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This Spotlight reviews non-invasive optical methods to determine the blood flow velocity in microcirculation, with an emphasis on the most recent developments, that may be routinely applied in cellular biology laboratories. The methods covered in this book include both scanning and whole-field optical imaging techniques based on endogenous or exogenous (fluorescent) contrast agents for plasma and red-blood-cell visualization, such as laser Doppler velocimetry, laser speckle contrast imaging, diffuse correlation spectroscopy, particle image velocimetry, and two-photon microscopy. Several methods have been developed to quantify the blood flow velocity within microvessels by exploiting the cross-correlation concept applied to optical scanning microscopes: dual-foci cross-correlation, spatio-temporal image correlation spectroscopy, line-scanning techniques, and a recently developed single raster-scanned xy-image technique (FLICS). The basic operational principles, the advantages and disadvantages, and the technical issues of these methods will be discussed and compared, and their applications in biomedical research will be presented.
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