Abstract
Laser speckle is a statistical phenomenon and should be treated as such when establishing experimental parameters for using laser speckle techniques to infer information about the dynamics of a coherently illuminated medium, such as biological tissue. Herein, we demonstrate that when sampling (imaging) speckle patterns, it is critical that the sampled speckle patterns are unbiased estimators of the underlying random speckle field. If this condition is not met, the quality of the results will be compromised. Specifically, this study examines the effects of first and second order spatial statistics of speckle intensity images on laser speckle contrast imaging results. Finally, it is recommended that when using speckle techniques such as laser speckle contrast imaging, investigators should examine the first and second order spatial statistics of a speckle image prior to collecting actual data. If this examination reveals that the imaged speckle intensity image is not an unbiased estimator of the underlying random speckle field, then adjustments should be made to ensure that the images taken are unbiased estimators of the true speckle field.
Highlights
Laser speckle techniques are well established for investigating biophysical dynamics, including the motion of blood cells [1,2,3] and tissue mechanics [4,5]
A brief overview is necessary as the remainder of the present paper explores the effects of inadequate spatial sampling of laser speckle patterns such that the sample statistics are not unbiased estimators of the true underlying statistics
To examine the effects of second order statistics on the probability density function (PDF) of a speckle contrast image, we first present the results of results of local neighborhood size over which speckle contrast is calculated [12]
Summary
Laser speckle techniques are well established for investigating biophysical dynamics, including the motion of blood cells [1,2,3] and tissue mechanics [4,5]. Laser speckle techniques possess numerous advantages as tools for investigating dynamic biological systems, including but not limited to good spatial resolution, high sensitivity to motions, relative simplicity, and cost effectiveness. These same features, present a weakness of the techniques. The simple act of illuminating a scattering surface or volume with coherent radiation (i.e., laser light) will result in the appearance of a speckle pattern in the observation plane. If the scattering medium is dynamic, the observed speckle pattern will appear to be temporally dynamic and estimates of this motion are made [6]
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