Characterizing dynamic cerebral vascular reactivity using a hybrid system combining time-resolved near-infrared and diffuse correlation spectroscopy

Abstract

This study presents the characterization of dynamic cerebrovascular reactivity (CVR) in healthy adults by a hybrid optical system combining time-resolved (TR) near-infrared spectroscopy (NIRS) and diffuse correlation spectroscopy (DCS). Blood flow and oxygenation (oxy- and deoxy-hemoglobin) responses to a step hypercapnic challenge were recorded to characterize dynamic and static components of CVR. Data were acquired at short and long source-detector separations (r SD) to assess the impact of scalp hemodynamics, and moment analysis applied to the TR-NIRS to further enhance the sensitivity to the brain. Comparing blood flow and oxygenation responses acquired at short and long r SD demonstrated that scalp contamination distorted the CVR time courses, particularly for oxyhemoglobin. This effect was significantly diminished by the greater depth sensitivity of TR NIRS and less evident in the DCS data due to the higher blood flow in the brain compared to the scalp. The reactivity speed was similar for blood flow and oxygenation in the healthy brain. Given the ease-of-use, portability, and non-invasiveness of this hybrid approach, it is well suited to investigate if the temporal relationship between CBF and oxygenation is altered by factors such as age and cerebrovascular disease.

Conflict of interest statement

The authors declare that they have no conflict of interest.

© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement.

Figures

Fig. 1.

Average change in heart rate (ΔHR) and mean arterial pressure (ΔMAP) across all subjects. Grey region represents the 2-min hypercapnic challenge and shading surrounding each line represents the standard error of the mean.

Fig. 2.

TR-NIRS and DCS data from one participant. Top row displays ΔCHbO (red squares) derived from moment analysis (ΔN, Δ<t>, ΔV) applied to DTOFs acquired at rSD = 3.0 cm. Middle row displays the corresponding inverted ΔCHb (blue squares). Bottom row displays ΔCHbO and inverted ΔCHb determined from ΔN at rSD = 1.0 cm and ΔBFi (black squares) time courses measured at rSD = 1.0 and 2.7 cm. The best fit of the hemodynamic model is illustrated in each graph by the solid coloured line, and the grey line is the recorded ΔPETCO2.

Fig. 3.

Average ΔCHbO, ΔCHb and ΔBFi responses (red, blue and black, respectively) to the 2-min hypercpnic challenge (indicated by the shaded region). Time courses are presented for the two source-detector separations (rSD = 1 and 3 cm for TR NIRS; rSD = 1 and 2.7 cm for DCS). In addition, the second row provides the ΔCHbO and ΔCHb responses determined by regressing ΔN1cm from ΔN3cm. All time courses were averaged across nine subjects. Shading surrounding each line represents the standard error of the mean.

Fig. 4.

Box plots of the three fitting parameters. From left to right: time delay (t0), time constant (τ), and steady-state cerebrovascular reactivity (ssCVR). Results are presented for moment analysis applied to TR-NIRS data recorded at rSD = 1 cm (top row), rSD = 3 cm (middle row) and for DCS data acquired at the two rSD values (bottom row). For the oxygenation results, red indicates ΔCHbO and blue ΔCHb. Outliers are represented by the crosses, and in one case (τ for ΔCHbO,V), the outlier reached the upper fitting boundary of 250 s.

Fig. 5.

Relative changes in cerebral blood flow (ΔCBF), cerebral blood volume (ΔCBV) and tissue oxygen saturation (ΔStO2) during hypercapnia. Time courses were averaged across nine subjects. Shaded region represents hypercapnic period and the shading surrounding each line represents the standard error of the mean.