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The design of biplanar active shim coils for ultra low-field NMR

• 1、State Key Laboratory of Power Transmission Equipment & System Security and New Technology,Chongqing University, Chongqing 400044

Abstract：The ultra low-field nuclear magnetic resonance (NMR) is a popular research topic in the field of portable medical imaging. Due to the low value of main magnetic field in the ultra low-field nuclear magnetic resonance device（lower than 50mT）, a higher request to homogeneity of the magnet field has been put forward. The homogeneity of the magnet field impacts signal noise ratio (SNR) of nuclear magnetic resonance directly. An optimized target-field method is used to design bi-planar active shim coils, and the small magnetic field generated by the active shim coil can offset the inhomogeneous component of main magnetic field so that the homogeneity can be improved obviously. Shimming object is a spherical space with a diameter of 200mm where the magnetic field strength is 50mT. First, we analyse all inhomogeneous components of the permanent magnetic field which are expanded by the orthogonal spherical harmonic function. Second, mathematical expressions that relate the surface current distribution of shim coils to spherical harmonic terms in the magnetic field expansion are established by the optimized target-field method. And then a simple matrix inversion approach is used to obtain current distribution of shim coils. Last, an isoline discrete method with stream function was utilized to obtain the winding distribution on the coil plane. A range of active shim coils are designed by using this method, the verification experiment of Z2-(X2+Y2)/2 shim coil，which is the most inhomogeneous component of the permanent magnetic field, shows that the actual magnetic field produced by the coil agrees with the theoretical field, which can basically meet the design requirements.Magnetic field uniformity becomes 72.3911ppm from 174.4258ppm when the current of Z2-(X2+Y2)/2 shim coil is 1.4 A.

Keywords： Theory and New Technology of Electrical Engineering Ultra low-field NMR Target-field method Bi-planar shim coils