Pavel Zakharov

Dynamic laser speckle imaging of cerebral blood flow

Laser speckle imaging (LSI) based on the speckle contrast analysis is a simple and robust technique for imaging of heterogeneous dynamics. LSI finds frequent application for dynamical mapping of cerebral blood flow, as it features high spatial and temporal resolution. However, the quantitative interpretation of the acquired data is not straightforward for the common case of a speckle field formed by both by moving and localized scatterers such as blood cells and bone or tissue. Here we present a novel processing scheme, we call dynamic laser speckle imaging (dLSI), that can be used to correctly extract the temporal correlation parameters from the speckle contrast measured in the presence of a static or slow-evolving background. The static light contribution is derived from the measurements by cross-correlating sequential speckle images. In-vivo speckle imaging experiments performed in the rodent brain demonstrate that dLSI leads to improved results. The cerebral hemodynamic response observed through the thinned and intact skull are more pronounced in the dLSI images as compared to the standard speckle contrast analysis. The proposed method also yields benefits with respect to the quality of the speckle images by suppressing contributions of non-uniformly distributed specular reflections.

Quantitative modeling of laser speckle imaging

We have analyzed the image formation and dynamic properties in laser speckle imaging (LSI) both experimentally and with Monte Carlo simulation. We show for the case of a liquid inclusion that the spatial resolution and the signal itself are both significantly affected by scattering from the turbid environment. Multiple scattering leads to blurring of the dynamic inhomogeneity as detected by LSI. The presence of a nonfluctuating component of scattered light results in the significant increase in the measured image contrast and complicates the estimation of the relaxation time. We present a refined processing scheme that allows a correct estimation of the relaxation time from LSI data.

Laser speckle imaging with an active noise reduction scheme

We present an optical scheme to actively suppress statistical noise in Laser Speckle Imaging (LSI). This is achieved by illuminating the object surface through a diffuser. Slow rotation of the diffuser leads to statistically independent surface speckles on time scales that can be selected by the rotation speed. Active suppression of statistical noise is achieved by accumulating data over time. We present experimental data on speckle contrast and noise for a dynamically homogenous and a heterogeneous object made from Teflon. We show experimentally that for our scheme spatial and temporal averaging provide the same statistical weight to reduce the noise in LSI: The standard deviation of the speckle contrast value scales with the effective number N of independent speckle as 1/ radicalN.

Multispeckle diffusing-wave spectroscopy with a single-mode detection scheme

We present a detection scheme for diffusing-wave spectroscopy (DWS) based on a two-cell geometry that allows efficient ensemble averaging. This is achieved by putting a fast rotating diffuser in the optical path between laser and sample. We show that the recorded (multispeckle) correlation echoes provide an ensemble averaged signal that does not require additional time averaging. Furthermore, combined with traditional two-cell DWS, the full intensity autocorrelation function can be measured with a single experimental setup. The scheme provides access to a large range of correlation times thus opening an experimental window for the study of slowly relaxing and arrested systems, such as viscoelastic complex fluids, colloidal glasses, and gels.

Noise in laser speckle correlation and imaging techniques

We study the noise of the intensity variance and of the intensity correlation and structure functions measured in light scattering from a random medium in the case when these quantities are obtained by averaging over a finite number N of pixels of a digital camera. We show that the noise scales as 1/N in all cases and that it is sensitive to correlations of signals corresponding to adjacent pixels as well as to the effective time averaging (due to the finite integration time) and spatial averaging (due to the finite pixel size). Our results provide a guide to estimation of noise levels in such applications as multi-speckle dynamic light scattering, time-resolved correlation spectroscopy, speckle visibility spectroscopy, laser speckle imaging etc.

Multiple-scattering suppression in dynamic light scattering based on a digital camera detection scheme

We introduce a charge-coupled device (CCD) camera-based detection scheme in dynamic light scattering that provides information on the single-scattered autocorrelation function even for fairly turbid samples. It is based on the single focused laser beam geometry combined with the selective cross-correlation analysis of the scattered light intensity. Using a CCD camera as a multispeckle detector, we show how spatial correlations in the intensity pattern can be linked to single- and multiple-scattering processes. Multiple-scattering suppression is then achieved by an efficient cross-correlation algorithm working in real time with a temporal resolution down to 0.02 s. Our approach allows access to the extensive range of systems that show low-order scattering by selective detection of the singly scattered light. Model experiments on slowly relaxing suspensions of titanium dioxide in glycerol were carried out to establish the validity range of our approach. Successful application of the method is demonstrated up to a scattering coefficient of more than micro(S) = 5 cm(-1) for the sample size of L = 1 cm.

Monitoring spatially heterogeneous dynamics in a drying colloidal thin film

We report on a new type of experiment that enables us to monitor spatially and temporally heterogeneous dynamic properties in complex fluids. Our approach is based on the analysis of near-field speckles produced by light diffusely reflected from the superficial volume of a strongly scattering medium. By periodic modulation of an incident speckle beam we obtain pixel-wise ensemble averages of the structure function coefficient, a measure of the dynamic activity. To illustrate the application of our approach we follow the different stages in the drying process of a colloidal thin film. We show that we can access ensemble averaged dynamic properties on length scales as small as ten micrometers over the full field of view.

Advances in dynamic light scattering techniques

Recent developments in processing techniques and detection hardware have opened new horizons for the application of light scattering methods based on the dynamic analysis of coherent scattered light. The increased computational power of modern microprocessors allows real-time data evaluation on standard desktop computers. The continuous improvement of detector arrays, such as cameras based on CCD or CMOS technologies, facilitate space-resolved detection of scattering intensities, which can be used to boost the statistical weight accumulated in a single experiment. New methods and improved accuracy on the other hand also provide answers to questions concerning the quantitative data interpretation which were only partially addressed in some of the earlier work.

Diffusion approach for community discovering within the complex networks: LiveJournal study

The diffusion approach of concentration mapping is used to discover communities in the directional friendship network of LiveJournal users. We show that this Internet-based social network has a power-law region in degree distribution with exponent γ=3.45. It is also a small-world network with high clustering of nodes. To study the community structure we simulate diffusion of a virtual substance immersed in such a network as in a multi-dimensional porous system. By analyzing concentration profiles at intermediate stage of the diffusion process the well-interconnected cliques of users can be identified as nodes with equal values of concentration.

Follow-up of cortical activity and structure after lesion with laser speckle imaging and magnetic resonance imaging in nonhuman primates

The nonhuman primate model is suitable to study mechanisms of functional recovery following lesion of the cerebral cortex (motor cortex), on which therapeutic strategies can be tested. To interpret behavioral data (time course and extent of functional recovery), it is crucial to monitor the properties of the experimental cortical lesion, induced by infusion of the excitotoxin ibotenic acid. In two adult macaque monkeys, ibotenic acid infusions produced a restricted, permanent lesion of the motor cortex. In one monkey, the lesion was monitored over 3.5 weeks, combining laser speckle imaging (LSI) as metabolic readout (cerebral blood flow) and anatomical assessment with magnetic resonance imaging (T2-weighted MRI). The cerebral blood flow, measured online during subsequent injections of the ibotenic acid in the motor cortex, exhibited a dramatic increase, still present after one week, in parallel to a MRI hypersignal. After 3.5 weeks, the cerebral blood flow was strongly reduced (below reference level) and the hypersignal disappeared from the MRI scan, although the lesion was permanent as histologically assessed post-mortem. The MRI data were similar in the second monkey. Our experiments suggest that LSI and MRI, although they reflect different features, vary in parallel during a few weeks following an excitotoxic cortical lesion.