Elimination of mutual inductance in NMR phased arrays: The paddle design revisited (original) (raw)

2012, Journal of Magnetic Resonance

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 B 1-field simulations for resonant coils at f o = 300.5 MHz, having various sizes (3-10 cm), and for paddles with sizes ranging from 16 to 30 mm, and bench tests on constructed 4 Â 4 cm 2 two-(1 Â 2) and four-coil loop (2 Â 2) planar arrays. Simulation results yield total mean percentage B 1-field differences of only 7.03% between the two nonoverlapping coil array configurations (paddles vs. no-paddles). Pair-wise comparisons of elicited mean B 1-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 9.9 MHz were observed for the two coils in the 1 Â 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 S 11 /S 22 were À16.7/À16.2 dB and À28.2/À16.1 dB, for the two array loops, respectively. Under unloaded conditions and in the absence of paddles, split resonances were observed for the 1 Â 2 array, yielding transmission coefficients of À5.5 to À8.1 dB, reversing to single resonance responses upon paddle placements, with transmission coefficients of À14.4 to À15.6 dB.