Elimination of mutual inductance in NMR phased arrays: The paddle design revisited

Abstract

This study proposes a method to empirically minimize mutual inductance, using passive end-ring circular paddles, with neighboring coil loops placed in a non-overlapped configuration. The proposed concepts are validated through B1-field simulations for resonant coils at fo=300.5MHz, having various sizes (3–10cm), and for paddles with sizes ranging from 16 to 30mm, and bench tests on constructed 4×4cm2 two- (1×2) and four-coil loop (2×2) planar arrays.Simulation results yield total mean percentage B1-field differences of only 7.03% between the two non-overlapping coil array configurations (paddles vs. no-paddles). Pair-wise comparisons of elicited mean B1-field differences from the use of different circular and rectangular paddle sizes, yield values <5.3%. Theoretical calculation of the normalized mutual coupling coefficient in the non-overlapped coil configuration reduces to almost zero with optimally sized-paddles having a radius of approximately 28% the coil’s largest dimension.In the absence of paddles, differences in the split of resonance peaks of 9.9MHz were observed for the two coils in the 1×2 array, which vanished with paddle placement. Single coil responses (unloaded/loaded) without paddles, and responses from array coils with use of optimally-sized paddles yielded quality factor ratios that ranged between 1.1–1.86 and 1.0–1.5, respectively. Phantom and mouse loaded reflection coefficients S11/S22 were −16.7/−16.2dB and −28.2/−16.1dB, for the two array loops, respectively. Under unloaded conditions and in the absence of paddles, split resonances were observed for the 1×2 array, yielding transmission coefficients of −5.5 to −8.1dB, reversing to single resonance responses upon paddle placements, with transmission coefficients of −14.4 to −15.6dB.