Lock-in functional near-infrared spectroscopy for measurement of the haemodynamic brain response

• Published in: Biomedical optics express Vol. 13; no. 4; pp. 1869 - 1887
• Main Authors: Wojtkiewicz, Stanislaw, Bejm, Karolina, Liebert, Adam
• Format: Journal Article
• Language: English
• Published: United States Optica Publishing Group 01.04.2022
• ISSN: 2156-7085; 2156-7085
• DOI: 10.1364/BOE.448038

Here we show a method of the lock-in amplifying near-infrared signals originating within a human brain. It implies using two 90-degree rotated source-detector pairs fixed on a head surface. Both pairs have a joint sensitivity region located towards the brain. A direct application of the lock-in technique on both signals results in amplifying common frequency components, e.g. related to brain cortex stimulation and attenuating the rest, including all components not related to the stimulation: e.g. pulse, instrumental and biological noise or movement artefacts. This is a self-driven method as no prior assumptions are needed and the noise model is provided by the interfering signals themselves. We show the theory (classical modified Beer-Lambert law and diffuse optical tomography approaches), the algorithm implementation and tests on a finite element mathematical model and on healthy volunteers during visual cortex stimulation. The proposed hardware and algorithm complexity suit the entire spectrum of (continuous wave, frequency domain, time-resolved) near-infrared spectroscopy systems featuring real-time, direct, robust and low-noise brain activity registration tool. As such, this can be of special interest in optical brain computer interfaces and high reliability/stability monitors of tissue oxygenation.