Individualized brain monitoring spectroscopy with less hassle

Functional near-infrared spectroscopy (fNIRS) is used to monitor brain function in clinical and research settings by measuring oxygen levels in the brain. Using light sources placed on the scalp, near-infrared light can penetrate the skull and is absorbed and scattered by the oxygen-carrying protein hemoglobin, which can then be quantitatively measured.

This technique isn’t perfect, though, because accurate readings require that each person’s highly individualized differential pathlength factor (DPF) be collected via MRI. Liu et al. attempted to find these values in a more efficient way, improving the efficacy of fNIRS.

“Portable continuous wave fNIRS commonly relies on empirical DPFs that assume the head is homogeneous,” said author Dongyuan Liu. “This neglects layered anatomy and subject- and channel-specific variability, resulting in superficial–cortical crosstalk and systematic quantification errors.”

The researchers introduced a computational framework, known as an optical parameter inversion strategy, to the fNIRS technique. To do this, they first developed a statistically representative brain model and then used multilayer Monte Carlo simulations to identify time gates that can be sensitive to the different layers of the brain. Finally, they aligned this data with each test subject’s photon time-of-flight data and used a computational model to calculate individual DPFs.

In simulations, their results improved the spatial accuracy of fNIRS and decreased noise. In tests where participants held their breath to modulate oxygen levels in the brain, the researchers found their technique worked in efficiently measuring the brain’s hemodynamic responses.

They plan to refine their brain map with more precise anatomical models and increase their test subject pool.

“We will also investigate signal-processing and machine-learning methods to improve time-gate selection and inversion stability,” Liu said.

Source: “Resolving layer-specific optical properties with time-shift inversion to mitigate crosstalk in continuous-wave fNIRS optical topography,” by Dongyuan Liu, Xiaomeng Wang, Tong Zhang, Limin Zhang, and Feng Gao, APL Photonics (2026). The article can be accessed at https://doi.org/10.1063/5.0334744 .