Sensitivity of near-infrared spectroscopy and diffuse correlation spectroscopy to brain hemodynamics: simulations and experimental findings during hypercapnia

Abstract

Near-infrared spectroscopy (NIRS) and diffuse correlation spectroscopy (DCS) are two diffuse optical technologies for brain imaging that are sensitive to changes in hemoglobin concentrations and blood flow, respectively. Measurements for both modalities are acquired on the scalp, and therefore hemodynamic processes in the extracerebral vasculature confound the interpretation of cortical hemodynamic signals. The sensitivity of NIRS to the brain versus the extracerebral tissue and the contrast-to-noise ratio (CNR) of NIRS to cerebral hemodynamic responses have been well characterized, but the same has not been evaluated for DCS. This is important to assess in order to understand their relative capabilities in measuring cerebral physiological changes. We present Monte Carlo simulations on a head model that demonstrate that the relative brain-to-scalp sensitivity is about three times higher for DCS (0.3 at 3 cm) than for NIRS (0.1 at 3 cm). However, because DCS has higher levels of noise due to photon-counting detection, the CNR is similar for both modalities in response to a physiologically realistic simulation of brain activation. Even so, we also observed higher CNR of the hemodynamic response during graded hypercapnia in adult subjects with DCS than with NIRS.

Keywords: diffuse correlation spectroscopy; functional brain imaging; hypercapnia; near-infrared spectroscopy.

Figures

Fig. 1

Sensitivity of near-infrared spectroscopy (NIRS) (a) and diffuse correlation spectroscopy (DCS) (b) as simulated with Monte Carlo simulations on an MRI-based head structure, for source–detector separations of 1, 2, and 3 cm. We present the sensitivity to scalp (measured-over-true scalp change), sensitivity to brain (measured-over-true brain change), and the relative brain-to-scalp sensitivity. The NIRS sensitivity is based on a blood volume change only, and that of DCS is based on a blood flow change only. For DCS, we present the cases where the whole autocorrelation curve g1(τ) is fit (magenta, see first inset), and when only the early delays corresponding to g1(τ)>0.7 are fit (cyan, see second inset). In all cases (NIRS and DCS), the reconstruction is done using a homogeneous semi-infinite model. The bar heights present the median value across all eight probe locations, and the error bars extend from the 25th to 75th percentile of the values.

Fig. 2

Dependence on the baseline physiological parameters of NIRS and DCS sensitivities: (a) varying CBV, at constant brain-to-scalp volume ratio=2.5; (b) varying brain-to-scalp volume ratio, at constant CBV=75  μM; (c) varying CBF, at constant brain-to-scalp flow ratio=6; (d) varying brain-to-scalp flow ratio, at constant CBF=6×106  mm2 s−1. The dotted lines show the sensitivity to scalp, and the solid lines the sensitivity to brain. For DCS brain-to-scalp sensitivity, we present the results when the whole autocorrelation curve is fit (thick magenta lines), and when only the early delays corresponding to g1(τ)>0.7 are included (thin cyan lines).

Fig. 3

Monte Carlo simulations of the contrast (top) and CNR (bottom) of NIRS and DCS, in response to functional activation (left) and hypercapnia (right). The bar heights show the median value across the eight probe locations. For each parameter, the three bars of increased darkness in the same color show the results at 1, 2, and 3 cm, respectively. For DCS, we present the cases where the whole autocorrelation curve g1(τ) is fit (magenta), and when only the early delays corresponding to g1(τ)>0.7 are fit (cyan).

Fig. 4

Hemodynamic response function (HRF) to hypercapnia measured in subject 1, with NIRS (top) and DCS (bottom). The targeted hypercapnia duration is indicated with the shaded gray area. For DCS, the three colors indicate different range of delays included in the fit, corresponding to different thresholds on g1(τ)2 (0.01, 0.1, and 0.5).

Fig. 5

NIRS and DCS HRF to hypercapnia (8 mm Hg above baseline) measured at 3 cm for all four subjects.

Fig. 6

Median across four subjects of contrast (top) and CNR (bottom) of the HRF to 4 mm Hg hypercapnia (left) and 8 mm Hg hypercapnia (right). For each parameter, we present the results at 0.8 cm (light color) and at 3 cm (dark color). The DCS results are presented for two different ranges of delays incorporated in the fit, corresponding to two thresholds of the autocorrelation curve: g1(τ)2>0.01 (“All τ,” magenta), and g1(τ)2>0.5 (“Short τ,” cyan).