Many fNIRS systems use a few wavelengths between 650 and 850 nm (often only two), without intensity modulation; these data allow measurements of brain activation through changes in wavelength-dependent tissue absorption resulting from changes in the concentrations of oxy- and deoxy-hemoglobin. However, these measurements are vulnerable to spurious signal from the scalp, changes in optical coupling to the skin, etc. Quantification also requires assumptions about the baseline tissue scattering and absorption.
The system described by Lange and colleagues adds a wealth of spectral (in the range between 500 and 1000 nm with 20-nm resolution) and temporal (50 ps resolution over a 5-ns window) information to fNIRS measurements. These data allow quantitative measurement of tissue scattering, resulting in accurate assessment of the baseline concentrations of oxy- and deoxy-hemoglobin, and therefore more accurate measurements of blood oxygen saturation changes due to neural activity, as well as depth sensitivity. The development of this system, along with complementary work at Politecnico di Milano, brings quantitative, portable optical measurements of human brain function closer to common use in elucidating the mysteries of the human brain.
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