Research On Magneto-acoustic-electric Imaging Technology Based On Coding And Rotational Scanning

Magneto-acousto-electrical tomography(MAET),with the advantages of high contrast and high resolution,is a non-invasive electrical impedance tomography based on multi-coupling of magnetic field,acoustic field and electric field,which plays an important role in the diagnosis of early tumors.Due to the low conductivity of biological tissues,the imaging quality is affected by the weak signal and low signalnoise ratio(SNR).Therefore,how to improve the amplitude and SNR of magnetoacousto-electrical(MAE)signal is significant in the conductivity reconstruction.While the shape of actual tumor tissue is usually irregular.When the tissue surface is not perpendicular to the direction of acoustic propagation,the amplitude of the MAE signal is greatly reduced and the boundary of the tissue cannot be reconstructed integrally.Therefore,it is theoretically significant to map the boundary of the irregular tissue by analyzing the cause of tissue deformation and constructing a feasible system in MAET.In this paper,based on the Hall effect and electromagnetic induction law,a general theory of MAE focusing on the irregular shape tissue model is derived combining with acoustic radiation of the directional transducer and the acoustic propagation theory.In order to solve the problem of high-intensity ultrasonic excitation,low amplitude and low SNR in MAE detection technology,the formula of the detected MAE signal under the excitation of a sinusoid-Golay pulse is further derived,and the MAE signal decoded for the orthogonal complementary sinusoid-Golay pulses is realized by introducing a conversion factor.MAE signals generated in a layered gel phantom model are simulated under the condition with various SNR.The side lobes are effectively eliminated by the matched filter with significant pulse compression and superposition,which provide the improved accuracy of boundary positioning and conductivity change.With the experimental setup,the MAE measurement and imaging are performed for a three-layer gel phantom model with the excitation of a single 16-bit sinusoid-Golay pulse,and the influence of code length on the SNR gain of MAE detection is discussed The results show that,compared with the single-cycle sinusoidal excitation at an identical amplitude,the SNR of the detected MAE signal excited by the sinusoid-Golay pulse can be increased effectively by about 6.5 d B,and the SNR gain also increases with the increasing code length,showing the accurate information of boundary positioning and conductivity variation in both amplitude and polarity.The reconstructed MAE image with improved contrast and clarity suggests prosperous perspectives of the sinusoidGolay pulse excitation based MAE technique in the early detection of pathological changes of tissues in biomedical applications.In order to solve the problem of the accuracy in MAET,the characteristics of the MAE on the irregular tissue model are studied,and the root-cause of ill-posed in MAET is summarized.Then a rotary scanning-based MAET is proposed by combining the traditional MAET with the CT imaging.Firstly,the temporal and spatial characteristics of the MAE signal on irregular tissue model are analyzed by the general theory of the MAE detection.Considering the acoustic radiation and the system response function of the directional transducer,the MAE signals generated by the inclined model with various angle excited by the delta signal and 1-cycle sinusoidal wave are numerical simulated to analyze the relationship between the amplitude of the MAE and the incident angle.The two-dimensional(2-D)MAE reconstruction can be obtained by the1-D scanning,which proves that there is an obvious difference between the MAET imaging and the experimental model under the condition of non-perpendicular incidence.Furthermore,the experimental measurement of the inclined tissue model and the copper cylinder model are carried out to prove that the propagation time aliasing caused by wide angle acoustic radiation and the difference of MAE signal amplitude at the tissue boundary under non-perpendicular incidence are the main reasons for the ill condition of MAET imaging.In order to solve the ill-posed problem of MAET,the nonconcentric columnar tissue is established,and the MAE signal(2-D B-scan MAET image)under different rotation angles are simulated theoretically.The MAET image of the cross-section is obtained through the rotation and superposition of the image,then the irregular model was measured by rotating scanning MAET,which reflects the boundary and the difference of the conductivity of the irregular tissue model.The feasibility and validity of the rotary scanning MAET is demonstrated in solving the illposed problem of the traditional MAET.It provides a new technology for MAET in the conductivity reconstruction for the irregular tissue model,which has a good research and promotion value in biomedical imaging.