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

Area of Science:

• Neuroscience
• Biomedical Engineering
• Medical Physics

Background:

• Transcranial photobiomodulation (tPBM) is an emerging noninvasive technique using light to modulate brain activity.
• Increasing interest exists in tPBM's therapeutic potential, with key questions surrounding light penetration and tissue heating.
• Existing computational models for tPBM are often simplified, limiting accurate predictions.

Purpose of the Study:

• To computationally investigate light penetration and temperature increases in the brain during tPBM.
• To evaluate the impact of different power densities and stimulation durations on tPBM parameters.
• To assess the safety of tPBM parameters against established limits.

Main Methods:

• A 3D, high-resolution, anatomically realistic head model was used for simulations.
• Simulations targeted the F3 region with 800 nm wavelength light at power densities of 10, 100, and 1000 mW/cm².
• Time-variant applications and temperature increases relative to ANSI safety limits were analyzed.

Main Results:

• Cortical irradiance was focal, with ~1% reaching gray matter; scalp absorbed ~65%.
• Cerebrospinal fluid (CSF) contributed to a halo effect, extending light to the cortex.
• Scalp temperature increased up to 3.76°C, while brain temperature rose by 0.57°C at 1000 mW/cm².

Conclusions:

• tPBM allows spatial profile control by altering light source geometry.
• Power densities of 1000 mW/cm² surpass established safety limits for scalp and brain.
• Light penetration exceeding 10 cm from gray matter is possible, contrary to some previous findings.