Effect of Incidence Sites on Light Distribution at Different Wavelengths During Transcranial Photobiomodulation.

Rationale And Objectives: Transcranial photobiomodulation (tPBM) has emerged as a promising noninvasive therapeutic technique for neurological diseases, such as Alzheimer's Disease and Stroke. However, the optimal incidence site for precise stimulation remains unclear. To address this, we aimed to employ the high-resolution Visible Chinese Human (VCH) dataset and Monte Carlo simulation to identify the most suitable incidence site.

Materials And Methods: Monte Carlo model for photon migration in voxelized media (MCVM) was applied to visualize and compare the photon distribution across different incidence sites. We selected four representative incidence sites in the frontal, parietal, occipital, and temporal lobes and simulated photon propagation at four wavelengths commonly used in tPBM studies: 660 nm, 810 nm, 980 nm, and 1064 nm.

Results: For each wavelength, the light source incident from prefrontal lobe had the deepest penetration depth (7 cm, 7 cm, 5 cm, 5 cm for 660 cm, 810 nm, 980 nm, 1064 nm, respectively) and the widest irradiation range (15%, 20%, 13%, 14% of brain for 660 cm, 810 nm, 980 nm, 1064 nm, respectively), while that incident from temporal lobe ensured the highest photon fluence reaching brain parenchyma. When the same light source (the input power was normalized to 1) was respectively applied at four incidence sites, ∼1×10 1/cm of photon fluence reached brain parenchyma for prefrontal lobe, ∼7.5×10 1/cm for parietal lobe, ∼1.5×10 1/cm for occipital lobe, and ∼2.8×10 1/cm for temporal lobe. To achieve similar photon fluence reaching brain parenchyma across all brain regions during whole-brain tPBM stimulation, we recommended setting the input power ratios of light source at four sites as ∼17:280:20:1 (prefrontal: parietal: occipital: temporal) for 660 nm light, ∼22:250:18:1 for 810 nm, ∼60:1450:20:1 for 980 nm, and ∼54:830:17:1 for 1064 nm.

Conclusion: From the perspective of photon delivery to the brain, the prefrontal and temporal lobes were two more optimal locations for light source placement. This study provided a theoretical strategy for optimizing incidence sites in tPBM.