Flexible-circuit-based 3-D aware modular optical brain imaging system for high-density measurements in natural settings

Abstract

Significance: Functional near-infrared spectroscopy (fNIRS) presents an opportunity to study human brains in everyday activities and environments. However, achieving robust measurements under such dynamic condition remains a significant challenge.

Aim: The modular optical brain imaging (MOBI) system is designed to enhance optode-to-scalp coupling and provide real-time probe 3-D shape estimation to improve the use of fNIRS in everyday conditions.

Approach: The MOBI system utilizes a bendable and lightweight modular circuit-board design to enhance probe conformity to head surfaces and comfort for long-term wearability. Combined with automatic module connection recognition, the built-in orientation sensors on each module can be used to estimate optode 3-D positions in real-time to enable advanced tomographic data analysis and motion tracking.

Results: Optical characterization of the MOBI detector reports a noise equivalence power (NEP) of 8.9 and 7.3 $\text{pW}/\sqrt{Hz}$ at 735 nm and 850 nm, respectively, with a dynamic range of 88 dB. The 3-D optode shape acquisition yields an average error of 4.2 mm across 25 optodes in a phantom test compared to positions acquired from a digitizer. Results for initial in vivo validations, including a cuff occlusion and a finger-tapping test, are also provided.

Conclusions: To the best of our knowledge, the MOBI system is the first modular fNIRS system featuring fully flexible circuit boards. The self-organizing module sensor network and automatic 3-D optode position acquisition, combined with lightweight modules (18 g/module) and ergonomic designs, would greatly aid emerging explorations of brain function in naturalistic settings.

Keywords: brain imaging; modular fNIRS; naturalistic neuroimaging; optical tomography; wearable system.

Fig 1

Photos of the MOBI module showing the (a) top view without a silicone cover, (b) bottom view showing light guides attached to optodes and a black silicone enclosure, and (c) MOBI flexible-circuit-board before adding components.

Fig 2

Schematic diagram of a single MOBI module. The microcontroller uses an internal inter-integrated circuit (I²C) protocol to communicate with components on a single board. A peer-to-peer (P2P) network allows communication between neighboring modules. Associated components are color-coded: red for sources, blue for detectors, green for the IMU, and yellow for the microcontroller.

Fig 3

Schematic diagram of (a) a multi-module MOBI probe. Optional external power source and trigger board not shown. We also include in (b) a photo for the master node with its circuit board exposed and (c) a photo of the hardware trigger board.

Fig 4

Demonstration of automatic determination of module-to-module connectivity, shown in the top-left overlay, using two five-module configurations. [Video 1 (MP4, 5 MB)]

Fig 5

System characterization showing (a) measured signal vs. incident optical power, providing noise equivalent power at both 735 and 850 nm and (b) signal-to-noise ratio of versus source-detector separation from a 3-module probe, with each point representing a unique channel.

Fig 6

Three example probe layouts composed of five identical MOBI modules. Optodes are represented by small red circles (sources) and blue crosses (detectors). Each layout has multiple spatial multiplexing groups determined based on the global proximity of sources to each other. Red dashed circles show which sources are simultaneously on for each layout’s first spatial multiplexing group.

Fig 7

Comparison between (a) a rigid and (d) flexible module implementation in terms of conformity to a hemisphere phantom. Specifically, we report the distances to the sphere center measured for the (b) rigid and (e) flexible 5-module probes using a digitizer. The histogram of the resulting distances for a total of 25 optodes are shown for the (c) rigid and (f) flexible probes. With a 5 mm module thickness, a reading of 105 mm is expected for a perfectly conforming probe.

Fig 8

Schematic and results for automated optode position estimation, showing (a) a diagram of the piecewise-spherical algorithm to sequentially estimate module positions, p_i (i = 1,2, …), from a reference module location, p_R, using their quaternion-derived unitary normal vectors, ${\overrightarrow{n}}_i$, and the computed radius r_i between adjacent modules; (b) a photo of a V-shaped MOBI probe secured by a hair-net over a hemisphere phantom; (c) estimated module positions (green) and optode positions (red - source, blue - detector) using IMU data versus manually digitized positions (purple circles). We also report the error of IMU-derived optode positions (d) on the probe level and (e) per module.

Fig 9

Results from a dual-pressure blood occlusion experiment using (a) a single MOBI module and (b) a single Artinis channel placed on the forearm. Venous (100 mmHg) and arterial (220 mmHg) occlusions lasted 75 seconds each prior to release.

Fig 10

Validation of MOBI system in a finger-tapping experiment, showing (a) a photo of a two-module probe mounted over the wire-frame head-cap and (b) resulting hemodynamic responses for each channel, with red, green, and blue lines indicate HbO, HbR, and HbT changes, respectively. Source LEDs (red) and detectors (blue) are numbered in both plots.