Experimental implementation of array‐compressed parallel transmission at 7 tesla

Purpose To implement and validate a hardware‐based array‐compressed parallel transmission (acpTx) system. Methods In array‐compressed parallel transmission, a small number of transmit channels drive a larger number of transmit coils, which are connected via an array compression network that implements optimized coil‐to‐channel combinations. A two channel‐to‐eight coil array compression network was developed using power splitters, attenuators and phase shifters, and a simulation was performed to investigate the effects of coil coupling on power dissipation in a simplified network. An eight coil transmit array was constructed using induced current elimination decoupling, and the coil and network were validated in benchtop measurements,B 1 +mapping scans, and an accelerated spiral excitation experiment. Results The developed attenuators came within 0.08 dB of the desired attenuations, and reflection coefficients were −22 dB or better. The simulation demonstrated that up to 3× more power was dissipated in the network when coils were poorly isolated (−9.6 dB), versus well‐isolated (−31 dB). Compared to split circularly‐polarized coil combinations, the additional degrees of freedom provided by the array compression network led to 54% lower squared excitation error in the spiral experiment. Conclusion Array‐compressed parallel transmission was successfully implemented in a hardware system. Further work is needed to develop remote network tuning and to minimize network power dissipation. Magn Reson Med 75:2545–2552, 2016. © 2016 Wiley Periodicals, Inc.