Photon transport through the entire adult human head
Photon transport through the human head refers to the movement of light (typically in the near-infrared spectrum) as it passes through the complex anatomical structures of the skull, cerebrospinal fluid (CSF), and brain tissue. Historically limited by scattering and absorption, recent advancements in photonics and computational modeling have enabled researchers to track photons across the entire adult human head, offering a non-invasive, high-resolution alternative to traditional neuroimaging modalities.
Neuroimaging has long relied on modalities such as MRI, PET, and CT scans to visualize brain structure and function. However, these technologies come with limitations: high cost, limited portability, radiation exposure (in the case of PET and CT), and the need for patient immobility. Optical techniques, particularly diffuse optical tomography (DOT) and near-infrared spectroscopy (NIRS), offer non-ionizing, cost-effective, and portable alternatives, but have historically struggled with deep brain imaging due to the inherent challenges in photon transport through thick, scattering media like the adult skull and brain.
Recent research has achieved a breakthrough: photons have been successfully guided and detected through the entire adult human head, overcoming previous depth and resolution barriers.
Physics of Photon Transport in Biological Tissues
Photon transport in tissue is governed by scattering, absorption, and anisotropy:
- Scattering (μs): Dominates in biological tissues. Photons deviate from their path due to microscopic inhomogeneities.
- Absorption (μa): Due to chromophores like hemoglobin, water, and lipids.
- Anisotropy factor (g): Represents the average cosine of scattering angles. Brain tissue typically has g ≈ 0.9.
To model this behavior, researchers use:
- Radiative Transport Equation (RTE): Describes photon propagation considering scattering and absorption.
- Diffusion Approximation: A simplification of RTE for highly scattering tissues, useful for modeling deep photon penetration.
- Monte Carlo Simulations: Statistical modeling of individual photon paths through layered head models, widely regarded as the gold standard in simulation.
research team at the University of Glasgow demonstrated for the first time that near-infrared photons (typically 650–950 nm) can be successfully transmitted through the entire adult human head and detected using time-gated single-photon avalanche diode (SPAD) arrays.
Key Innovations:
- Ultrafast Time-Gated Detectors: SPADs with picosecond resolution isolate ballistic and snake photons that preserve directional information.
- Optimized Illumination: Use of structured light or diffuse wavefront shaping improves penetration and localization.
- Computational Backpropagation: Machine learning-assisted models reconstruct internal photon paths and infer absorption changes due to brain activity.
Experimental Design:
- Source-Detector Geometry: Light is injected at the frontotemporal area and detected at the occipital bone, spanning ~15–18 cm.
- Wavelength Optimization: 800–850 nm range balances hemoglobin absorption and scattering losses.
- Photon Counting Techniques: Integration over long durations enhances signal-to-noise ratio (SNR) while preserving spatial resolution.
Applications in Neuroimaging
With whole-head photon transport now viable, new frontiers in optical neuroimaging emerge:
- Functional Brain Mapping: Real-time detection of hemoglobin oxygenation changes across cortical regions.
- Neurovascular Coupling Studies: Improved temporal resolution compared to fMRI.
- Portable Brain-Computer Interfaces (BCIs): Wearable optical devices for clinical or consumer use.
- Pediatric and Geriatric Neurocare: Safer, mobile diagnostics for vulnerable populations.
- Traumatic Brain Injury (TBI): Detection of hematomas or oxygen deficits non-invasively in emergency settings.
The successful transmission of photons through the entire adult human head marks a pivotal advancement in optical imaging. By leveraging state-of-the-art detectors, computational modeling, and a deep understanding of photon-tissue interaction, researchers have paved the way for safer, more accessible, and real-time brain imaging modalities. This progress promises to transform neurodiagnostics, cognitive science, and personalized healthcare.
Photon transport through the entire adult human head
The University of Glasgow, Scotland, UK. A world top 100 university