Simultaneous Discrimination of Multiple Chromophores With Frequency Division Multiplexed Four-Color Functional Near-Infrared Spectroscopy

Herein, we implement a four-color functional near-infrared spectroscopy (fNIRS) system using frequency division multiplexing (FDM) to demonstrate simultaneous discrimination of multiple chromophores. The proposed fNIRS system consists of a four-channel flexible patch with two light-emitting diode (LED) modules, four photodiodes (PDs), and a control board for FDM. Unlike the conventional time division multiplexing (TDM) method that turns on/off individual LEDs sequentially, the FDM method can simultaneously turn on/off the entire LEDs using different modulation frequencies. Therefore, the sampling rate of FDM can be increased by eight times compared to the case of TDM under the same condition of two light sources with four wavelengths. First, for validation of the developed fNIRS system, we conducted a comparative experiment with a commercial spectrometer using two types of dyes in a customized phantom. By comparing the absorbance spectra for each wavelength using both instruments, we confirmed that the correlation between the absorbance measured by the two instruments was high, with an R-square value of up to 0.99662. Second, three chromophores of oxyhemoglobin (HbO), deoxyhemoglobin (HbR), and the oxidation state of cytochrome-c-oxidase (oxCCO) were discriminated in vivo using four wavelengths for the cerebral blood flow response with correlation coefficients and effect sizes. The average hemodynamic response during cerebral activity clearly presented an increased HbO and a decreased HbR simultaneously. We also confirmed that the strong correlation between oxCCO (directly involved in oxygen metabolism) and hemoglobin difference (cerebral oxygenation) showed a Pearson's correlation coefficient of 0.7342. These results demonstrate the validity of the FDM-based fNIRS system and the successful acquisition of brain signals. This reveals that FDM can be applied to various bio-engineering applications in the future.

Automated summary

This study presents a four-color functional near-infrared spectroscopy (fNIRS) system using frequency division multiplexing (FDM) to simultaneously discriminate multiple chromophores. The system achieves a higher sampling rate compared to the conventional time division multiplexing (TDM) method. Experimental results show a high correlation between the developed fNIRS system and a commercial spectrometer, validating its performance. In vivo discrimination of three chromophores and the successful acquisition of brain signals further demonstrate the effectiveness of the FDM-based fNIRS system.