| Magnetic Induction Tomography (MIT) detects the eddy current magnetic field on theboundary of the imaging area based on electromagnetic induction method, and gets thedetecting object tomography of the conductivity distribution by the reconstructionalgorithm. It meets requirements of a modern medical device that is contactless,non-invasive, and imaging-functional. However, it has not yet been used in the clinicalpractice due to the limitation in imaging resolution, so high precision scanningmeasurement methods and high resolution image system are still in the research phases. Inthis paper, the research on theory modeling, system design, forward and inverse problem ofmagnetic field, and imaging method was carried out, which aims to provide solutions to thekey problems inherited in MIT, and a sector array MIT system is presented in this paper,which includes single excting and multichannel harmonic resonance detecting systemstructure, integrated data acquisition system, data processing methond and a new nolinearback-projection reconstruction algorithm for the system. The main work as follows:(1) The distribution and the propertis of the eddy currents and the magnetic fieldsinduced by the imaging object may provide important message for the MIT system design.This paper simulated the3-D MIT model and mearsuring expriments according to the brainconductivity distribution, and studied on the electric field distribution of detecting erea, theeddy current magnetic field with the different object, the change in magnetic field andboundary mearsuring data.(2) For the induction signal is feeble and corrupted badly, it allows separation of thedesign of detecting coils and exciting coil, and adopts the large coil to get strong incentivemagnetic field, and small detecting coils to get the high accuracy of position and smallinterference. Meanwhile harmonic resonance detecting method is adopted to enhance theexciting magnetic fields and improve the stability due to the frequency selectivity of thedetecting coils. In the process of signal detecting, the reference signal is separated frominduction signal, and transmits to phase discriminator circuit byindependent channel, and itimproves the accuracy of phase discriminate signal. (3) A multichannel array signal acquisition system is designed to obtain measurementdata used for image reconstruction, and we design the detecting coils semicirculardistribution opposite the exciting coil. The relative rotational scanning method is used toget multiangular measurement data in the system. The raw detecting data is interpolated,adjusted, and standardizated, and got the reconstructed data.(4) The magnetic field of imaging area in MIT is nonlinear, with increase thedifficulty of image reconstruction and leads to the poor positioning accuracy. The fastimage reconstruction algorithm for sector MIT system and its implement method wasstudied in this paper based on back-projection algorithm. A new data standardizationmethod is presented according to the measureing data sensitivity with the conductivitychange in the imaging area, and then an improved back-projection image reconstructionalgorithm fit for the magnetic induction tomography is presented, based on the theory thatthe phase difference can be described as the projection of the conductivity distributionalong the magnetic field lines. The back-projection path was determined by the magneticfield lines in the imaging area, and then a complete set of sector multichannel MIT systedis enstablished. |
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