Quantifying 808 nm Laser Penetration in Live Mouse Brain: A Comparison of Photodiode and Fiber Probe Methods for...

Accurate dosimetry is critical for the safe and effective application of transcranial photobiomodulation (tPBM), particularly when targeting deep brain regions. This study quantitatively evaluates 808 nm light transmittance in mouse cranial tissue using direct photodiode detection and a fiber-optic probe coupled to a photodiode, and further assesses depth-resolved attenuation using a fiber-optic probe in the live mouse brain tissue.

Detector-comparison experiments were conducted in freshly prepared mouse head specimens, and depth-resolved measurements were performed in vivo under physiologically preserved conditions using anesthetized C57BL/6 mice.

The photodiode detector collected an estimated 8.6-fold higher optical signal than the fiber optic probe under the present setup. This difference is attributed to the fiber's limited numerical aperture and reduced acceptance angle in tissue. Additional depth-resolved measurements revealed an exponential decline in transmittance, with significant attenuation observed in deep brain regions.

These findings emphasize the need for calibration when using fiber-based detection and provide essential parameters for optimizing tPBM treatment protocols. This work contributes to improving transcranial PBM dose accuracy for mouse models.