Abstract

In contrast to photon-migration imaging which is playing an increasingly important role as an imaging technique for neurophysiology, experiments using diffusing-wave spectroscopy (DWS) [1, 2] for brain imaging have been scarce: aiming at mapping blood flow velocities in cortical vessels, Lohwasser et al. have tried to account for the effects of multiple light scattering in laser Doppler spectra from brain phantoms [3]. Using a CCD camera, Dunn et al. [4] mapped the cerebral blood flow with spatially resolved measurements of the speckle contrast from an exposed rat cortex. A recent experiment by Cheung et al. [5] on a semi-exposed mouse brain revealed that the field autocorrelation function shows a faster decay with a complex shape upon stimulation by increased CO₂ partial pressure. This suggests that, using DWS to investigate functional brain activity in an entirely non-invasive way is likely to be complicated by the heterogenous optical and dynamical properties of the head consisting of scalp, skull, cerebrospinal fluid and white and gray matter. The scattering of light from subcellular organelles in cortical tissue should give rise to speckle fluctuations that are largely unrelated to blood flow. From isolated neural tissue it is known that neurosecretory release enhances the mobility of neurosynaptic vesicles [6–8], which might be detectable by DWS from cortical tissue entirely non-invasively.

© 2004 Optical Society of America