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The Improved Back Projection Algorithm of Magnetic Induction Tomography with Uniform Magnetic Excitation

Author: LiZuo
Tutor: HeWei
School: Chongqing University
Course: Electrical Engineering
Keywords: Magnetic Induction Tomography Helmholtz coils Back projectionalgorithm Mixed-weighted Filtered back projection
CLC: TP391.41
Type: Master's thesis
Year: 2012
Downloads: 40
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Abstract


Magnetic Induction Tomography(MIT)is an emerging bio-impedance tomography.It applies a magnetic field from an excitation coil to induce eddy currents in the materialto be studied, and the magnetic field from these is then detected by detecting coils. MITuses a magnetic field as the excitation source. Compared with CT, MRI and otherimaging techniques, MIT will not harm the human body. It is safe and reliable. MIT usethe magnetic field as the excitation source, excitation coil does not contact with thehuman body. This avoids the impact of contact resistance on the imaging. In addition,magnetic fields can pass through the skull with low conductivity. So, it is very suitablefor imaging the head. MIT has become a hot research topic for the today’s biomedicalimaging field.This thesis aims to the inverse problem of MIT; focus on the image reconstructionalgorithm. By analyzing the traditional spiral coil measurement system, found thatexcitation magnetic field spatial distribution is uneven, and proposed a new MIT systemmodel of using the Helmholtz coil as an excitation coil. Use back projection algorithmfor image reconstruction, and propose some method to improve the algorithm. The maincontents are as follows:(1) Use double sphere model to simulate the head and its lesions. Use the separationof variables method for solving the magnetic vector potential and detection voltage. Thesimulation results show that the linear change of the detection voltage with the electricalconductivity of measured object.(2) Simulation results show that the incentive magnetic field of the traditionalmeasurement system is divergent in space. So, the excitation field in the measured areais not equal, and the projection path for back projection algorithm is curved.(3) Using the back projection algorithm to reconstruct the image of the model. Theresults can accurately reflect the location of the foreign body, but there is a seriousartifact.(4) Proposed to improve the quality of the reconstructed image from the two aspects.First, the methods to improve the image quality are non-algorithm factors. Thesimulation results show that by increasing the number of detection coil and detectioncoil rotary stepper can eliminate image artifacts and improve image resolution. Second,from the angle of correcting algorithm, it proposed mixed-weighted back projection algorithm and filter back projection algorithm. The results showed that, the twoimproved algorithms can eliminate the artifacts of the image, the position of foreigntarget would be also very accurate.(5) It compared the sensitivity and resistance to noise to foreign target’s location, sizeand number with back projection, mixed-weighted back projection and filtered backprojection. The results showed these three algorithms can accurately position foreigntarget. But when the foreign bodies near the edge of the measured region, the foreignbody bias to the edge of the measured region in the reconstructed image withmixed-weighted back projection algorithm. When the size of foreign targets is small, itcan distinguish the difference of different foreign body. When two foreign bodies in themeasured region, back projection algorithm can’t separate the two foreign bodies. Threealgorithms have good noise immunity.(6) Use the actual measurement system for the physics experiment. Reconstruct theimage of single target and dual targets agar models with three algorithms. The resultsobtained and the simulation results are the same.

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