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Accurate Shim-Coil Design and Magnet-Field Profiling by a Power-Minimization-Matrix Method
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| Journal of Magnetic Resonance, Series A |
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The design of a single correction coil that annuls, with minimal power consumption, the field inhomogeneity associated with a specific magnet is described. The design strategy used is also shown to be advantageous for the production of high-accuracy, power-efficient shim coils, "drift-free" shims (i.e., no slow main-field drift following a change of shim current), and high-homogeneity :shielded" magnets. Starting with a description of field inhomogeneity in a pherical-harmonic basis set, the cylindrical-surface current-density function needed to annul inhomogeneity is calculated, with minimization of electrical power dissipation, by a simple matrix formulation. The inclusion of design constraints, such as the annulment of mutual inductance between zonal shims and the magnet, is highlighted and production of the current-density function with both wire and cut sheet is briefly discussed. Insights are presented as to why the method, unlike some, gives a smooth current-density function lacking spurious high-frequency ripples, and experimental and numerical tests are reported that reveal the efficacy of the computational procedures.
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