In vivo estimation of light scattering and absorption properties of rat brain using a single-reflectance fiber probe during cortical spreading depression.

Diffuse reflectance spectroscopy using a fiber optic probe is a promising technique for evaluating the optical properties of biological tissue. We herein present a method for determining the reduced scattering coefficient, μ's, the absorption coefficient, μa, and the tissue oxygen saturation, StO2, of in vivo brain tissue using a single-reflectance fiber probe with two source-collector geometries. We performed in vivo recordings of diffuse reflectance spectra and of the electrophysiological signals for exposed rat brain during the cortical spreading depression evoked by the topical application of KCl. The time courses of μa at 500, 570, and 584 nm indicated the hemodynamic change in the cerebral cortex as well as StO2. At 570 nm, the time course of μ's was well correlated with that of μa, which also reflects the scattering by RBCs. On the other hand, increases in μ's at 500 and 584 nm and a decrease in μ's at 800 nm were observed before the profound increase in μa, and these occurrences were synchronized with the negative dc shift of the local field potential. The resultant change in the slope of μ's ðλÞ is indicative of the morphological changes in the cellular and subcellular structures induced by the depolarization due to the temporal depression of the neuronal bioelectrical activity. The results of the present study indicate the potential application of the proposed method in evaluating the pathophysiological conditions of in vivo brain.