Contents

📒 Bhattacharya 2019

Computational modeling of the photon transport, tissue heating, and cytochrome C oxidase absorption during transcranial near-infrared stimulation1

Goal

Develop a head model for near-infrared (NIR) absorption and scattering with thermal effects to derive the dosage cytochrome c oxidase (CCO) receives for evaluation of photomodulation and photothermal neurostimulation.

Methods

Head model

Tetrahedral mesh 3D FEM model with COMSOL Multiphysics software using the Partial Differentiation Equation (PDE) toolbox.

Photon transfer

Radiative transfer equation (RTE) for the scattering (2nd order PDE). https://www.biorxiv.org/sites/default/files/highwire/biorxiv/early/2019/07/19/708362/embed/graphic-6.gif https://www.biorxiv.org/sites/default/files/highwire/biorxiv/early/2019/07/19/708362/embed/graphic-7.gif https://www.biorxiv.org/sites/default/files/highwire/biorxiv/early/2019/07/19/708362/embed/graphic-8.gif

The anisotropy factor, g = 0.89 has been assumed for all the tissue layers

With boundary condition: https://www.biorxiv.org/sites/default/files/highwire/biorxiv/early/2019/07/19/708362/embed/graphic-9.gif

Heat transfer

The Pennes Bio-heat equation: https://www.biorxiv.org/sites/default/files/highwire/biorxiv/early/2019/07/19/708362/embed/graphic-11.gif

Tissue absorption

  • Absorption by CCO at 630nm, 700nm, and 810nm with photomodulation effects.
  • Other chromophores: HbO2, Hb, lipids

Results

810nm comparatively shows a higher absorption of power at the gray matter, and thus the authers hypothesized that this wavelength a better choice for photothermal neuromodulation.

Discussions

Limitations

  • The brain has been assumed as a highly scattering medium which is not true for CSF which is a low scattering medium where RTE can produce erroneous results, compared to Monte Carlo methods
  • Some computational limits of FEM modeling, discretization, memory limits.

photothermal vs photomodulation by chromophores

  • Temperature change in the scalp is well within 1 degree Celsius. The simulated results showed insignificant temperature change (0.033°C) in the grey matter to cause photothermal neuromodulation.

Conclusion

  • the biochemical effects of CCO absorption need further investigation in conjunction with the heating effects since a small, steady state temperature change can affect the kinetics of photobiomodulation
  • 810nm has higher penetration depth than the 630nm and 700nm, which supports the use of tNIRS for non-invasive brain stimulation.

Reference


  1. Bhattacharya, M., & Dutta, A. (2019). Computational modeling of the photon transport, tissue heating, and cytochrome C oxidase absorption during transcranial near-infrared stimulation. BioRxiv. https://www.biorxiv.org/content/10.1101/708362v1↩︎