Research On Gradient Coil Design For Open Magnetic Resonance Imaging And Method Of Magnetic Actuation Systems

Magnetic resonance imaging(MRI)system is a novel medical imaging instrument which is composed of main magnet,radio frequency coil system and gradient coil system.Most importantly,gradient coil system is a core component of the MRI instrument,provides three-dimensional linearly varied magnetic gradient field in the imaging volume,for the encoding of the spatial information of the imaged object,whose performance directly affects the MR image quality.Nowadays,the research about MRI technology is developed slowly,especially for the gradient coil in country.Therefore,it is necessary to design gradient coil with reasonable structure and significant performance,which not only improve the performance of the whole MRI system and image quality,but also demonstrate the ability of the independently research for the MRI systems component in our country.To achieve a faster slew rate for rapid imaging,the gradient coils were designed with higher performance in the modern MRI applications.However,the rapid switching on and off of the gradient coils induces secondary fields and eddy currents in the human tissues and surrounding conducting structures.These magnetic field that can produce interference for the spatial encoding of gradient coils,seriously affected the image quality for the MRI system.This work is part of the research about image-guided for the MRI-Linac real time ablation(supported by ARC,Australia Research Council),although the image guided robot technology has been developed in the biomedical engineering over the past decades,the consequent actuation related safety problem have become an important influence factor which limits the applications,thus it is necessary to further analyze and discuss the compatibility and safety for the systems under the magnetic resonance imaging.To address the poor capability of independent research,this thesis attempts to design a novel gradient coil design method for the open MRI system,,aims at resolving the currents problems that exists in the gradient coil design process to improve performance for the gradient coils and to reduce the eddy currents in the gradient coils design process,an magnetic actuation systems was proposed with a particular focus on the investigation of power dissipation and the design scheme of MRI-guided.A brief summary of the thesis work as follows.(1)Gradient coil designA conventional gradient coil based on stream function is widely used in the coil design,however an issue with this design is a connection problem between coil loops.To eliminate the field errors,this thesis proposed a novel spiral coil design scheme using limacon curve.The proposed coil design method was not only able to improve the magnetic field performance of the coil,but also could integrate the cooling system into the coil design for the controlled line spacing.An improved stream function combined with finite difference method coil design was proposed based on magnetic mirror principle.Considering the magnet pole effect for the target field,multi-layer magnetic materials was used as anti-eddy current plate to eliminate the secondary magnetic field during the design process.(2)Inductive coupling between coils caused eddy currentsDue to the complexity of the coil geometry,it is difficult to predict and analysis quantitative for the eddy current produced by coil system.In this thesis,a multi-layer integration method was proposed to effectively analyze eddy current field produced by the interactions between the gradient coil and surrounding objects,then theoretical analysis and experimental validation were provided.(3)Magnetic actuation system design and key technology of MRI-guidedAn external magnetic actuation was to proposed with focus on addressing the issues related to power dissipation and less workspace for current electromagnetic actuation system.The influence factors for the magnetic driving force produced by the proposed by the actuation system,and an multi-parameter prediction model for the magnetic force and magnetic torque were established to provide the theoretical and data guidance for the actuator design process.(4)MR-compatibility assessment and establishment for the MRI-guided test deviceA MRI-guided testing device driven by the ultrasonic motor was manufactured based on previously study.The results show the presented prototype of the device is MR-compatibility under different scanning sequence,did not create image distortion and artifacts.Under the guidance of MR image,the puncture experiment of the proposed porotype was completed,and the security of the image-guided was verified based on MRI-guided technology.Meanwhile,the results were proven as the precise position in the guided test.