Simulation Study on the Optimization of Photon Energy Delivered to the Prefrontal Cortex in Low-Level-Light Therapy Using Red to Near-Infrared Light

Brain functions have been proved to be affected by external stimuli. Low-Level-Light Therapy (LLLT) using near-infrared photons is one of the effective ways to modulate the hemodynamic activities in the brain. However, the biphasic hormetic dose-response where bioenergetics are stimulated at a low dose and inhibited at a high dose is well observed in all photon stimulations. The amount of photon energy delivered to the brain are affected by the wavelength as well as the multilayered head structure with variations of optical parameters (OPs). A real 3D volume head model is built for each participant in this study, and the boundary conditions of each OP in each layer is considered. The Monte Carlo simulation with wavelengths ranging from 650 nm to 1064 nm is implemented to investigate the energy delivered to the brain under different radiation profiles. Results show that 1064-nm photons penetrate deeper than 810-nm photons except for scalp absorption at the lower bound due to low melanin content. Collimated-beam radiation is better than diverging-beam due to a more uniform intensity distribution at the scalp surface. Further research to optimize LLLT dosage for each individual is imperative due to the high inter-person variability in structure and OPs.