Research On Magnetic Induction Magnetoacoustic Three-dimensional Electrical Impedance Imaging Based On Rotating Magnetic Field

Magnetoacoustic tomography with magnetic induction(MAT-MI)is a new electrical impedance imaging technique for biological tissues,which can detect the changes of electrical impedance inside object and reconstruct magnetoacoustic or impedance images.MAT-MI exhibits the advantages of high spatial resolution of ultrasonic imaging and good contrast of electrical impedance tomography(EIT),which has broad application prospects in the field of medical imaging and diagnostic analyses.By combining the theory of acoustic dipole radiation and reception directivity of ultrasonic transducer,the formulae of magnetoacoustic pressure and waveform detected by the transducer are deduced based on the basic principle of MAT-MI.The influence of transducer directivity on magnetoacoustic detection and image reconstruction is analyzed.Aiming at the weakness of low-level waveform and signal-to-noise ratio(SNR)in three-dimensional imaging using the traditional cylindrical scanning,a new three-dimensional conductivity imaging method using rotating magnetic fields for two-dimensional scanning of MAT-MI is proposed.By applying coordinate transform for magnetic field rotation with the two-dimensional conductivity reconstruction algorithm,three-dimensional conductivity reconstruction inside the object is realized based on the magnetoacoustic detection with a strong directional transducer,successfully eliminating the influences introduced by shape and size of object.With the traditional cylindrical scanning structure,MAT-MI simulations and image reconstruction are conducted for a two-layer cylindrical model.Experimental measurements are also performed for a cylindrical metal model with high conductivity.The theoretical and experimental results show that MAT-MI can effectively distinguish conductivity changes at tissue boundaries,which shows the application feasibility in non-destructive testing and medical measurement.Then,numerical simulations for a double-layer eccentric spherical phantom model are carried out for the transducers with different directivity.It is proved that the transducer in large diameter with strong directional directivity can improve the quality and accuracy of reconstruction images with suppressed image artifacts,which provides the basis for transducer selection in MAT-MI applications.By applying rotating magnetic fields,three-dimensional image reconstruction is conducted for double-layer eccentric spherical phantom model using a strong directional transducer,and also compared with the results of the traditional cylindrical scanning method.The favorable results show that the SNR of the magnetoacoustic detection is significantly affected by the size and shape of the object in cylindrical scanning,which is not applicable to the measurements of three-dimensional image reconstruction for the object in arbitrary shape.Through the rotation of the scanned layer,three-dimensional conductivity reconstruction can be realized by the rotating superposition of two-dimensional MAT-MI scanning,effectively elimimating the weakness of low-level signal produced by the small size of the object in the scanning layer.Based on the two-dimensional MAT-MI scanning,the proposed three-dimensional conductivity reconstruction algorithm with rotating magnetic fields provides a new imaging modality for accurate measurement and electrical impedance imaging,which exhibits great significance for the practical application of MAT-MI.