Beam Of Light Shone All The Way Through A Human Head For The Very First Time

There are lots of situations where getting a look at the human brain can be very helpful. Unfortunately, our heads are famously opaque, so decades of research and development have gone into finding ways to break through this barrier and catch a glimpse of the workings underneath. A recent study has taken this one step further by shining a beam of light all the way through the bones and tissues of the head and out the other side.

The new technique, developed by experts at the University of Glasgow, builds on an established method called functional near-infrared spectroscopy (fNIRS) that measures how a beam of light is absorbed by the brain.

This has been in use for some years as a portable, low-cost, and noninvasive way of imaging the brain, but it has some drawbacks. The major one is that it can’t look very deep, generally only allowing for a view of the top 4 centimeters (about 1.5 inches) of the brain’s surface. So there’s a limit to how useful it can be, and sometimes it’s still necessary to bring in the big guns like MRI scanners. 

It was thought that it would be impossible to re-engineer the light beam to pass all the way through the brain to the other side – but the scientists at Glasgow have done it. 

“Detecting photons in the extreme case through an entire adult head explores the limits of photon transport in the brain,” they write in their paper. “We overcome attenuation of ∼10¹⁸ and detect photons transmitted through an entire adult human head for a subject with fair skin and no hair.”

“Photons measured in this regime explore regions of the brain currently inaccessible with noninvasive optical brain imaging.”

A powerful pulsed laser beam was directed at one side of the head, with a detector placed on the other side. Only a few photons made it through, but when you consider the amount of skin, bone, and brain tissue they had to navigate, it’s still seriously impressive.

Tracking the path of the plucky photons, the scientists found it matched closely with computer simulations, and that light tends to follow a predictable route through certain components of the brain like the cerebrospinal fluid. 

The team tried the technique on a range of people with different combinations of skin tones, hair colors, and hair types, but the landmark signal detections were achieved with an adult man with fair skin and no hair. 

“We speculate that the participant’s fair skin and lack of hair were significant factors that reduced the attenuation of light to feasibly detect a signal,” they write, acknowledging that further refinement will be needed to reliably use the method on people with different characteristics. It was also time-consuming, taking 30 minutes to collect all the data, and had to be conducted in darkness to minimize the impact of other light sources in the environment.

It does, however, show that such a thing could be possible in the future, and “may inspire the community to rethink what is possible for the next generation of fNIRS systems,” reads a press release about the breakthrough. 

The study is published in the journal Neurophotonics.