| Breast cancer has a high incidence and high lethality,but early detection can remarkably reduce its lethality.In recent decades,microwave thermoacoustic imaging（MTI）technology,which assemblies the superiority of the high spatial resolution of ultrasound（US）imaging and the high spatial contrast of microwave imaging（MI）,has received more expectations in early breast cancer screening and is considered a prospective non-destructive imaging technique for early detection of breast cancer.In MTI of breast cancer,variations in the dielectric and acoustic properties of different mammary tissues are the source of their contrast,while the distribution and propagation of externally excited microwave fields and internally generated microwave thermoacoustic（MTA）fields are the key to the quality of MTA images.The joint microwave and US multi-physics field analysis can help provide the optimal microwave parameters required for obtaining high-resolution and high-contrast MTA images,as well as US detection methods.In addition,experimental studies have shown that MTI is useful for accurate positioning of the needle tip in breast cancer puncture biopsiy（PB）.In this thesis,the correspondence between puncture needle diameter and length and positioning accuracy is investigated in detail based on the simulation,which provides a theoretical basis and reference for more accurate MTA puncture needle tip positioning.For the purpose of these unresolved issues mentioned above,the main research of this thesis can be elaborated as follows:1.Based on the theory of MTI of breast cancer,the generation and propagation process of MTA signal is analyzed,the 3-D simulation model of MTI of breast cancer is well established by the utilization of a finite element method（FEM）software,COMSOL.The coupling and transformation of electromagnetic heat,thermal expansion and pressure wave propagation are simulated and reverted respectively.2.Using the above-mentioned MTI model of breast cancer,the spatial distribution features of electric field,temperature field,displacement field and acoustic field in different tissues are simulated and investigated,and the simulation results are evaluated in detail to determine that the amplitude distributions of each physical field of the tumor is strong.It has the maximum temperature rising of 0.023 m K,the biggest displacement of 1.65×10-8 mm,and the highest pressure rising of 3.22×10-3 Pa.Furthermore,the variation charts of time and frequency domain simulations were analyzed.It was found that the changes in tissue displacement lagged behind the changes in temperature and pressure,and the MTA response at the tumor site was faster,and the frequency of the MTA signals generated by it was the highest,which is 24 k Hz.3.The influence of external microwave excitation on the MTI process was suffered relevant investigation.Using COMSOL software,different types of microwave pulses with different pulse widths were used to excite breast cancer tissues.Through simulation researches,the rectangular pulse has the highest energy efficiency,the energy efficiency of Gaussian,sine and triangular pulses is 0.39,0.61 and 0.48 times higher respectively.The larger the pulse width,the stronger the MTA signals generated by the tissues.4.The influence of tumor acoustic characteristics on MTA process was also suffered relevant investigation,and breast cancer models with different sizes and Young’s modulus were constructed using COMSOL software.It was found that the MTA signal amplitude and frequency of the tumors were positively correlated with its Young’s modulus（YM）.The MTA signal frequency of the tumor increases with the YM and linearly becomes larger with a scale factor of about 1.4.5.It is proposed that the MTA response characteristics of the puncture needle are equivalent to a half wave dipole antenna,and verified by using the simulation software,CST.A model of microwave antennas with different operating frequencies irradiating puncture needles with different lengths was established,and the surface specific absorptivity distribution and energy coupling efficiency of the puncture needle were obtained.Finally,it was determined that the length of the puncture needle should be half of the excitation wavelength under the optimal needle tip visualization condition.This thesis not only theoretically simulates the realization of 3-D breast cancer MTA process,but also further investigates the influence of microwave and biometric multi-physical parameters on MTA images,which provides more possibilities for the realization of MTI technology for early detection of breast cancer in the clinical setting.In addition,this thesis also solves the problem of MTA visualization of the needle tip in breast PB,which will have a contribution to the enhancement of exactness and productivity of breast cancer PB in the clinical setting. |
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