Transcranial Near-Infrared Laser Transmission (NILT) Profiles (800 nm): Systematic Comparison in Four Common Research Species.
Level 5 - mechanism / opinion, no new human data
Ex vivo bench study evaluating physical light transmission through animal and human skull specimens.
PubMed 26039354 · doi:10.1371/journal.pone.0127580
What was done
Researchers measured near-infrared laser transmission (800 nm, 700 mW/cm² surface power density) through ex vivo skull specimens from four species: mouse, rat, rabbit, and human. Transmission percentages were evaluated in both dehydrated and hydrated states and compared against skull thickness and anatomical region (bregma and parietal areas).
What was found
In dehydrated animal skulls, transmission decreased as thickness increased: mouse (0.44 mm thickness, 40.10% transmission), rat (0.83 mm, 21.24%), and rabbit (2.11 mm, 11.36%). Hydration significantly increased transmission across animal skulls (p < 0.05) without altering thickness. In human calvaria (mean thickness 7.19 mm at bregma, 5.91 mm at parietal skull), light transmission was 4.18% at bregma and 4.24% at the parietal skull. Hydration in human skulls produced a non-significant 9.2% to 13.4% increase in transmission. Transmission correlated with thickness in human specimens but not with skull density.
Why it matters
Translational transcranial photobiomodulation dosing cannot be directly scaled from small rodents to humans without accounting for dramatic attenuation differences (~40% transmission in mice versus ~4% in humans). Preclinical protocols must account for human skull thickness and hydration properties to achieve biologically relevant brain tissue fluence.
Limits
The abstract does not disclose the exact sample size (number of skulls tested per species). Measurements were conducted ex vivo on bare bone, excluding overlying soft tissue, skin, hair, and underlying dura or cerebrospinal fluid, which substantially alter in vivo photon scattering and absorption.
Cited by
- partial Long-wavelength red and infrared light can penetrate through human bone and skull tissue.