Abstract
This study characterizes the sensitivity of noninvasive measurements of cerebral blood flow (CBF) by using frequency-domain near-infrared spectroscopy (FD-NIRS) and coherent hemodynamics spectroscopy (CHS). We considered six FD-NIRS methods: single-distance intensity and phase (SDI and SDϕ), single-slope intensity and phase (SSI and SSϕ), and dual-slope intensity and phase (DSI and DSϕ). Cerebrovascular reactivity (CVR) was obtained from the relative change in measured CBF during a step hypercapnic challenge. Greater measured values of CVR are assigned to a greater sensitivity to cerebral hemodynamics. In a first experiment with eight subjects, CVR(SDϕ) was greater than CVR(SDI) (p < 0.01), whereas CVR(DSI) and CVR(DSϕ) showed no significant difference (p > 0.5). In a second experiment with four subjects, a 5 mm scattering layer was added between the optical probe and the scalp tissue to increase the extracerebral layer thickness (L (ec) ), which caused CVR(DSϕ) to become significantly greater than CVR(DSI) (p < 0.05). CVR(SS) measurements yielded similar results as CVR(DS) measurements but with a greater variability, possibly resulting from instrumental artifacts in SS measurements. Theoretical simulations with two-layered media confirmed that, if the top (extracerebral) layer is more scattering than the bottom (brain) layer, the relative values of CVR(DSI) and CVR(DSϕ) depend on L (ec) . Specifically, the sensitivity to the brain is greater for DSI than DSϕ for a thin extracerebral layer (L (ec) < 13 mm), whereas it is greater for DSϕ than DSI for a thicker extracerebral layer.