For decades, scientists have used near-infrared light to study the brain in a noninvasive way. This optical technique, known as "fNIRS" (short for "functional near-infrared spectroscopy"), measures how light is absorbed by blood in the brain, to infer activity. Valued for portability and low cost, fNIRS has a major drawback: it can't see very deep into the brain. Light typically only reaches the outermost layers of the brain, about 4 centimeters deep—enough to study the surface of the brain, but not deeper regions involved in critical functions like memory, emotion, and movement. This drawback has restricted the ability to study deeper brain regions without expensive and bulky equipment like MRI machines.
Now, researchers at the University of Glasgow have demonstrated something previously thought impossible: detecting light that has traveled all the way through an adult human head. Their study, published in Neurophotonics , shows that with the right setup, it is possible to measure photons that pass from one side of the head to the other, even across its widest point.
To achieve this, the team used powerful lasers and highly sensitive detectors in a carefully controlled experiment. They directed a pulsed laser beam at one side of a volunteer's head and placed a detector on the opposite side. The setup was designed to block out all other light and maximize the chances of catching the few photons that made the full journey through the skull and brain.
The researchers also ran detailed computer simulations to predict how light would move through the complex layers of the head. These simulations matched the experimental results closely, confirming that the detected photons had indeed traveled through the entire head. Interestingly, the simulations revealed that light tends to follow specific paths, guided by regions of the brain with lower scattering, such as the cerebrospinal fluid.
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This breakthrough suggests that it may be possible to design new optical devices that can reach deeper brain areas than current technologies allow. While the current method is not yet practical for everyday use—it required 30 minutes of data collection and worked only on a subject with fair skin and no hair—this extreme case of detecting light diametrically across the head may inspire the community to rethink what is possible for the next generation of fNIRS systems.
With further development, this approach might help bring deep brain imaging into clinics and homes in a more affordable and portable form. This could eventually lead to better tools for diagnosing and monitoring conditions like strokes, brain injuries, or tumors, especially in settings where access to MRI or CT scans is limited.
For details see the original Gold Open Access article by J. Radford et al., " Photon transport through the entire adult human head ," Neurophotonics 12(2), 025014 (2025), doi 10.1117/1.NPh.12.2.025014
