Simulation Of Tissue Temperature Of Multiphoton Absorption Based Spatially Selective Photothermolysis

Spatially selective photothermolysis based on multiphoton absorption irradiates tightly focused near-infrared high power femtosecond laser on target tissue to denature it by increasing temperature.The spatially selective photothermolysis breaks the limitation of traditional laser therapy,which usually requires the absorption of target tissue is higher than the surrounding tissue.Simulation of temperature distribution of spatially selective photothermolysis has not been reported so far.In this study,we simulated the temperature distribution of spatially selective photothermolysis with finite element method.First,we built skin model including epidermis and dermis.The effects of laser power,irradiation time on temperature distribution were investigated.The results show that the temperature at irradiation point increases to 147? with 200 m W laser irradiation.Furthermore,the temperature increases with laser power.The temperature distribution with irradiation time from 1ms to 10 ms was also simulated.The results show that the area with increasing temperature is smaller than 20 um when the irradiation time is 1ms.The areas will increase to 32 um and 48 um when the irradiation time increases to 5ms and 10 ms,respectively.Second,we built brain model including gray matter and white matter.The effects of laser power and laser modulation mode on temperature distribution were explored.The results show that the temperature distribution of brain model is similar to that of skin model.However,the temperature of brain model is far lower than that of skin model irradiated with same power laser.Moreover,the increasing temperature area of brain model is bigger than that of skin model,which indicates that brain tissue should be treated with more delicate light dose in order to keep the surrounding normal tissues intact.The increasing temperature area will decrease by increasing thermal relaxation time to achieve higher accuracy treatment,which is consistent with our preview experiment results.Third,we built two brain models with one blood vessel and two blood vessels,respectively.Laser attenuation by scattering was introduced into the heat source item in the heat transfer function to simulate the temperature of blood vessel located at different depth.The blood perfusion rate was also introduced into the heat transfer function.We studied the effects of blood perfusion rate and blood vessel size on the temperature distribution and analyzed the relationship between depth of blood vessel and temperature distribution.This study reveals the temperature distribution of spatially selective photothermolysis,which will benefit the application of spatially selective photothermolysis on laser therapy and building disease model.