Sensitivity of frequency-domain optical measurements to brain hemodynamics: simulations and human study of cerebral blood flow during hypercapnia

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, CVRSDϕ was greater than CVRSDI (p < 0.01), whereas CVRDSI and CVRDSϕ 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 CVRDSϕ to become significantly greater than CVRDSI (p < 0.05). CVRSS measurements yielded similar results as CVRDS 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 CVRDSI and CVRDSϕ 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.

Noninvasive Optical Measurements of Dynamic Cerebral Autoregulation by Inducing Oscillatory Cerebral Hemodynamics

“…In the experiment on healthy subjects, the subject breathed through a facemask (AFT25, BIOPAC Systems, Inc., Goleta, CA) medical air (21% O 2 , 79% N 2 ) for 7 min (normocapnic baseline), 5% CO 2 mixed with 21% O 2 and balanced N 2 (5% CO 2 , 21% O 2 , 74% N 2 ) for 3 min (hypercapnic interval), and then medical air for 5 min of recovery. The setup for the normocapnia-hypercapnia experiment on healthy subjects was described elsewhere in detail ( 43 ). An end-tidal carbon dioxide pressure (PETCO2) signal was collected by using an infrared-based CO 2 monitor module (CO2100C, BIOPAC Systems, Inc., Goleta, CA) connected to the facemask.…”

“…SDI and SDϕ measurements refer to the I and ϕ data, respectively, collected by a single source-detector pair. SSI and SSϕ refer to the linear dependence of an I-based function ( ) and ϕ on source-detector distance ρ, respectively ( 43 ). Δμ a from SDI and SDϕ were found by using the differential pathlength factors (DPF: DPF I for I and DPF ϕ for ϕ), and from SSI and SSϕ by using the differential slope factors (DSF: DSF I for I and DSF ϕ for ϕ).…”

“…]) and φ on source-detector distance ρ, respectively (43). µ a from SDI and SDφ were found by using the differential pathlength factors (DPF: DPF I for I and DPF φ for φ), and from SSI and SSφ by using the differential slope factors (DSF: DSF I for I and DSF φ for φ).…”

Data_Sheet_1.PDF

“…This may tell us that this measurement is sensitive to different dynamics of blood volume compared to other methods. Finally, a greater sensitivity to deeper tissue (compared to superficial tissue) has been demonstrated for the slope measurements, especially for DSφ, in a homogeneous tissue setting (18,38), but the situation can be more complicated in the presence of tissue heterogeneity and different scalp/skull anatomy of the subjects (43). We also stress that the SSφ data was not reported in both healthy subjects and NCCU patients due to poor signal-to-noise ratio of the phase measurements, particularly in the SS configuration.…”

“…In the experiment on healthy subjects, the subject breathed through a facemask (AFT25, BIOPAC Systems, Inc., Goleta, CA) medical air (21% O 2 , 79% N 2 ) for 7 min (normocapnic baseline), 5% CO 2 mixed with 21% O 2 and balanced N 2 (5% CO 2 , 21% O 2 , 74% N 2 ) for 3 min (hypercapnic interval), and then medical air for 5 min of recovery. The setup for the normocapniahypercapnia experiment on healthy subjects was described elsewhere in detail (43). An end-tidal carbon dioxide pressure (P ET CO 2 ) signal was collected by using an infrared-based CO 2 monitor module (CO2100C, BIOPAC Systems, Inc., Goleta, CA) connected to the facemask.…”