Abstract
Laser speckle flowmetry (LSF) was initially developed to measure blood flow in the retina. More recently, its primary application has been to image baseline blood flow and activity-dependent changes in blood flow in the brain. We now describe experiments in the rat retina in which LSF was used in conjunction with confocal microscopy to monitor light-evoked changes in blood flow in retinal vessels. This dual imaging technique permitted us to stimulate retinal photoreceptors and measure vessel diameter with confocal microscopy while simultaneously monitoring blood flow with LSF. We found that a flickering light dilated retinal arterioles and evoked increases in retinal blood velocity with similar time courses. In addition, focal light stimulation evoked local increases in blood velocity. The spatial distribution of these increases depended on the location of the stimulus relative to retinal arterioles and venules. The results suggest that capillaries are largely unresponsive to local neuronal activity and that hemodynamic responses are mediated primarily by arterioles. The use of LSF to image retinal blood flow holds promise in elucidating the mechanisms mediating functional hyperemia in the retina and in characterizing changes in blood flow that occur during retinal pathology.
Highlights
Laser speckle flowmetry (LSF) has been successfully employed to measure blood flow in the CNS for three decades
Arterioles and venules, which radiate from the optic disc, can be visualized simultaneously by confocal microscopy and LSF
Using this combined imaging system, blood velocity changes were measured with LSF and arteriole diameter monitored with confocal line scans while retinal photoreceptors were stimulated by modulating the laser light from the secondary scanner of the confocal microscope
Summary
Laser speckle flowmetry (LSF) has been successfully employed to measure blood flow in the CNS for three decades. It has the distinct advantage over other methods of monitoring blood flow, such as laser Doppler flowmetry, as it can produce two-dimensional images of blood flow with high spatial and temporal resolution. If the surface is stationary, the speckle pattern of bright and dark spots will remain stationary as well. If the surface moves (e.g., blood cells moving through vessels), the speckle pattern will blur with time. Local speckle contrast is calculated by imaging a surface with a digital camera and computing the standard deviation of brightness divided by the mean brightness over a small array of pixels. Speckle contrast can be calculated and speckle images analyzed almost instantaneously
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