Innovative Microwave Thermoacoustic Imaging Techniques And Its Experimental Research

When the tissue is irradiated by microwave,the microwave energy is converted into mechanical waves by thermal expansion and other effects.The method of reconstructing the internal features of the tissue by collecting the mechanical waves is called microwave thermoacoustic imaging.It can be seen from the generation process of microwave thermoacoustic signal that thermoacoustic signal is a physical quantity carrying dielectric parameters and mechanical properties of the tissue.It can be seen that microwave thermoacoustic imaging technology is a new multi-physical field imaging technology which combines the characteristics of high resolution and high contrast of ultrasonic imaging technology.However,at present,on the one hand,microwave thermoacoustic imaging technology is limited by the uneven distribution of microwave field,and it is unable to achieve accurate reconstruction of structural information and functional information and lacks of effective cross-validation method.On the other hand,the signalto-noise ratio of thermoacoustic signals is often low,which requires sacrificing the temporal resolution to improve the signal quality.In addition,some literatures have shown that: on the one hand,the dielectric properties of diseased liver tissues are quite different from those of normal liver tissues.On the other hand,its mechanical properties are quite different from normal liver tissues.Based on the microwave thermoacoustic imaging technology and the characteristics of liver diseases,it is very suitable for the diagnosis of liver diseases.Chronic liver diseases such as fatty liver disease are a cause of liver cancer.Liver cancer is a kind of cancer with high death rate and high death toll,and this phenomenon is particularly serious in China.So the realization of fatty liver screening and monitoring is a very important thing,is also an important means to prevent liver cancer to improve people's health.Similarly,blood vessels and blood,as a kind of tissues that maintain life and function normally and are related to many diseases,have great differences in dielectric properties between normal and diseased tissues.Therefore,it is meaningful to realize the imaging of blood vessels and blood based on microwave thermoacoustic imaging technology.Based on the above problems,this dissertation will carry out new microwave thermoacoustic imaging technology and experimental research from the aspects of imaging algorithm,imaging system optimization,fatty liver grading and angiography.The specific contents can be summarized as:1)Quantitative conductivity reconstruction technology and quantitative elastic modulus reconstruction technologyAiming at the problems of the existing quantitative conductivity reconstruction methods,such as the low stability of the algorithm,the complexity of the method and the small effective calculation area,the quantitative conductivity reconstruction system is reconstructed by the two-dimensional waveguide model,the average dielectric constant and the adaptive algorithm.The performance of the new conductivity reconstruction system is discussed by simulation and simulation experiments.As a mechanical wave,the propagation process of thermoacoustic signal is affected by the mechanical parameters of the tissue and is reflected in the phase and amplitude of the signal.Without considering the sound loss,the difference of volumetric modulus of elasticity is considered as the physical quantity that leads to the change of signal.Then the volumetric elastic modulus is constructed in the wave equation and solved quantitatively with the energy loss density as unknown parameters.The elastic modulus and energy loss density can be reconstructed simultaneously.In addition,the elastic modulus,as an intrinsic property of the tissue,is not affected by the distribution uniformity of microwave field.Therefore,the reconstructed elastic quantity can remove the influence of microwave field distribution interference.The performance of quantitative elastic modulus reconstruction technique is also discussed from simulation and simulation experiments.2)Imaging system optimizationThermoacoustic imaging technology is limited by signal to noise ratio(SNR).As a result,the temporal resolution of microwave thermoacoustic imaging is often not high.In order to solve this problem,on the one hand,the energy loss of specific absorber is increased by means of microwave regulation,so as to improve the signal-to-noise ratio.On the other hand,the parallel circuit is used to superimpose the thermoacoustic signal through the multiplex amplifier at the same time,so as to realize the average signal without sacrificing the time resolution.In addition,both thermoacoustic imaging technology and ultrasonic imaging technology are based on the collection of mechanical wave signals for imaging.This facilitates thermoacoustic/ultrasonic dual mode imaging and provides a cross-validation platform for thermoacoustic imaging technology.A set of independent thermoacoustic/ultrasonic dual mode imaging system was built by using the existing equipment in the laboratory and applied in angiography.The imaging of the blood vessels of the human arm is realized through ultrasound imaging and microwave regulation technology,which opens up a new research direction of microwave thermoacoustic imaging technology.3)Liver imaging and fatty liver grading in rabbitsBased on the array probe,a thermoacoustic system was designed to perform in vivo liver imaging in rabbits.The cross validation of liver region was carried out by positioning crochet through eddy current effect.The imaging region was proved to be liver.Based on the above results,the semi-quantitative fatty liver grading method was firstly established by the simulation model,and then the feasibility of the method was proved by the fatty liver model rabbit experiment and pathological results.In this dissertation,aiming at the shortcomings and problems of microwave thermoacoustic imaging technology,quantitative imaging technology and system optimization were studied,and the application of microwave thermoacoustic imaging technology in liver and angiography was explored.