Compact, Portable, High-Density Functional Near-Infrared Spectroscopy System for Brain Imaging

Abstract

Advancements in functional near-infrared spectroscopy (fNIRS) neuroimaging have extended its usage from clinical settings to real-life applications. We developed a wearable and wireless fNIRS-based neuroimaging system that can be applied for brain disease diagnostics and during daily life activities. We utilized 128 compact dual-wavelength LEDs of 735 nm and 850 nm and a silicon photodiode (SiPD). The LEDs and SiPD were closely assembled in a 3D-printed flexible configuration holder fitted in a soft neoprene cap. The developed configuration has 128 channels in a $7\times 7$ cm area with source-detector separations ranging from 5 mm to 38 mm, which intends to provide a high-density device. The system acquires data from multiple brain depths to generate a 3D activation image. Short-separation channels provide the physiological noise in the superficial layer, which can be used to enhance fNIRS signals. The experimental data are stored in a USB flash disk attached to the device. We developed software that is connected to the device using Wi-Fi to display the oxyhemoglobin (HbO) and deoxyhemoglobin (HbR) levels in real-time. The system architecture allows the selection of channels and the use of multiple devices simultaneously. The system was tested using a silicone phantom as well as human subjects. The LED intensity is dynamically calibrated within the safety range once the device is set up, so that every channel can acquire stable data. The system provided high-quality signals with a signal-to-noise ratio of more than 64 dB, high optical sensitivity, and dynamic optical range of 140 dB in the phantom test. A sampling rate of 30 Hz was achieved with all the channels used. The system exhibited valid HbO and HbR responses during the arterial occlusion test and a motor task. These results show that the developed system can provide precise, robust, and multi-depth measurements resulting in high-density images. With these novel characteristics, the system favors clinical usage and outdoor activities as well as brain-computer interface applications.