Phase dual‐slopes in frequency‐domain near‐infrared spectroscopy for enhanced sensitivity to brain tissue: First applications to human subjects

Abstract

We present a first in vivo application of phase dual‐slopes (DSϕ), measured with frequency‐domain near‐infrared spectroscopy on four healthy human subjects, to demonstrate their enhanced sensitivity to cerebral hemodynamics. During arterial blood pressure oscillations elicited at a frequency of 0.1 Hz, we compare three different ways to analyze either intensity (I) or phase (ϕ) data collected on the subject's forehead at multiple source‐detector distances: Single‐distance, single‐slope and DS. Theoretical calculations based on diffusion theory show that the method with the deepest maximal sensitivity (at about 11 mm) is DSϕ. The in vivo results indicate a qualitative difference of phase data (especially DSϕ) and intensity data (especially single‐distance intensity [SDI]), which we assign to stronger contributions from scalp hemodynamics to SDI and from cortical hemodynamics to DSϕ. Our findings suggest that scalp hemodynamic oscillations may be dominated by blood volume dynamics, whereas cortical hemodynamics may be dominated by blood flow velocity dynamics.

Transformational change in the field of diffuse optics: From going bananas to going nuts

“…15 Despite the e®ectiveness of these approaches in reducing the sensitivity to thin, uniform super¯cial layers, they still feature banana-shaped regions of sensitivity (albeit asymmetrical and bimodal) with maximal sensitivity close to the tissue surface for localized inhomogeneities. [16][17][18] In this paper, we consider an extension of single-slope (SS) methods that signi¯cantly improves upon them in terms of depth sensitivity.…”

Frequency-Domain Techniques for Cerebral and Functional Near-Infrared Spectroscopy

“…Furthermore, it is evident from Figures 5C,D that the sensitivity of phase dual-slope extends deeper than the sensitivity of intensity dualslope. An FD-NIRS study on human subjects showed the different information content of intensity and phase collected with singledistance, single-slope, and dual-slope arrangements, consistent with a deeper sensitivity of phase slope methods, and more robust dual-slope vs. single-slope data (Blaney et al, 2020).…”

“…One possible exception is the measurement of the fast optical signal, which will be presented in Section 3.6. In the case of a homogeneous absorption change, ∆µ a , in a semiinfinite medium, one can translate relative changes in the DC Intensity (∆I DC /I DC0 , where we indicate with I DC0 the detected DC intensity in a reference condition, say at baseline, and with ∆I DC the intensity change with respect to the reference condition as a result of an absorption change ∆µ a ) and absolute changes in the phase (∆ϕ) into the corresponding absorption change as follows (Blaney et al, 2020;Fantini et al, 2020):…”

“…A correction to Eq. (5) is needed for the AC amplitude of intensity (I AC ); such correction can be found in Sassaroli et al (2019) and Blaney et al (2020). The effect of localized absorption changes on the DC Intensity (and similarly on the AC amplitude) and on the phase can be described in terms of spatial regions of sensitivity, which are introduced in Section "The Spatial Region of Sensitivity of DC, AC, and Phase.…”

“…Such hemodynamic changes affect both intensity and phase data and are mostly associated with absorption changes due to blood volume and blood oxygenation dynamics induced by brain activity. Under the assumption of negligible scattering changes, temporal phase data can be translated into measurements of [Hb] and [HbO 2 ] (concentration changes with respect to baseline) (Toronov et al, 2001;Blaney et al, 2020) [see Eq. (6)], similar to the way intensity changes are translated into…”