Frequency domain measurements of changes of optical pathlength during spreading depression in a rodent brain model

In this study, we have monitored mean photon migration pathlengths 〈L〉 in the frequency doman in the rodent brain insulted by hypoxia, ischemia and spreading depression (SD) using phase modulated spectroscopy (PMS). This technique consisted of monitoring light which emerged from the exposed rodent skull at 8 mm from an incident light source of 754 nm and 816 nm whose intensity was modulated at 220 MHz. The changes in phase-shift, θ, of the emergent light with respect to the incident light are reflective of the photon pathlengths and hemoglobin absorbance. Correlations between changes in intracellular deoxygenation (NADH) and hemoglobin deoxygenation as measured by PMS 〈L〉 changes at 754 nm and 816 nm during hypoxia, and ischemia were found. The depolarization phase of spreading depression resulted in a similar increase 〈L〉 at both 754 nm and 816 nm. We attribute this result to vasoconstriction and/or the decrease of extracellular space due to water shift in the rodent brain.