With the appropriate optical setup, it is possible to measure photons as they travel diametrically across the inside of the adult human head. This discovery, made by scientists at the University of Glasgow, could contribute to the development of noninvasive optical techniques for deep brain imaging. For decades, scientists have used functional near-infrared spectroscopy (fNIRS) to measure and study brain activity. This approach is noninvasive, low-cost, and portable, but it extends only as far as the outermost region of the cortex, due to the scattering nature of human tissue. To explore photon transport through the entire human head, the researchers directed a pulsed laser beam at one side of a subject’s head and positioned a detector on the opposite side. They arranged an optical setup that blocked out all other light to increase the likelihood of capturing the small number of photons that would complete the journey through the subject’s skull and brain. Researchers explored the limits of photon transport in the brain and detected photons through an entire adult head. The findings could lead to access to regions of the brain currently inaccessible with noninvasive optical brain imaging. Courtesy of J. Radford et al., doi: 10.1117/1.NPh.12.2.025014. The researchers overcame attenuation of approximately 1018 and detected photons transmitted through an entire adult human head for a subject with fair skin and no hair. The photons measured in this regime explored regions of the brain currently inaccessible with noninvasive optical brain imaging. The experimental measurements of the photons transmitted through the adult human head suggest that, although the attenuation of light is challenging, in principle, it is possible to detect light even from the most extreme source-detector separations. Light is not back-reflected from shallow regions but migrates through the entirety of the head. The researchers surmise that the subject’s fair skin and lack of hair were significant factors causing the attenuation of light to be decreased enough to detect a signal. From computer simulations, to predict how light would move through the layers of the head, the researchers observed that photons moved through the head along specific trajectories. They identified various light propagation pathways and observed that these pathways were largely guided by cerebrospinal fluid, which is found in regions of the brain with relatively low scattering. Simulation results correlated with the experimental results. And an analysis of the photon migration pathways indicated sensitivity to regions of the brain well beyond accepted limits. The team also found that source repositioning can be used to isolate sensitivity to targeted regions of the brain, including under the cerebrum. This finding suggests that optical techniques could be used to monitor activity in the sulci, midbrain, and deep regions of the cerebellum, which are currently inaccessible with fNIRS. While this method is not yet practical for everyday use — it required 30 min 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 could inspire new approaches for the next generation of fNIRS systems. With further development, this approach could potentially provide clinics and even home services with a portable, affordable option for deep brain imaging. In the future, it could lead to better tools for diagnosing and monitoring conditions like strokes, brain injuries, and tumors, especially in settings where access to bulky equipment for conducting MRI or CT scans is limited. This approach could be especially useful in static imaging applications that do not require fast sample rates, according to the researchers. The research was published in Neurophotonics (www.doi.org/10.1117/1.NPh.12.2.025014).