Transcranial photobiomodulation (t-PBM), which delivers red/infrared light to the target regions of the brain responsible for emotion regulation, has been proven to be an effective and safe technique for the treatment of MDD. Here we create 18 5-layer head models from neurodevelopment MRI brain atlas ranging from 5 to 89 years old and perform Monte Carlo simulations to derive the energy deposition for two extracranial source positions that are commonly used in clinical treatment--F3-F4 and Fpz in 10-20 EEG system.

In the results, we show a decreasing trend of energy deposition with the increase of age for all parcellations. The increasing scalp and skull thickness is found to be primarily responsible for the energy decline. Furthermore, the Fpz position is less efficient than F3-F4 position in delivering light to the target region. The exposure time for Fpz position to achieve the effective fluence at target region per treatment session is 10-fold longer than the F3-F4 position. The long distance between the source and the target region is the main cause of the low efficiency of Fpz position. Finally we demonstrate there is an inversely proportional relationship between the exposure time and the skin irradiance.