| Background and objective:Photobiomodulation (PBM) therapy is a promising and noninvasive approach to
stimulate neuronal function and improve brain repair. The optimization of PBM parameters is important to maximize
effectiveness and tolerability. Several studies have reported on the penetration of visible-to-near-infrared (NIR) light
through various animal and human tissues. Scientific findings on the penetration of PBM light vary, likely due to use
of different irradiation parameters and to different characteristics of the subject such as species, age, and gender.
Materials and methods:In this article, we review published data on PBM penetration through the tissues of the
head in both animal and human species. The patterns of visible-to-NIR light penetration are summarized based
on the following study specifications: wavelength, coherence, operation mode, beam type and size, irradiation
site, species, age, and gender.
Results:The average penetration of transcranial red/NIR (630–810 nm) light ranged 60–70% in C57BL/6 mouse
(skull), 1–10% in BALB/c mouse (skull), 10–40% in Sprague–Dawley rats (scalp plus skull), 20% inOryctolagus
cuniculusrabbit (skull), 0.11% in pig (scalp plus skull), and 0.2–10% in humans (scalp plus skull). The observed
variation in the reported values is due to the differencein factors (e.g., wavelengths, light coherence, tissue
thickness, and anatomic irradiation site) used by researchers. It seems that these data challenge the applicability of
the animal model data on transcranial PBM to humans. Nevertheless, two animal models seem particularly
promising, as they approximate penetration in humans: (I) Penetration of 808 nm laser through the scalp plus skull
was 0.11% in the pig head; (II) Penetration of 810 nm laser through intact skull was 1.75% in BALB/c mouse.
Conclusions:In conclusion, it is worthwhile mentioning that since the effectiveness of brain PBM is closely
dependent on the amount of light energy reaching the target neurons, further quantitative estimation of light
penetration depth should be performed to validate the current findings. |