Research On Key Technologies Of Magnetic Induction Tomography

Magnetic induction tomography is a new reconstruction image technology with advantage of non-invasive, non-destructive and without contacting the target object directly, especially suitable for medical application. MIT has positive meaning of prevention and diagnosis for the disease not yet occurred but exist potential danger such as cancer. The magnetic field generated by MIT easily penetrates the human tissue such as skull. Compared to the existing medical image technique such as CT, MRI, PET and ultrasound, it is non-radiation, inexpensive, without implanting drugs in the human body, portable, and long time monitoring easily. Therefore, the research on the lab experimental model is the key for MIT to apply in medical human measurement.For the existing problem of magnetic induction tomography, the thesis does the research on key technologies for MIT based on the theory of measurement model. The main works are as follows:The MIT measurement model is analyzed. According to the own characteristics of MIT, the MIT electromagnetic field equations of the forward problem is derived by Maxwell equations and is transformed by the finite element method. A three dimensional measurement model is established, and the eddy current in the detection domain is calculated. After solving the magnetic flux density from different surfaces and from each axis, the experiment results are analyzed, which provide a strong basis for all aspects of the follow-up study of MIT.The magnetic field generated by the annular excitation coil used widespread in MIT is analyzed. On the application background of the focusing magnetic field, the plane circular spiral coil, conical spiral coil and the lib shape spiral coil are proposed which can produce the focusing magnetic field, and the half spherical array composed of small plane circular coils is also proposed in order to focus more areas in the target object. The measurement model with the target object is established for the focusing excitation coil and the array. The primary field and the eddy current field produced within the target object are calculated and analyzed comparatively. The results show that the proposed focusing excitation coils and the relative array can produce the focusing eddy current field in the target object, providing an effect method for MIT from the eddy current source.The characteristics of eddy current in MIT are analyzed, and the methods of increasing eddy current field that utilize the detection coil to offset the primary field are introduced. It puts forward a method of reducing the primary magnetic field as much as possible and the corresponding detection coil. A variety of methods of offsetting the primary field with single detection coil are compared and analyzed. According to the proposed principle, the model including detection coil is established. The eddy current in the excitation coil and the target object is calculated. After optimizing the distance and the size of the detection coil, signal linearity and sensitivity are tested under this mode.Based on the analysis and derivation of the reciprocity principle, an method of calculating the sensitivity matrix of MIT fast is proposed after expanding the principle to apply in the derivation of the sensitivity matrix of MIT. Experiment and analysis are carried out using the proposed method based on the established model. The sensitivity matrix with single perturbation and multiple perturbations are respectively solved, and the influence of the value of perturbation conductivity to the experiment is analyzed. The results prove that the sensitivity matrix can solve fast and effectively using this method, which bring a great help to MIT image reconstruction.Based on the description of the reconstruction problem of MIT, Gauss-Newton one-step dynamic algorithm in combination with regularization technique is proposed and used to experiment for obtaining optimized solutions of relative parameters. The model is solved by the finite element method, and the obtained sensitivity is applied in the reconstruction algorithm. The method presented in the paper realizes the reconstruction of the absolute conductivity, and it is implemented that adding different ratios of signal to noise to measurement voltage for simulating real measurement value. It proves that the algorithm in the paper gets better effect in precision and speed than others.The low sensitivity in the central region of the target object is always restricting the application of MIT to the real measurement. It analyzes the biological tissue structure of the human brain. On the basis of detecting sensitivity of no perturbation in the target domain with four different cases, the perturbations with the different cases under the same condition are added in the area. Change the conductivity, amount and radius of the perturbations to do measurement experiment and comparative analysis, the results prove that adding perturbation with a suitable value of conductivity for different cases can increase the sensitivity of the central region.It designs and realizes an experiment system for detecting the propagation velocity of electromagnetic field in the target object. It puts forward a detection parameter of the electromagnetic velocity which can reflect the electrical property of the target object based on the principle of the velocity image. The relationship between the propagation velocity of the electromagnetic field and the electrical characteristics of the passed material is introduced and derived. An oscillation circuit of detecting the propagation velocity of the electromagnetic field is designed which measures respectively both without the object and with the water. It gains the oscillation frequency of the circuit, and thus the propagation velocity of the electromagnetic field in the water. The results show that it is feasible to design the oscillation circuit to obtain the propagation velocity which reflects the electrical parameters information of the material through the electromagnetic field.