Study On The Influence Of The Contact Temperature On Near Infrared Noninvasive Diffuse Reflectance Detection

Near infrared non-invasive detection technology is rapid,convenient and noninvasive.It is the main research direction of noninvasive detection in the future.However,under the interference of uncertainty of measurement conditions,it can not be applied to the clinic at present.Especially in contact diffuse reflectance spectrum detection,the contact pressure and temperature of optical fiber probe will affect the accuracy and stability of the spectrum.In this paper,a theoretical analysis and experimental study on the change of the temperature field of the skin layer in the contact process between the probe and the tissue is made,and an optimization scheme of the optical fiber probe is proposed.Finally,the optimization scheme is verified by the experiment of the near infrared diffuse reflectance spectroscopy.In the natural state,there is a certain difference in temperature between the optical fiber probe and the skin tissue.After contact,the temperature field of the skin tissue will be changed,which will affect the accuracy of the spectrum detection.Based on the classical heat transfer theory and the heat conduction equation of Pennes,we established a simplified temperature conduction model of epidermal layer.Then by the establishment of the partial differential equations and method of the finite element analysis,we studies the effect of probe endothermic coefficient and probe temperature on the temperature field of skin tissue.It is found that the greater the probe endothermic coefficient is and the lower the initial temperature of the probe is,the change of the temperature field of the tissue after contact is more intense,and the difference between the temperature and the initial temperature is greater.The temperature field of the tissue is always in the dynamic fluctuation,and the influence of the probe contact on the temperature of tissue not only change the surface temperature of the skin,but also make the internal temperature field fluctuate.When the surface temperature is stable,the internal temperature will continue to fluctuate for a period of time to form a new thermal equilibrium state.Based on the simplified model,a theoretical calculation method for the optimum preheating temperature of the probe is proposed.The optimum preheating temperature is also different under different probe materials and ambient temperature.Then,we build up the temperature detection system to do the experiment.The results of the influence of the material and the temperature of the probe on the surfacetemperature of the tissue were compared,and they were in agreement with the simulation results.Finally,a probe optimization scheme is proposed: the best material of the probe is stainless steel and the best preheating temperature is 31~32? when the environment temperature is 24?.The validity of the optimized scheme is proved by the test of near infrared spectrum detection in vivo: The signal to noise ratio of the spectral data can be stabilized faster;and the coefficient of variation of the spectral data of repeated measurement has been reduced by 2 times.