A 32-Channel Combined RF and B0 Shim Array for 3T Brain Imaging

Purpose: We add user-controllable direct currents (DC) to the individual elements of a 32-channel radio-frequency (RF) receive array to provide B-0 shimming ability while preserving the array's reception sensitivity and parallel imaging performance. Methods: Shim performance using constrained DC current (+/- 2.5A) is simulated for brain arrays ranging from 8 to 128 elements. A 32-channel 3-tesla brain array is realized using inductive chokes to bridge the tuning capacitors on each RF loop. The RF and B-0 shimming performance is assessed in bench and imaging measurements. Results: The addition of DC currents to the 32-channel RF array is achieved with minimal disruption of the RF performance and/or negative side effects such as conductor heating or mechanical torques. The shimming results agree well with simulations and show performance superior to third-order spherical harmonic (SH) shimming. Imaging tests show the ability to reduce the standard frontal lobe susceptibility-induced fields and improve echo planar imaging geometric distortion. The simulation of 64- and 128-channel brain arrays suggest that even further shimming improvement is possible (equivalent to up to 6th-order SH shim coils). Conclusion: Including user-controlled shim currents on the loops of a conventional highly parallel brain array coil is feasible with modest current levels and produces improved B-0 shimming performance over standard second-order SH shimming. (C) 2015 Wiley Periodicals, Inc.