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Speckle-related scaling effects in laser Doppler perfusion imaging

We study the consequences of the laser speckle phenomenon for the quantitative properties of laser Doppler perfusion imaging (LDPI) systems. These consequences are suggested by our earlier presented theory that allows for the prediction of the power of the photocurrent fluctuations in laser Doppler blood flowmetry (LDF). The speckle behaviour is particularly important in LDPI since tissue is illuminated by a free laser beam. In that case, the optical properties of the tissue govern the size of the speckles, together with the beam size and shape. Hence, the type of tissue affects the instrumental response through the speckles in an independent way. In this paper, this effect will be demonstrated experimentally, using diluted suspensions of Intralipid. We demonstrate that the photocurrent fluctuations decrease with increasing beam size and decreasing scattering coefficient of the Intralipid. The dependency on scattering becomes smaller with increasing beam size. Furthermore, we show that the power spectrum, which represents the Doppler shifts, becomes more narrow when the beam size is reduced. This can be explained by the relatively smaller speckle size associated with light that a longer path through the medium.