Monte Carlo simulations of a multisource transcranial photobiomodulation helmet device: application to young and aged brains

Transcranial photobiomodulation is an emerging non-invasive technique that uses near-infrared light to stimulate brain function and offer protection against damage. Although this method is safe, there is much uncertainty concerning the optimum dosage, in particular, the quantity of energy that, when applied transcranially from a multisource helmet device, reaches the brain parenchyma and produces a change in neural activity. In this study, we used the Monte Carlo method to simulate a typical session of transcranial photobiomodulation using an existing multisource helmet device and to quantify the energy deposition region by region in the brain. We simulated two commonly used wavelengths, 670 and 810 nm, and examined light propagation in young and aged brains. Our results showed that, from the multisource helmet device, light at both wavelengths could indeed reach superficial regions of the brain parenchyma, up to 3–4 cm beneath the scalp surface. Overall, the 810 nm light penetrated deeper than the 670 nm light. From approximately 1 cm beneath the scalp, the light distribution became almost uniform for both wavelengths. However, regional analysis revealed that light penetrated better into the frontal and parietal lobes than into other regions (e.g., the temporal lobe). Furthermore, the light from this multisource device did not reach deeper structures (e.g., subcortical). Finally, the Monte Carlo simulations from the device revealed only subtle differences between the young brain and the aged brain. In summary, our results contribute to a better understanding of the quantification of regional light deposition during a typical session of transcranial photobiomodulation using a multisource helmet device.