Computational analysis of light diffusion and thermal effects during Transcranial Photobiomodulation.

Transcranial Photobiomodulation (tPBM) is a non-invasive procedure where light is applied to the scalp to modulate underlying brain activity. tPBM has recently attracted immense interest as a potential therapeutic option for a range of neurological and neuropsychiatric conditions. The common technological questions related to this modality are extent of light penetration and associated scalp and brain temperature increases. Limited computational efforts to quantify these aspects are restricted to simplified models. We consider here a 3D high-resolution (1 mm) and anatomically realistic model to simulate light propagation and thermal effects. We consider a dose of 100 mW /cm² and use a single light source targeting the F3 location based on 10-20 EEG. Our simulations reveal that while the induced irradiance distribution largely mimics the shape and extent of the source, there is a blurring effect at the brain. This diffusion is attributed to the scalp, skull, and compounded at the surface of the cerebrospinal fluid. Around 1% of the injected irradiance reaches the gray matter. As expected and aligned with previous efforts, the scalp accounts for the greatest loss (~65%). We observe a nominal 0.38 °C rise in the scalp in regions directly underneath the source. There is negligible temperature rise in the brain. Finally, irradiance reduces to 0.01 mW /cm² at ~13.5 cm from the scalp surface.