NIR light penetration depth in the rat peripheral nerve and brain cortex.

Near infrared (NIR) light energy has been used in medical applications both for diagnostic and treatment purposes. A priory knowledge of optical tissue properties is necessary in these applications; not only of human but also in animals for testing of devices. However, published data on the optical properties of neural tissue in rodents are rare. The aim of this study was to measure the penetration depth of light into the rat peripheral nerve and brain cortex at NIR wavelengths. Penetration depth was calculated from measurements of transmitted light for various thicknesses of the neural tissue. We found the penetration depth in the rat sciatic nerve to be 0.35 +/- 0.023 mm and in the white matter 0.35 +/- 0.026 mm. The penetration depth of the gray matter was 0.41+/-0.029 mm. Compared to the data reported in literature for the human brain, the rat peripheral and the brain cortex attenuate the NIR light much more strongly.