Design Of Ultra-thin RF Coil For Magnetic Resonance Based On Partial Equivalent Circuit Method

During the field of unilateral nuclear magnetic resonance,PCB ultra-thin RF coils with excellent performance,less space,and simple manufacturing process are often used as RF magnetic field excitation and receiving coils.The working frequency of the RF coil in the unilateral magnetic resonance sensor is generally several hundred k Hz to several MHz.The skin effect and proximity effect make the valid current density close to the copper wire surface;at the same time,the polarization of the dielectric substrate and the eddy current induced in the surrounding metals also affect the impedance of the coil and the distribution of the RF magnetic field.Accurately and quickly calculating the AC resistance,inductance and RF magnetic field distribution in region of interest is beneficial to the design of the coil size.The thickness of the ultra-thin RF coil is only tens of ?m,however,the length is often on the order of centimeters or even meters.When the conventional finite element and other numerical methods deal with this multi-scale electromagnetic field problem,the number of meshing units is huge,and the hardware resources and calculation time consumed exceed acceptable range.In addition,because the RF coil model under the influence of metal eddy currents is often not symmetrical,it is impossible to establish a concise and usable analytical formula for calculation.Therefore,it is necessary to find a fast calculation method of impedance and RF magnetic field distribution for the ultra-thin RF coil in the unilateral nuclear magnetic resonance sensor.The main works of this paper are as follows:(1)The common electromagnetic field calculation methods were summarized,and the partial element equivalent circuit(PEEC)model was utilized to calculate the ultra-thin RF coil impedance and RF magnetic field in unilateral nuclear magnetic resonance sensors.Partial equivalent circuit models considering resistance and inductance effects and partial equivalent circuit models considering resistance,inductance,and capacitance effects were respectively deduced,and the transition from electromagnetic field problems to pure circuit problems was realized at last.(2)The meshing method of the PEEC model was given,so that the number of meshing elements was much lower than the commonly used finite element methods.The PEEC models of ultra-thin planar rectangular spiral coil,the circular cross-section lead wire at both ends of the coil,the dielectric substrate and the metal at the bottom of the coil were derived.The selection method of the PEEC meshing parameters was summarized.The accuracy and calculation time of the PEEC impedance calculation of the coil were compared with measurement and commercial finite element simulation software.(3)Based on Biosav's law,the calculation of the magnetic field generated by the RF coil under the PEEC model was derived,and the influence of the metal eddy current at the bottom of the RF coil on the coil impedance and the RF magnetic field in the region of interest was analyzed and compared,and the simplified equivalent circuit and the tuning and matching circuit were derived.(4)For a unilateral nuclear magnetic resonance sensor,the optimization goal was to maximize the signal-to-noise ratio of the RF coil.Based on the advantages of the precise and fast calculation of the PEEC method,parametric scan on the size of the ultra-thin planar rectangular spiral RF coil was performed,and finally,the optimal coil structure was obtained.The NMR test of a silicone rubber composite insulator was performed,and the CPMG echo signal of the shed with high signal-to-noise ratio was successfully obtained.The results show that the PEEC method is suitable for the design of ultra-thin RF coils in unilateral magnetic resonance sensors.