Near infrared laser propagation and absorption analysis in tissues using forward and inverse Monte Carlo methods

For understanding the mechanisms of low level laser/light therapy (LLLT), accurate knowledge of light interaction with tissue is necessary. In order to have a successful therapy, laser energy needs to be delivered effectively to the target location which depending on the application can be within various layers of skin or deeper. The energy deposition is controlled by input parameters such as wavelength, beam profile and laser power, which should be selected appropriately. This thesis reports a numerical study that investigates the laser penetration through the human skin and also provides a scale for selection of wavelength, beam profile and laser power for therapeutic applications. First, human skin is modeled as a three-layer participating medium, namely epidermis, dermis, and subcutaneous, where its geometrical and optical properties were obtained from the literature. Both refraction and reflection are taken into account at the boundaries according to Snell’s law and Fresnel relations. Then, a three dimensional multi-layer reduced-variance Monte Carlo tool was implemented to simulate the laser penetration and absorption through the skin. Local profiles of light penetration and volumetric absorption densities were simulated for uniform as well as Gaussian profile beams with different spreads at 155 mW average power over the spectral range from 1000 nm to 1900 nm. The results showed that lasers within this wavelength range could be used to effectively and safely deliver energy to specific skin layers as well as to achieve large penetration depths for treating deep tissues, without causing any skin damage. In addition, by changing the beam profile from uniform to Gaussian, the local volumetric dosage could be increased as much as three times for otherwise similar lasers. In the second part of this thesis, a three-dimensional single-layer reduced-variance inverse Monte Carlo method was developed to find the optical properties of the skin using the experimental values of transmittance and reflectance. The results showed that both transmittance and reflectance scale well with transport optical thickness. Moreover, it was also shown that penetration depth is highly sensitive to the laser wavelength and varied within the range from 1.7 mm to 4.5 mm.