Abstract

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/√N.

© 2005 Optical Society of America

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References

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    [CrossRef] [PubMed]
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    [CrossRef] [PubMed]
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    [CrossRef] [PubMed]
  7. B. J. Berne, and R. Pecora, Dynamic Light Scattering (John Wileys and Sons, New York, 1976).
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    [CrossRef]
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    [CrossRef]
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  22. A. C. Völker, P. Zakharov, B. Weber, A. Buck, and F. Scheffold, "Dynamic contrast resolution in laser speckle imaging," in preparation (2005).
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Appl. Opt.

P. Zakharov, S. Bhat, P. Schurtenberger, and F. Scheffold, "Multiple scattering suppression in dynamic light scattering based on a digital camera detection scheme," Appl. Opt. in press, (2005).

S. Yuan, A. Devor, D. A. Boas, and A. K. Dunn, "Determination of optimal exposure time for imaging of blood flow changes with laser speckle contrast imaging," Appl. Opt. 44, 1823-1830 (2005).
[CrossRef] [PubMed]

Eur. J. Neurosci.

B. Weber, C. Burger, M. T. Wyss, G. K. von Schulthess, F. Scheffold, and A. Buck, "Optical imaging of the spatiotemporal dynamics of cerebral blood flow and oxidative metabolism in the rat barrel cortex," Eur. J. Neurosci. 20, 2664-2670 (2004).
[CrossRef] [PubMed]

J Exp. Theor. Phys.

S. E. Skipetrov, and I. V. Meglinskii, "Diffusing-wave spectroscopy in randomly inhomogeneous media with spatially localized scatterer flows," J Exp. Theor. Phys. 86, 661-665 (1998).
[CrossRef]

J. Biomed. Opt.

H. Cheng, et. al., "Modified laser speckle imaging method with improved spatial resolution," J. Biomed. Opt. 8, 559-564 (2003).
[CrossRef] [PubMed]

J. D. Briers, and S. Webster, "Laser speckle contrast analysis (LASCA): A nonscanning, full-field technique for monitoring capillary blood flow," J. Biomed. Opt. 1, 174-179 (1996).
[CrossRef]

J. Cereb. Blood Flow Metab.

A. K. Dunn, B. Hayrunnisa, M. A. Moskowitz, and D. A. Boas, "Dynamic imaging of cerebral blood flow using laser speckle," J. Cereb. Blood Flow Metab. 21, 195-201 (2001).
[CrossRef] [PubMed]

J. Opt. Soc. Am. A

J. Phys. I

E. Jakeman, and E. R. Pike, "Intensity-fluctuation distribution of Gaussian Light," J. Phys. I 1, 128-138 (1968).

Phys. Rev. B

F. C. Mackintosh, J. X. Zhu, D. J. Pine, and D. A. Weitz, "Polarization memory of multiply scattered-light," Phys. Rev. B 40, 9342-9345 (1989).
[CrossRef]

Phys. Rev. E

F. Scheffold, S. E. Skipetrov, S. Romer, and P. Schurtenberger, "Diffusing-wave spectroscopy of nonergodic media," Phys. Rev. E 6306, art. no. 061404 (2001).

Phys. Rev. Lett.

D. J. Pine, D. A. Weitz, P. M. Chaikin, and E. Herbolzheimer, "Diffusing-wave spectroscopy," Phys. Rev. Lett. 60, 1134-1137 (1988).
[CrossRef] [PubMed]

Physica A

P. N. Pusey, and W. Vanmegen, "Dynamic light-scattering by Non-Ergodic Media," Physica A 157, 705(1989).
[CrossRef]

Physiological Measurement

J. D. Briers, "Laser Doppler, speckle and related techniques for blood perfusion mapping and imaging," Physiological Measurement 22, R35-R66 (2001).
[CrossRef]

Rev. Sci. Instrum.

R. Bandyopadhyay, A. S. Gittings, S. S. Suh, P.K. Dixon, and D. J. Durian, "Speckle-visibility spectroscopy: A tool to study time-varying dynamics," Rev. Sci. Instrum. 76, art. no. 093110 (2005).
[CrossRef]

L. Cipelletti, and D. A. Weitz, "Ultralow-angle dynamic light scattering with a charge coupled device camera based multispeckle, multitau correlator," Rev. Sci. Instrum. 70, 3214-3221 (1999).
[CrossRef]

Stroke

C. Ayata, Y. Ozdemir, A. Dunn, D. N. Atochin, P. L. Huang, V. R. Muzykantov, J. C. Murciano, D. A. Boas, and M. A. Moskowitz, "Laser speckle-flowmetry: A novel two-dimensional technique for the study of cerebral blood flow in normal and ischemic mouse brain, in vivo," Stroke 34, 251-251 (2003).

Other

J. C. Dainty, Laser speckle and related phenomena (Springer-Verlag, Berlin ; New York, 1984).

B. J. Berne, and R. Pecora, Dynamic Light Scattering (John Wileys and Sons, New York, 1976).

A. C. Völker, P. Zakharov, B. Weber, A. Buck, and F. Scheffold, "Dynamic contrast resolution in laser speckle imaging," in preparation (2005).

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Figures (4)

Fig. 1.
Fig. 1.

Experimental setup: A laser beam (785 nm) is incident on ground glass mounted on a motor (rotation velocity one rph). Light passing the ground glass is moderately divergent and illuminates the sample surface as an expanded light spot. The illuminated surface is imaged via a beam-splitter by the camera objective onto the CCD chip of the digital camera. Right: Heterogeneous sample obtained by milling a cylindrical void (diameter D0 = 3 mm) into a solid block of Teflon. The inclusion is filled with an aqueous suspension of 710 nm polystyrene latex spheres that matches the optical properties (l*=250±30 μm) and creates dynamic contrast. A layer of thickness d = 0.45 mm separates the inclusion from the imaging surface.

Fig. 2.
Fig. 2.

Characteristics of the laser speckle imaging setup obtained from a rigid sample (Teflon block) for different LASCA square sizes. a) mean speckle contrast K as a function of the speckle size (in units of the pixel size rP ). b) Standard deviation of the contrast σK as a function of rS . To achieve approximately the same intensity for all apertures the exposure time was varied between 200 and 750 ms.

Fig. 3.
Fig. 3.

Three dimensional plot of a laser speckle contrast image of a liquid inclusion in a solid block of Teflon (scale inverted). a) LASCA image using 8×8 square size, image resolution 80×60 pixel averaged over 500 individual measurement. b) Same sample using the active noise reduction. c) Full resolutions 640×480 pixel with active noise reduction scheme.

Fig. 4.
Fig. 4.

Left: Noise in speckle contrast, expressed by the standard deviation sk, as a function of the number of frames taken, different square sizes are displayed (where #/pixel is the number of pixels in the corresponding square size). b) σK scaling as a function of “effective pixels” N. It is shown that spatial and temporal averaging give identical results.

Equations (3)

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K = β I 1 [ i = 1 N ( I i I ) 2 N 1 ] 1 2 .
K = β 0 T 2 ( 1 t T ) C ( τ ) T = β e 2 x 1 + 2 x 2 x 2 with x = T τ c .
r s = 2 l k 0 q = 4 × f # × l k 0 f .

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