Amazing stuff!
"The first step toward an inexpensive new medical imager was proving that near-infrared photons can fly all the way through your head. It was a comically difficult experiment. ..."
"... Optical brain imaging primarily uses near-infrared light, to which human tissue is relatively transparent. But human heads are so good at blocking even those wavelengths that ... only a billionth of a billionth of all near-infrared photons make it through an entire adult human head from one side to the other. ...
the difficult trade-off of expensive, deeper-penetrating imaging equipment versus cheaper but shallower sensors is starting to break down. ..."
From the abstract:
"Significance
The highly scattering nature of near-infrared light in human tissue makes it challenging to collect photons using source-detector separations larger than several centimeters. The limits of detectability of light transmitted through the head remain unknown. Detecting photons in the extreme case through an entire adult head explores the limits of photon transport in the brain.
Aim
We explore the physical limits of photon transport in the head in the extreme case wherein the source and detector are diametrically opposite.
Approach
Simulations uncover possible migration pathways of photons from source to detector. We compare simulations with time-resolved photon counting experiments that measure pulsed light transmitted through the head.
Results
We observe good agreement between the peak delay time and width of the time-correlated histograms in experiments and simulations. Analysis of the photon migration pathways indicates sensitivity to regions of the brain well beyond accepted limits. Source repositioning can isolate sensitivity to targeted regions of the brain, including under the cerebrum.
Conclusions
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."
Photon transport through the entire adult human head (open access)
Fig. 2
Experimental results.
(a) The experimental configuration used in laboratory experiments—an ultrafast pulsed laser (1.2 W power, 800 nm wavelength, 140 fs pulse duration, 80 MHz repetition rate) is expanded to a uniformly distributed 1 in. diameter circle and projected against the side of the head. Diametrically opposite the source, a PMT is synchronized to the laser trigger such that a histogram of photon ToF can be measured using TCSPC.
(b) The normalized simulated ToF (black dashed) and the mean of 15 experimental trials (2 min exposure) on day 1 (blue) and day 2 (red). The uncertainty bands indicate the standard error of the mean at each time stamp. The IRF (black) with the peak centered at time = 0 ns is shown for comparison. The start time of all time traces are aligned to the peak of the IRF.
(c) The mean of the background-subtracted counts (black) of day 1 data in each timebin for 15 2-min exposures. The filtered mean counts for each timebin using a Savitzky–Golay filter (window = 251, order = 3). The normalized IRF is shown in red for comparison.
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