<title>Capillary-blood-flow monitoring using laser speckle contrast analysis (LASCA): improving the dynamic range</title>

Abstract

We have developed a technique, called Laser Speckle Contrast Analysis (LASCA), to monitor capillary blood flow. Like the earlier method 'single-exposure speckle photography' it uses the phenomenon of time-varying speckle, drawing on models that relate the statistics ofthe speckle pattern, especially the contrast, to the velocity ofthe scatterers. Temporal variations in the pattern blur the captured image, whether photographic or digital, reducing the contrast. LASCA is a fully-digitized, fullfield, non-invasive technique and operates in quasi-real-time: it grabs an image and then produces a 2-dimensional map of contrast variations representing differing velocities. Previously the data had been stretched, as a full range of contrasts could not be achieved. This problem has been addressed and we believe solved, thus giving us a full range of contrasts. In this paper we look at the effects ofchanging the exposure time. If it is varied it should be possible, according to the models used, to look at different ranges of velocity. Also we hope to show that by using different wavelengths together it is possible to distinguish between flow at different depths in the skin. We illustrate the validity of our technique with some of our results and then compare these to other methods ofmeasuring capillary blood flow.

Coherent Optical Techniques for Diagnostics of Retinal Blood Flow

“…The technique ingeniously utilizes the two-dimensionality of the speckle pattern and, at this point, it is of considerable use for future applications in comparison with point measurements by the laser Doppler or biospeckle technique. The photographic process is certainly disadvantageous for practical uses, but Briers and his co-workers 71,72 have recently developed a fully digitized imaging/processing technique called ''laser speckle contrast analysis (LASCA),'' instead of using photography. This approach may be one of the promising steps in the development of this technique.…”

High speed perfusion image based on laser speckle fluctuations

“…Richards and Briers [45] showed that contrast images obtained using LASCA give a good picture of the movement of red blood cells in the hand of a volunteer. Cheng and Duong [56] and Konishi et al [34] even used LSI and LSFG respectively to map the ocular blood flow in the retina.…”

“…Richards and Briers [44,45] suggested this was due to an offset in the pixel values termed pedestal and introduced by the CCD camera. Manually removing this offset resulted in an increase in contrast from 0.41 to 0.95 for a static speckle pattern.…”

“…Besides improving the computer hardware and software, the development of the LASCA technique continued. An improved version was described by Richards and Briers who implemented a camera with a variable exposure time and ran trials with lasers in the green wavelength range instead of in the red [21,44,45]. Several researchers used a slightly different LASCA-setup.…”

“…For example, in one setup, the backscattered light was collected on the camera without making use of a lens but by making use of a single-mode fiber [47] or adjustable iris and camera in the diffraction plane [48]. Furthermore a polarizer was positioned in between sample and camera to select the linearly polarized light [49] to increase the contrast of the grabbed speckle pattern [21,44,45,47].…”