Cutting-Edge Near-infrared Wearable Neuroimaging Technologies Promise New Insights

A brain-monitoring headset analyzing brain scans © Ritthichai - stock.adobe.com

Recent technological advancements in optoelectronics have enabled the development of wearable and high-density functional near-infrared spectroscopy (fNIRS) and diffuse optical tomography (DOT) technologies. These innovations promise to revolutionize the field of neuroscience by allowing functional neuroimaging of the human cortex with a resolution comparable to fMRI, applicable in almost any environment and population.

An article published in the SPIE Digital Library by Ernesto E. Vidal-Rosas, Alexander von Lühmann, Paola Pinti, and Robert J. Cooper, provides a comprehensive overview of the history, status and future potential of these technologies (1). Functional near-infrared spectroscopy (fNIRS) utilizes light in the near-infrared wavelength region, typically between 650 nm and 900 nanometers (nm); and specifically, 735 and 850 nm (2). This range is selected because it allows near-infrared light to penetrate biological tissues, such as the human scalp and skull, to reach the cerebral cortex, where it can then be absorbed by hemoglobin in the blood (2).

The absorption spectra of oxyhemoglobin and deoxyhemoglobin differ, enabling the differentiation and quantification of these two forms of hemoglobin based on the changes in light absorption at specific wavelengths within the near-infrared range. Near-infrared light wavelengths ranging from 700–1000 (up to 1200) nanometers (nm) is measured to detect changes in oxyhemoglobin (HbO2) and deoxyhemoglobin (Hb) absorption in the brain.

Changes in the signal level at 808 nm are related to the total amount of hemoglobin in the tissue (3).

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Wearable fNIRS and HD-DOT: A New Era of Brain Imaging

Wearable fNIRS) and diffuse optical imaging methods have gained significant interest for the ability to noninvasively monitor cerebral hemodynamics. These techniques use the near-infrared light to measure changes in hemoglobin concentrations, providing insights into brain blood circulation, health, and functionality. From initial work, high-density fNIRS (HD-fNIRS) and high-density diffuse optical tomography (HD-DOT) have emerged by using dense arrays of sources and detectors at various separations, enhancing lateral spatial resolution and depth specificity when used with specialized imaging tomography software (1).

The advancements in miniaturization and wearability of fNIRS devices over the past decade have led to the development of fully wearable, wireless, and unobtrusive systems. These systems are not only lightweight but also capable of high-density tissue measurements, allowing researchers to study brain function during cognitive tasks that involve movement, exercise, and dynamic balance (1).

Pioneering Research and Technological Innovations

HD-DOT technology was introduced in 2007, demonstrating its feasibility for retinotopic functional mapping with superior spatial resolution compared to earlier fNIRS or DOT studies. Continuous-wave HD-DOT devices cover the occipital cortex with a dense array of source-detector channels. Later, the technology was improved by incorporating null source-detector separation measurements to achieve somatotopic mapping of the motor cortex (1). Further advancements in 2014 have expanded the field of view, enabling mapping of higher-order cognitive functions and increased connectivity networks. These innovations have allowed for more comprehensive and precise brain imaging, paving the way for several real-world applications (1).

Challenges and Future Directions

Despite significant progress, several challenges remain in developing wearable HD-fNIRS and HD-DOT technologies. These include ensuring robust optical coupling, minimizing motion artifacts, and balancing the need for high sampling density with the ergonomic requirements of a wearable system. The move from fiber-based to fiberless designs has been crucial, as it reduces the weight and fragility of the devices, enhancing their practicality for real-world monitoring use.

Modular system architectures have emerged as a promising solution, allowing dense networks of channels to be formed while maintaining flexibility and comfort. Modular HD-DOT systems have been demonstrated for imaging tomographic hemodynamic changes in the motor cortex with high-quality reconstructions. This approach has shown great potential for imaging brain function in various experimental settings (1).

Summary

The future of wearable HD-DOT and fNIRS technologies lies in the ability to provide high-quality imaging of cortical hemodynamics in naturalistic environments. These advancements will improve the ecological validity of neuroimaging studies, enabling researchers to study brain function during real-world activities. The development of multimodal, wearable systems will open new possibilities in clinical settings, including the study of neurodegenerative diseases and neurodevelopmental conditions.

As wearable neuroimaging technologies continue to advance, the focus will be on improving optoelectronics, data streaming, and optical coupling solutions. The fNIRS community is actively working to address these challenges, setting the stage for a new era of neuroscience research that can answer both long-standing and emerging questions about brain function.

References

(1) Vidal-Rosas, E. E.; von Lühmann, A.; Pinti, P.; Cooper, R. J. Wearable, High-Density fNIRS and Diffuse Optical Tomography Technologies: A Perspective. Neurophotonics 2023, 10 (2), 023513. https://doi.org/10.1117/1.NPh.10.2.023513.

(2) Liu, Z.; Si, L.; Shi, S.; Li, J.; Zhu, J.; Lee, W. H.; Lo, S. L.; Yan, X.; Chen, B.; Fu, F.; Zheng, Y. Classification of Three Anesthesia Stages Based on Near-Infrared Spectroscopy Signals. IEEE Journal of Biomedical and Health Informatics 2024. DOI: 10.1109/JBHI.2024.3409163

(3) Zuniga, K. B.; Ackerman, A. L.; Torosis, M.; Nitti, V.; Macnab, A.; Stothers, L. PD43-06 Methodology to Establish Non-Invasive Synchronous Heart Rate and Heart Rate Variability Measurement During Uroflowmetry to Examine Associations Between Cardiovascular Disease and Lower Urinary Tract Symptoms. J. Urol.2024, 211(5S), e900. DOI: 10.1097/01.JU.0001009568.19060.25.06