High-Resolution Computational Modeling of Transcranial Photobiomodulation: Light Propagation and Thermal Effects

Transcranial photobiomodulation (tPBM) is the noninvasive application of light to modulate underlying brain activity. There is increasing interest in evaluating tPBM as a therapeutic option. The typical technological questions are extent of light penetration and associated tissue temperature increases. Limited computational efforts to quantify these aspects are restricted to simplified models.

We considered a three-dimensional high-resolution (1 mm) anatomically realistic head model to simulate tPBM with the light source targeting the F3 region at 800 nm wavelength. Power densities spanning three decades (10, 100, and 1000) mW/cm² were investigated. We also tested time-variant application at 100 mW/cm² for up to 20 minutes. Finally, tissue temperature increases for the American National Standards Institute safety limit of 330 mW/cm² also were determined at a test case.

Our predictions reveal that the induced cortical irradiance is largely focal, demarcated by the shape and extent of the source. Approximately 1% of the injected irradiance reaches the gray matter. Aligned with previous efforts, the scalp accounts for the greatest loss (∼65%). The irradiance reduces to a hundredth of the value from gray matter at an approximately 113-mm perpendicular distance from its surface. There is a growing halo-like effect at the level of cerebrospinal fluid (CSF), which is extended down to the underlying cortex. The CSF was found to be mainly responsible for this effect. We observe scalp temperature increases of 0.38 °C and 3.76 °C for 100 and 1000 mW/cm² power density, respectively. The corresponding brain temperature increases are predicted to be 0.06 °C and 0.57 °C. As expected, irradiance absorption is linear with applied power density. Although the maximum induced scalp temperature increases linearly with power density, maximum brain temperature increases less slowly with power density. Transient analysis at 100 mW/cm² power density indicates expected scalp temperature increase with increasing stimulation duration. Temperature increases asymptote in approximately 10 minutes.

tPBM presents unique potential to directly impose a desired spatial profile using simple alteration of the shape and size of the source. Usage of power density of 1000 mW/cm² exceeds scalp and brain temperature safety limits. Contrary to prior reports, light penetration can exceed >10 cm from gray matter surface.