Abstract
Modern studies of the penetration of light into biological tissues is becoming very important in various medical applications. This is an important factor for determining the optical dose in many diagnostic and therapeutic procedures. The absorption and scattering properties of the tissue under study determine how deeply the light will penetrate into the tissue. However, these optical properties are highly dependent on the wavelength of the light source and tissue condition. This overview paper analyzes the transmission of light through different areas of human and animal head tissues, and the optimal laser wavelength and power density required to reach different parts of the brain are determined using lasers with different wavelengths by comparing the distribution of fluence, penetration depth and the mechanism of interaction between laser light and head tissues. The power variation in different regions of the head is presented, as estimated using Monte Carlo (MC) simulations. Data are analyzed for the absorption and scattering coefficients of the head tissue layers (scalp, skull, brain), calculated using integrating sphere measurements and inverse problem solving algorithms such as inverse MC (IMC) and adding-doubling (IAD). This study not only offered a quantitative comparison between wavelengths in terms of light transmission efficiency, but also anticipated the exciting opportunity for online, accurate and visible optimization of LLLT lighting parameters.
Highlights
In this year, 2020, the laser celebrates its 60th anniversary
Because transcranial laser therapy (TLT) has been shown to be safe in rodents, rabbits and humans under the specific treatment regimen used in translational studies and clinical trials, TLT neuroprotection should be further developed using an “optimized treatment regimen” that should take into consideration the architecture, anatomy and substantial barriers associated with the human scalp, skull and brain [20,21,22,23,24]
The results showed that the measured beam bandwidth reduced from 5.2 ± 0.3 to 2.0 ± 0.2 mm; and the scattering coefficient decreased nearly three-fold after the treatment of skull optical clearing solution (SOCS) on skull [37]
Summary
2020, the laser celebrates its 60th anniversary. While in the beginning, it was a solution looking for a problem; it is widely spread in industry, science, medicine. We will discuss several recent experiences, including the optical permeability of the human and animal head tissues, such as brain cortex, cranial bone and skin which has been estimated, aiming to the transcranial light applications, such as brain imaging or therapy. Measurements of the optical properties (diffuse reflectance, total and collimated transmittance) of brain tissues in healthy rats and rats with C6-glioma were performed in the spectral range from 350 to 1800 nm [6] It was obtained that the 10 day development of glioma led to increase of absorption coefficient, which was associated with the water content elevation in the tumor. This study offered a quantitative guide for optimization of the light stimulation parameters for medical treatments [7]
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