Study On Design Method Of ~3He Polarization Based Ultra-low-field Vertical Magnetic Resonance Imaging System

Research on lung diseases has long been dependent on X-ray and its related technologies.However,X-ray is an ionizing radiation and should not be exposed repeatedly in a short term,which consequently limits the dynamic tracking research on diseases.Magnetic resonance(MR)imaging is such a technology that uses nuclear spin resonance excited by electromagnetic wave to extract information.It is characterized by no ionizing radiation and multi-parameter information extraction,which can contribute to the observation of the physiological and pathological changes of biological tissues repeatedly,dynamically and continuously in a short term.Nevertheless,the hydrogen proton density of the lung is much lower than that of other surrounding tissues,and the interface between massive alveolar cavities and tissues generates an extremely uneven lung permeability.Therefore,it is difficult for traditional medium-and high-field MR imaging equipment with ~1H nuclear spin to realize a clear imaging of lungs.Significantly,MR with hyper-polarized ~3He gas as the medium can be used for clear lung imaging,with corresponding feasibility and reliability been confirmed preliminarily.In order to provide special instruments with continuous and dynamic tracking performance for lung disease research,National Natural Science Foundation of China has supported the major scientific instrument special project of"research and development of special equipment for low-field MR imaging of lung based on ~3He polarization".In view of the above interpretation,the present study was conducted focusing on design method of ~3He polarization based ultra-low-field vertical magnetic resonance imaging system,with the specific contents described as follows:(1)The input parameter of simulation method in traditional MR imaging is flip angle,which produces two problems.The one is not suitable for complex and changeable magnetic field environment,and the other is not being able to directly provide information about experimental phenomena caused by RF intensity changes.In order to address these two problems,this paper proposed a chain MR imaging simulation method and accuracy conditions,with the construction of a stable,accurate and efficient MR imaging simulation system simultaneously.Furthermore,the parameters of traditional hyper-polarized ~3He imaging variable-angle excitation sequence depend heavily on hardware parameters and require manual calibration.For this problem,our research proposed a real-time calibration sequence and corresponding imaging algorithm for hyper-polarized ~3He imaging,and verified these by simulation system.(2)In order to formulate a magnet solution with wider openness,this paper studied the basic configuration of the four-coil structure,proposed a unified structure constraint equation of the four-coil structure,and condensed the unified four-coil structure solving equation.Meanwhile,a manufacturing-oriented optimization method of ampere-turn ratio was proposed to minimize the rounding error.Further analysis and summary were made on the trend of opening degree and the performance change after the introduction of section size.In addition,our study put forward the uniform magnetic field design method of four-coil and six-coil with similar model.Using this method,this paper characterized circular(four-coil and six-coil)and polygonal coil in designing open magnet.The simulation results showed that the circular four-coil and polygonal four-coil had stronger openness than the six-coil and the circular four-coil,respectively.Given the uniform area size and uniformity,the octagonal structure showed the optimal comprehensive performance,while the quadrilateral structure had the strongest openness(slightly poor uniformity).According to the requirement of the project,our research designed an ultra-low-field octagonal main magnet with natural cooling.Moreover,in order to design the octagonal longitudinal gradient coil according to the shape of the main magnet,our study proposed a new design method of longitudinal gradient coil that separated the structure design from the magnetic field calculation,with a successful application of the scheme finally.(3)Furthermore,this study also completed the construction of an open ultra-low-field MR imaging system.On the basis of this system,water phantom(maximum size,310mm×310mm×400mm)and in vitro animal organ imaging experiments were carried out for over 60 hours.Corresponding experimental results revealed that the instrument worked stably and could correctly present two-dimensional and three-dimensional images without distortion.Meanwhile,the system could facilitate spin-echo and fast low-angle shot(FLASH)MR imaging.Further imaging experiment with hyper-polarized ~3He gas was performed by using this system.It was found that the polarized imaging was clear within 6.4s,which verified the significance of the new sequence and imaging algorithm.The equipment possessed the ability of breath-holding lung imaging in comparison with the performance of equipment developed by the Harvard team.To sum up,the ultra-low-field vertical MR imaging system based on ~3He polarizationdeveloped in this project has the characteristics of high signal-to-noise ratio,no radiation,easy operation and cost-efficient,which has a broad application prospect in the tracking diagnosis and treatment of lung diseases.