Monte-Carlo Simulation Of Laser Energy Profile In Skin Model

Port-wine stains are benign congenital malformation,characterized by ecstatic dermal blood vessel,which increases in size with increasing age.Most of them present on the face and neck,but can be located anywhere on the body.Generally,the treatment of Port-wine stain by laser couldn't remove the lesion through single period.At the same time,there could be a lot of side effects,like scaring,hyperpigmentation,as well as hypopigmentation. Eventually,most of the lesions couldn't be blanched completely.Therefore,port-wine stain is not cured satisfactorily.And it is one of the challenges which medical laser has been in front.Base on the Photothermolysis,pulsed laser with appropriate wavelength could induce selective destruction of tissue,cell or organdie.The energy deposition induced by laser in objective tissue dominates that if the objective tissue could be destroyed.Therefore, mathematic model was constructed to simulate the interaction between laser and tissue by Monte-Carlo method,and improved tissue geometry was used in the simulation,which cylinder vessel embedded in multi-layered tissue model.A stochastic model is constructed in which the expected value of a certain random variable(or of a combination of several variables) is equivalent to the value of a physical quantity to be determined.The computer gives a pseudo-random number to evaluate the variable.Then the free step size is computed and the interaction site is recorded in the Cartesian coordinate system.The deflection angle and azimuthal angle were sampled in a moving spherical coordinate system,at the same time an assistant Cartesian coordinate on the surface of vessel is used to calculate the reflection and refraction event on the interface between the dermis and the vessel.At the end,the energy profile in the tissue was discussed.The simulation reveals that the maximal energy density is located the upside of the vessel which is closed to the Laser,and the energy density profile of the section of the vessel was decreased acutely along with the direction that the beam transmitted into the surface of the tissue when the short wavelength was injected,such as 532nm,577nm,585nm.On the vessel section when the energy density is reduced into a half of the maximal,the attenuation distance was 10-17μm,and the attenuation distance when it reduced to 1/e is 13-26μm.And at center of the vessel section,along with the axis that beam propagated,the attenuation rate is corresponding to the absorption coefficient of the blood at the wavelength;on the outline of the vessel section,the half attenuation distance on which the energy distribution along with the increased depth is 34-43μm,the upper energy area covers a small part of the section. However the energy profile of 595nm is different,the maximal energy density is less than others wavelength,and the difference between the upside and the bottom of the section is more tiny,the half attenuation distance and 1/e attenuation distance is 76μm and 96μm.Take the energy density as the height,the energy profile of section is an uneven surface comprised of different exponential attenuation curves,and the surface is cupped,the more little absorption at the wavelength,the surface is more even,when 595nm was used,the surface almost is a inclined plane.Monte-Carlo method is used for modeling the interaction between laser and skin with discrete vessel model,could learn about the details of the energy profile of the vessel;with changing size and depth of the vessel could be adapt to multiple tissue model,makes for comprehending energy distribution in different tissue with special structure.It is comparatively compliant.