Université de Fribourg

Laser speckle analysis synchronised with cardiac cycle

Zakharov, Pavel ; Scheffold, Frank ; Weber, Bruno

In: Proceedings SPIE, 2015, vol. 9540, p. 954008–954008–9

We present an improved Laser speckle imaging approach to investigate the cerebral blood flow response following function stimulation of a single vibrissa. By synchronising speckle analysis with the cardiac cycle we are able to obtain robust averaging of the correlation signals while at the same time removing the contributions due to the pulsation of blood flow and associated tissue adaptation....

Université de Fribourg

Laser Speckle Imaging of Cerebral Blood Flow

Zakharov, Pavel ; Scheffold, Frank ; Weber, Bruno

In: Optical Imaging of Neocortical Dynamics, 2014, vol. 85, p. 255–271

Optical imaging of cerebral hemodynamics has been used extensively to investigate the complex interplay between neurons and the blood-borne supply of oxygen and energy substrates. Among several techniques, the so-called laser speckle imaging (LSI) has found widespread applications for studying (patho-)physiological aspects of neurovascular coupling. LSI is a rather simple and cost-effective...

Université de Fribourg

Numerical study of Anderson localization of terahertz waves in disordered waveguides

Lapointe, Clayton P. ; Zakharov, Pavel ; Enderli, F. ; Feurer, T. ; Skipetrov, S. E. ; Scheffold, Frank

In: EPL (Europhysics Letters), 2014, vol. 105, no. 3, p. 34002

We present a numerical study of electromagnetic wave transport in disordered quasi–one-dimensional waveguides at terahertz frequencies. Finite element method calculations of terahertz wave propagation within LiNbO3 waveguides with randomly arranged air-filled circular scatterers exhibit an onset of Anderson localization at experimentally accessible length scales. Results for the average...

Université de Fribourg

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

Peuser, Jörn ; Belhaj-Saif, Abderraouf ; Hamadjida, Adjia ; Schmidlin, Eric ; Gindrat, Anne-Dominique ; Völker, Andreas Charles ; Zakharov, Pavel ; Hoogewoud, Henri-Marcel ; Rouiller, Eric M. ; Scheffold, Frank

In: Journal of Biomedical Optics, 2011, vol. 16, no. 09, p. 096011

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...

Université de Fribourg

Monitoring spatially heterogeneous dynamics in a drying colloidal thin film

Zakharov, Pavel ; Scheffold, Frank

In: Soft Materials, 2010, vol. 8, no. 2, p. 102-113

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...

Université de Fribourg

Noise in laser speckle correlation and imaging techniques

Skipetrov, S. E. ; Peuser, Joern ; Cerbino, Roberto ; Zakharov, Pavel ; Weber, Bruno ; Scheffold, Frank

In: Optics Express, 2010, vol. 18, no. 14, p. 14519-14534

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...

Université de Fribourg

Dynamic laser speckle imaging of cerebral blood flow

Zakharov, Pavel ; Völker, A. C. ; Wyss, M. T. ; Haiss, F. ; Calcinaghi, N. ; Zunzunegui, C. ; Buck, Alfred ; Scheffold, Frank ; Weber, Bruno

In: Optics Express, 2009, vol. 17, no. 16, p. 13904-13917

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...

Université de Fribourg

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

Zakharov, Pavel

In: Physica A: Statistical Mechanics and its Applications, 2007, vol. 378, no. 2, p. 550-560

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...

Université de Fribourg

Quantitative modeling of laser speckle imaging

Zakharov, Pavel ; Völker, Andreas ; Buck, Alfred ; Weber, Bruno ; Scheffold, Frank

In: Optics Letters, 2006, vol. 31, no. 23, p. 3465-3467

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....

Université de Fribourg

Photocount statistics in mesoscopic optics

Balog, Sandor ; Zakharov, Pavel ; Scheffold, Frank ; Skipetrov, S. E.

In: Physical Review Letters, 2006, vol. 97, p. 103901

We report the first observation of the impact of mesoscopic fluctuations on the photocount statistics of coherent light scattered in a random medium. A Poisson photocount distribution of the incident light widens and gains additional asymmetry upon transmission through a suspension of small dielectric spheres. The effect is only appreciable when the average number of photocounts becomes...