Abstract
This review describes the unique opportunities and challenges for noninvasive optical mapping of human brain function. Diffuse optical methods offer safe, portable, and radiation free alternatives to traditional technologies like positron emission tomography or functional magnetic resonance imaging (fMRI). Recent developments in high-density diffuse optical tomography (HD-DOT) have demonstrated capabilities for mapping human cortical brain function over an extended field of view with image quality approaching that of fMRI. In this review, we cover fundamental principles of the diffusion of near infrared light in biological tissue. We discuss the challenges involved in the HD-DOT system design and implementation that must be overcome to acquire the signal-to-noise necessary to measure and locate brain function at the depth of the cortex. We discuss strategies for validation of the sensitivity, specificity, and reliability of HD-DOT acquired maps of cortical brain function. We then provide a brief overview of some clinical applications of HD-DOT. Though diffuse optical measurements of neurophysiology have existed for several decades, tremendous opportunity remains to advance optical imaging of brain function to address a crucial niche in basic and clinical neuroscience: that of bedside and minimally constrained high fidelity imaging of brain function.
Highlights
This review will discuss recent advancements in high-density diffuse optical tomography (HD-DOT) methods that have led to improved image quality and reliability in noninvasive optical mapping of human brain function
To improve the image quality of sparse functional near infrared spectroscopy (fNIRS), spatially overlapping fNIRS measurements can be tomographically reconstructed to produce three-dimensional maps of brain function [Fig. 2(d)], a technique known as diffuse optical tomography (DOT).[31,44,57]
We summarized papers highlighting the unique potential for HD-DOT methods to profoundly impact clinical care
Summary
Imaging spatially and temporally distributed brain activity has revolutionized our understanding of the brain.[1,2,3,4] The interacting brain systems supporting our thoughts and actions—from sensing the visual world, to communicating, to maintaining attention and control, to daydreaming or sleeping—are accessible to quantitative investigation through functional imaging techniques.[4,5,6] functional brain imaging has provided insight into neurological and psychiatric disorders such as Alzheimer’s disease,[7] autism spectrum disorder (ASD),[8,9,10] and stroke.[11,12] optimizing neuroimaging technologies as tools for understanding these disorders and tracking their progression presents significant challenges. To improve the image quality of sparse fNIRS, spatially overlapping fNIRS measurements can be tomographically reconstructed to produce three-dimensional maps of brain function [Fig. 2(d)], a technique known as diffuse optical tomography (DOT).[31,44,57] To further improve image quality, HD-DOT systems use a dense regular array of sources and detectors to obtain overlapping measurements at multiple distances. High-density is defined as a regular array, typically an interlaced lattice of sources and detectors, with a closest (a.k.a., nearest neighbor) SD distance of at most 15 mm45 [Fig. 2(e)] This maximum distance of 15 mm for the nearest neighbor SD separation makes possible access to multiple SD distances, including out to 40 mm and beyond, that together provide measurements crucial for obtaining spatial maps of brain function comparable to fMRI. We will briefly consider opportunities to further improve image quality, anatomical specificity, and reliability so that HD-DOT methods can realize their true potential in unconstrained and noninvasive assessment of human brain function in the clinic, in naturalistic and even remote settings, and in sensitive populations
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