Key Techniques Of Microwave Inducedthermoacoustic Tomography Andexperimental Research

Breast cancer is the most frequently diagnosed cancer in women and it is a serious threat to human health. Early diagnosis is very important to the treatment and recovery of the patient. Therefore, the new imaging detection techniques that can be applied in the early breast cancer detection attract wide interests of the researchers at home and abroad.Microwave induced thermo-acoustic tomography(MITAT) has the both advantages of microwave imaging and ultrasound imaging as an emerging imaging method. It has great potential in the biomediacl application especially in the early breast cancer detection.Aiming to the mechanism of MITAT and key techniques in the system development,the strategy of combining the numerical simulations and experiments is employed to promote the research. The main research contents of this thesis are as follows:There involves microwave engineering, array signal processing, ultrasound imaging and biomedical engineering in MITAT system. Thus, there are some issues such as the interfere to the weak TA signal due to the strong EM radiation,the interaction between EM and acoustic components and the non-uniform distribution of the microwave power density in the antenna near field.These issues can be solved through optimizing the layout of ultrasound transducers and the microwave radiation subsystem. The performances of the improved MITAT system are verified by the imaging experiments for tissue mimicking(TM) material samples.According to the characteristics of MITAT, an integrated simulation framework is proposed. In the integrated simulation, in order to improve the simulation efficiency and considering of the multiple physical characters, finite integration time domain(FITD)method and k-space pseudo-spectral(PS) method are employed to solve the electromagnetic(EM) and acoustic issues,respectively. The effectiveness of the integrated simulation is verified through the comparisons between the simulations and experimental results. Based on the integrated simulation of MITAT system, an image correction method is proposed. For eliminating the influence caused by the non-uniform distribution of the microwave power density in the near filed of the antenna,the microwave power density distribution in the tomography plane can be obtained by the simulation instead of the complex actual measurements. The TA image can be corrected by utilizing the simulated distribution of microwave power density and the “missing detection”and “false detection”of the system due to the non-uniform power density distribution are eliminated.For solving the insufficient TA contrast between the tumor tissue and the normal glandular tissue caused by the similarity of the dielectric properties, the feasibility of carbon nanotubes(CNTs) being used as imaging agents is evaluated. The influences of CNTs to the dielectric and acoustic properties are investigated through the measurements for the TM samples with different CNTs contents. Meanwhile, it is founded that the 1%weight concentration of CNTs can increase the TA response more than one times through evaluating the contrast enhancement of different CNTs concentrations. Considering of the other imaging applications of CNTs and the research contents in this thesis, CNTs has the potential to be developed as the TA imaging agents to overcome the limitation due to the low TA contrast between the tumor and gland and provide basis for the further development of MITAT.Different breast samples including normal adipose tissue,benign fibroma and tumor samples in different clinical stages are employed to perform the TA imaging experiments in the established MITAT system and the TA responses and images of these samples are obtained. In these experiments, multiple sample forms including single sample, multiple samples, hybrid sample and tumor sample embedded in the normal tissue are utilized to verify the imaging ability of the established MITAT system and these experiments provide early verification for the clinical application. Meanwhile, in order to improve the image resolution of the MITAT system, TA imaging with high-power short microwave pulse which is utilized as the radiation source is researched. The characteristics of the TA signals excited by the current microwave source and the nanosecond level microwave source are compared. The results show that the high-power short pulse can effectively overcome the contradiction between the energy accumulation and the image resolution.Under the premise of the signal to noise ratio(SNR) of TA signal is satisfactory, high-power short pulse can extend the frequency spectrum of TA signals and increase the resolution.In this thesis, system development,integrated simulation and image contrast enhancements are investigated. An image correction method is proposed and the feasibility of CNTs being used as the imaging agents is verified. Meanwhile, the potential of MITAT to the breast cancer detection is proved through the imaging experiments for the real ex-vivo samples. The contrast enhancement of CNTs is evaluated and the feasibility of the CNTs is used as imaging agents is proved. Finally, the experimental results show that TA imaging with high-power short pulse has advantages in the spectral bandwidth and predict the further developing direction of MITAT.