Key Technology Research On Magneto-acoustic Imaging Based On Acoustic Source Of Lorentz Force Divergence

Imaging by magneto-acoustic coupling effect is a new functional imaging modality for diagnosis of malignant lesions in the early stage. In this way, the tissue is excited by magnetic field energy, and the magnetic energy is transformed to acoustic vibration based on magneto-acoustic coupling effect, whereas, acoustic signal carried the electric information of the tissue is detected by transducer. Then, reconstruction algorithm is conducted to the collected data, and electric conductivity of the tissue could be obtained. The key issue in this method is acoustic source as well as its mechanism of generation and propagation. Since this method involve and cross the research of theories multi-disciplinarily and many kinds of technology synthetically, many problems have not been solved, study still in stage of probing both in theory and practice. Up to now, only the electric conductive boundary of the experimental phantom is reconstructed with the experimental signal, conductive distribution inside the tissue couldn’t be reconstructed yet.In order to study the technology of magneto-acoustic tomography, simulation of theoretical model and physical experiments are performed. At first, model is established with cuboid shape which simulating the acoustic characteristics and electric parameter of the experimental sample. Secondly, considering the characteristics of acoustic transducer, the numerical simulation method is established to solve the direct-inverse problems in electro-magnetic field and acoustic field, according to acoustic source of Lorentz force divergence. After that, the distribution of acoustic source and electric conductivity are reconstructed. In the simulation, the differences are analyzed between the acoustic sources induced by magnetic pulse and that excited by electric pulse current. The analyzing results display that, there is acoustic source inside the boundary obtained in direct problem, which is induced by magnetic pulse, simultaneously, only electric conductive boundary of the model could be reconstructed either excited by magnetic pulse or electric pulse.Next, the experimental system is set up to detect the acoustic pressure signal, also, the SNR and solution of this system is tested practically. For the purpose of comparing the relationship between the exciting signal and acoustic signal, experiment is carried out to impose electric current pulse to samples, including direct copper wire and copper wire circle as well as conductive rubber with the concentric circles structure. In this experiment, while electric current pulse with frequency of 0.7MHz,1MHz and1.3MHz are applied to the samples, acoustic signals are recorded by the detecting system, respectively. After the signal collected, frequency-spectrum method is used to get the amplitude-frequency response of the input/output system, also, the relationship between the exciting signal and acoustic signal are studied. Moreover, correlation estimate method is adopted here to obtain the correlation degree between acoustic signal and transducer response as well as exciting signal, so as to explore the relationship between the acoustic signal and exiting signal together with transducer response. As a result, both of acoustic signals induced in copper wire and copper wire circle get high correlation degree with the exiting signal and transducer response, however, acoustic signal of conductive rubber get poor correlation degree with the other two signals.Last, according to the analyzing results of the acoustic signal, reverse convolution projection algorithm is given to reconstruct the distribution of acoustic source. Adopted the algorithm, the distribution of acoustic sources are obtained with the data of copper wire circle and conductive rubber, respectively. The results suggest that when considering the experimental system error, the reconstructed boundary of acoustic source is basically same as that of the samples. And hence, the results agree with the simulation, which validated that only the boundary could be reconstructed based on acoustic source of Lorentz force divergence.Nevertheless, limited by the bandwidth of transducer, acoustic pressure detected by the system is filtered by the transducer’s characteristics. So, the effect frequency segment of the received signal distribute from200KHz to1.8MHz. On the other hand, since the SNR of the detecting system is not high enough, more noise is involved in the process of reverse convolution, and produce system errors in reconstruction.Furthermore, key technology should be researched to promote SNR of the detecting system promptly. In the next work, how to widen the bandwidth of the transducer should be considered as an important issue. Additional, reverse convolution algorithm with suppressing noise function should be developed to get reconstruction image with high quality. Therefore, distribution of conductivity inside the tissue might be reconstructed in the near future.