The Development Of RF Coil In Ultra-low Field MRI

Magnetic resonance imaging（MRI）has the advantages of multi-parameters,no ionizing radiation,and excellent soft-tissue image contrast.MRI based on ³He polarized gas can provide high-quality image data for the research and diagnosis and treatment of lung diseases,so it can assist doctors in visual diagnosis to avoid delayed treatment.As the key component of the MRI system,the RF coil plays the role of transmitting pulse excitation and detect nuclear magnetic signals.According to the need of clinical medicine for ultra-low field MRI in human lungs,this dissertation takes the magnetic resonance RF coil of an ultra-low field environment as the research object,combined with the theory of MRI and RF coil,conducts an in-depth study on the ultra-low frequency RF coil and its peripheral interface circuit.The main work includes:（1）The RF coil is connected to the magnetic resonance system through the peripheral interface circuit.The RF T/R switch and power divider are analyzed,simulated,designed,and debugged.The results show that the active RF T/R switch can well isolate the transmitting end and the receiving end,and realize the working mode switching of the RF coil.The dual-channel power divider can realize equal-amplitude and in-phase distribution of the input power signal.The S-parameters of both meets the requirements.（2）The target field method was applied to the design of ultra-low frequency RF coils.By setting the region of interest（ROI）and its internal target magnetic field in advance,and then constructing a constraint function to inversely solve the source current density distribution required to generate the target magnetic field in the ROI,and finally using the stream function to obtain the static profile of the RF coil,completing the structure of the coil design.The control variable method is used to study the influence of penalty factors on the uniformity of the RF field and the complexity of the coil.The results show that the larger the penalty factor is,the simpler the structure of the coil will be,but the uniformity of the RF field will become worse,so it is necessary to make a reasonable choice in design.（3）For the 6.6m T magnetic resonance system,a solenoid coil and a transmitter-rec-eiver integrated cylindrical coil designed using the target field method were fabricated.After instrument debugging and experiment,the two prototype coils have successfully obtained the images of water mode and ³He cell.The SNR of cylindrical coil based on³He polarized gas is obviously better than that of water model imaging,which proves the feasibility of ³He polarized gas MRI in the ultra-low field.（4）The method of using the target field method combined with CST software simulation was proposed,and a ³He RF prototype coil for 60m T ultra-low field MRI of the lung was designed and manufactured.The coil structure was designed by the target field method,and the CST field-circuit co-simulation was carried out.In this dissertation,the RF field B₁and SAR distribution of the load RF coil with different quality factors（Q）are studied for the first time.The experimental results show that the change of Q value has an impact on the performance of the coil to some extent.When designing,we should take comprehensive consideration of various indicators such as the B₁field and transmission efficiency.Through simulation experiments and actual tests,the S-parameters of the ³He RF coil designed by the target field method combined with CST simulation meet the performance requirements,which verifies the feasibility and versatility of the method,and can be extended to the development of other low-field RF coils.