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Recent success in reconciling laser Doppler and speckle measurements of dermal perfusion by the use of multi-exposure
speckle has prompted an investigation of speckle effects arising from directed blood flow which might be expected in the
small blood vessels of the eye.
Unlike dermal scatter, the blood in retinal vessels is surrounded by few small and stationary scatterers able to assist the
return of light energy by large-angle scatter. Returning light is expected to come from multiple small angle scatter from
the large red blood cells which dominate the fluid.
This work compares speckle measurements on highly scattering skin, with measurements on flow in a retinal phantom
consisting of a glass capillary which is itself immersed in an index matching fluid to provide a flat air-phantom interface.
Brownian motion dominated measurements when small easily levitated scatters were used, and flow was undetectable.
With whole-blood, Brownian motion was small and directed flows in the expected region of tens of mm/s were
detectable. The nominal flow speed relates to the known pump rate; within the capillary the flow will have a profile
reducing toward the walls.
The pulsatile effects on laser speckle contrast in the retina are discussed with preliminary multi-exposure measurements
on retinal vessels using a fundus camera. Differences between the multiple exposure curves and power spectra of
perfused tissue and ordered flow are discussed.
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Evan R. Hirst, Oliver B. Thompson, Michael K. Andrews, "Measurement of directed blood flow by laser speckle," Proc. SPIE 7898, Dynamics and Fluctuations in Biomedical Photonics VIII, 78980D (10 February 2011); https://doi.org/10.1117/12.873664