The Design Method Of Gradient Coils In Permanent MRI

More than one hundred years ago, Roentgen discovered X-ray. Ever since then a variety of medical imaging technologies have come out and have achieved unprecedented development. Medical treatment level has also been improved greatly. Magnetic resonance imaging (MRI) is an emerging medical imaging method, which imposing a RF signal to the human organs in a static magnetic field, acquiring the resonance signals of hydrogen atomic nuclei in the tissues and organs and achieving image reconstruction through the signals, according to the principle of nuclear magnetic resonance. MRI is widely used in the field of modern medical treatment with the advantages of multi-parameter imaging, without injury to the human body and higher resolution in soft tissues. Gradient coil system is an important component of the MRI system, with the functions of supplying a linear gradient magnetic field to the MRI signals, which can realize the spatial positioning, phase encoding and frequency encoding. And its performance will directly affect the MRI imaging speed and image quality. So the designing the gradient coils with a rational structure and good performance is of great significance to improve the overall performance of MRI systems and the quality of MRI images.Because the research work on MRI theory are started late in our country, we have lagged behind in this technology field. At present, most MRI equipments need to be imported from foreign companies. Although some enterprises have already developed a number of products in the field of permanent magnet-type magnetic resonance system, but the products are still with a lack of sufficient competitiveness in core technologies. To improve the capability of MRI system made in China, this thesis has a deeply research on the design methods for gradient coil system, and complete a lot of work in permanent magnet-type magnetic resonance system and finish the design of a gradient coil system for open permanent magnet-type magnetic resonance system. The investigation contents include two parts: longitudinal and transverse gradient coil design, longitudinal and transverse shielded coil design. Primary works are described as the following:(1) Gradient coil design methodThis thesis does lots of research on gradient coil design method, according to the classical electromagnetic theory. Based on the analysis and comparison of previous methods, this thesis applies the wire separation method to the gradient coil system design and achieves some improvement to the design. Depending on the different design objective of gradient magnetic field, the design process is divided into two parts: longitudinal and transverse gradient coil designs. The application result of the design method in this thesis demonstrates that we can get simple longitudinal and transverse gradient coil structures with good performance.(2) Self-shielded coil design methodEddy currents will be caused in ambient conducting parts because of the rapid switch speed of current pulse in gradient coils. The desired fast rise of the gradient magnetic field is suppressed by the eddy currents, so that the imaging process is affected. To decrease the influence of the eddy currents, a design method of self-shielded gradient coils is studied and the self-shielded gradient coils are designed in this thesis. The application result of the design method demonstrates that we can get simple longitudinal and transverse self-shielded coil structures with decreasing eddy current effectively and maintaining the performance of gradient magnetic field.