Numerical Simulation Research On Dielectric Properties Of Biological Tissues In Teraherz Band

Terahertz wave has no damage on biological tissue, and has a better ability to penetrate. When terahertz rays used for monitoring biological tissue, analyzing its structure and composition, it can overcome many problems raised by usual visible or ultraviolet light such as the image blurring problem and weak analysis signal. The research on dielectric properties of biological tissue in terahertz band is of important medical application. Many dielectric properties of biological tissues in terahertz band is still unknown. This thesis studied on the model that describes the dielectric properties of biological tissue, including the following aspects:The thesis provides a brief introduction of the terahertz science and terahertz time-domain spectroscopy technology. Microscopic mechanism of dielectric polarization, macroscopic description of polarization and dispersion and relaxation characteristics of biological tissue have been described in detail. The thesis also presents the detailed description of double Debye model and Cole-Cole model.Debye model can describe the dielectric properties of skin tissue in terahertz band. The existing extraction procedures of nonlinear least squares could generate the double Debye model of skin tissue, but the extraction procedures need an exact initial value of the parameter, the model track the measurements poorly at low frequencies. The Genetic Algorithm had adopted for data fitting in this paper to extract the parameters of double Debye model of skin tissue. There was no need for Genetic Algorithm to set initial value of the parameter because it could generated possible initial parameters randomly, which was simplified parameters extraction process. The search process was started from a number of possible parameters, rather than a single initial value. So it could be uneasy to fall into local optimum, and achieve the global optimization. The model optimized by Genetic Algorithm not only reduced the deviation at the low frequencies from0.15THz-0.6THz, but also was consistent with the original data at the whole frequencies. The results confirm the effectiveness of the method and viability of the double Debye model.The study on dielectric properties of biological tissues in terahertz band is comprehensive. There are large numbers of experimental data, and the fourth-order Cole-Cole model can describe the dielectric properties of biological tissues in 10Hz-100 GHz band. In order to obtain dielectric properties of biological tissue in the terahertz band, the fourth Cole-Cole model was modified, so that the model could not only describe the modified the dielectric properties of biological tissue in 10Hz-100 GHz band, but also could describe the dielectric properties of biological tissues in the terahertz band. To make the calculation data of modified fourth Cole-Cole model agree with experimental data, the fourth Cole-Cole model increased a ?ki term, and according to the way that the reference used fit the permittivity and conductivity of normal skin tissue in 10Hz-20 GHz and 0.15THz-1.95 THz bands with the fourth Cole-Cole modified model, and the model parameters were extracted. The percentage of relative error between data calculated by the fourth Cole-Cole model modified model and fourth Cole-Cole model was less than 5%. The percentage of relative error between the relative permittivity and conductivity of terahertz normal skin tissue calculated by fourth Cole-Cole model modified model and experimental data was less than 10%. It verified that the fourth Cole-Cole model could describe the dielectric properties. When the fourth Cole-Cole modified model applied in the calculation of brain tissue’s permittivity and conductivity, it can calculate the permittivity and conductivity of white matter and grey matter in 0.15THz-1.95 THz band. It can also used to predict the dielectric properties of other biological tissues.