(τ)〉 obeys a transport equation [16]. Using this model, the autocorrelation function can be calculated and compared to experimental measurements in order to determine the dynamics of the light scattering structures. The autocorrelation function can be calculated and compared to experimental measurements in order to determine the dynamics of the light scattering structures This approach has been used with simple, point like sources extensively for fiber based techniques such as DWS, in which the distance between the light source and detector is clearly defined. By taking advantage of the spatial filtering characteristics that turbid media enact onto structured light, one can model correlation transport as a function of the spatial frequencies in the structured light projected on to the sample
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