Feynman Lectures on Computation
Feynman Lectures on Computation
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In this work, we present the design and test of two prototype probes with high B1 field homogeneity for MRI, comprising four coaxial rings of wire in a symmetric arrangement on a spherical and on an ellipsoidal surface. Compared to Helmholtz pair which presents a 2nd-order homogeneity, the aim is first an improvement of the field homogeneity, while preserving the simplicity of implementation. So, the two neighbouring rings, on the same side, are mounted in series and share the same tuning capacitor. Thus, tuning of the coils is achieved as simply as for Helmholtz pair. The position and diameter of each ring are optimized so that the assembly produces a fourth- or sixth-order homogeneous field. A complete electrical model of the four-coil system taking into account all coupling effects permits to determine the resonance frequency in the co-current mode, and consequently to predict the value of the tuning capacitors. Results of comparison tests, in free space and in a spectro-imager, between a Helmholtz pair and our two prototype coils are presented, and appear to be in good agreement with theoretical predictions. Moreover, noticeable improvements of the quality factor (+37% on average) and the sensitivity (+40%) are observed.