Optical properties of neonatal rat head tissues in the visible and near-infrared spectral regions
Perinatal asphyxia is a condition responsible for a significant portion of neonatal deaths worldwide. The oxygen deprivation caused by this insult can lead to neurological sequelae in survivors, including impairment of motor and cognitive functions. On the other hand, recent studies have shown that photobiomodulation therapy can promote the protection and restoration of neuronal tissue. With the rise of this new therapeutic approach, a need arises to study the interaction of light with cranial and brain tissues in neonatal animals in order to improve light application methodologies. Therefore, the objective of this project is to investigate the interaction of light with the head tissues of neonatal rats during transcranial laser application emitting near-infrared radiation. In this study, the light distribution in the heads of 7-day-old neonatal rats was evaluated during laser application at wavelengths of 660 nm (red) and 808 nm (near-infrared), positioned over the animal's bregma region. Lambert-Beer's law was applied to images obtained at a 90º angle from the irradiated surface. Additionally, analyses were performed using a spectrophotometer coupled with an integrating sphere, acquiring diffuse reflectance and transmittance spectra between 350 nm and 1200 nm from three different tissues present in the animal's head (skin, bone, and brain). Absorption, scattering, and attenuation coefficients were calculated using the Kubelka-Munk model. The coefficients obtained through the Kubelka-Munk model showed higher scattering in the skin, while higher absorption was observed in brain tissue. When comparing data from two widely used spectral regions, red and near-infrared, it was observed that both interactions were stronger in the red region. Based on the data obtained so far, it can be concluded that radiation attenuation (absorption + scattering) is lower in the near-infrared region.