Separation of changes in light scattering and chromophore concentrations during cortical spreading depression in rats

We used near-infrared spectroscopy to separate tissue scattering changes from changes in cerebral oxyhemoglobin and deoxyhemoglobin and the redox state of cytochrome-$c$-oxidase. A separate term of the transport scattering coefficient $\left({{\mu }_s}^{\prime }\right)$ was included in a modified Lambert–Beer equation. It is shown by diffusion equation analysis that there is a simple relationship between the differential path-length factor $\left(D_a\right)$ and its scattering equivalent $\left(D_s\right)$. The method was applied to cortical spreading depression (CSD) data recorded through the skulls of rats. Biphasic changes in ${{\mu }_s}^{\prime }$ of $\pm 0.1 {\mathrm{mm}}^{-1}$ were observed during CSD's that spread with a velocity of $\sim 5 \mathrm{mm}/min$. The method proposed has the promise to permit monitoring of scattering changes noninvasively in humans during cortical activation or pathophysiological conditions.