Analysis Of Magnetic Induction And Magnetic Imaging Of Acoustic Signals

The electrical impedance of biological tissues varies greatly with the changes of type, structure, moisture content, temperature, and pathological condition. This property can be used to distinguish the physiological and pathological state of biological tissues, which can help us to get the pathological changes of the tissues and provide direct basis for the early diagnosis or treatment of cancer. Magnetoacoustic tomography with magnetic induction (MAT-MI) is a recently proposed imaging modality to image the electrical impedance of biological tissue. In MAT-MI, we apply a time-varying pulse magnetic field on biological tissues in the static magnetic field to induce eddy currents in the object. Then, ultrasound waves produced by the Lorentz force induced tissue vibration can be generated due to the interaction between eddy currents and the static magnetic field. Through image reconstruction algorithm for the received acoustic signals, two-dimensional tomographic images can be reconstructed, which is relating to the conductivity distribution of the biological tissue.Firstly, the mechanism of source generation for MAT-MI has been studied and the formulae of internal source with uniform conductivity distribution and boundary source with conductivity saltation have been put forward. With a concentric cylindrical model for different model radius, transducer surface radius and model conductivities, the ratios of acoustic pressure and wave cluster for boundary/internal sources have been simulated to make comparison for sources. The simulation results coincide well with experimental results and the acoustic dipole radiation based theory has been demonstrated for wave cluster analysis. Because the boundary magnetoacoustic signal is much greater than the internal, the received acoustic signal can be considered as the boundary magnetoacoustic signal produced by conductivity saltation.Secondly, based on the theory of time reversal, the acoustic sources have been derived with traditional reconstruction formulae for the received acoustic waveforms. Based on the acoustic dipole radiation theory, a tomographic conductivity reconstruction algorithm of MAT-MI has been proposed for a cylindrical measurement configuration. Simulation results show that not only the configuration but also the inner conductivity can be reconstructed, and the spatial resolution is greatly improved without the limitation of acoustic vibration.Finally, a MAT-MI experiment has been carried out for a square aluminum shell model. The comparison between the experimental and the numerical simulation results has been conducted. The existing problems and the insufficiency of the experiment have been discussed and further improvements have been pointed out. The favorable results suggest the feasibility and verification of the acoustic dipole radiation based theory of MAT-MI.