title: Experimental implementation of array-compressed parallel transmission at 7 tesla.
abstract: To implement and validate a hardware-based array-compressed parallel transmission (acpTx) system.
  
  
  
  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, B1+ mapping scans, and an accelerated spiral excitation experiment.
  
  
  
  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.
  
  
  
  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.
authors.affiliation: Vanderbilt University Institute of Imaging Science, Nashville, Tennessee, USA.|Department of Radiology, Vanderbilt University, Nashville, Tennessee, USA.
authors.forename: Xinqiang
authors.initials: X
authors.lastname: Yan
authors.affiliation: Vanderbilt University Institute of Imaging Science, Nashville, Tennessee, USA.|Department of Biomedical Engineering, Vanderbilt University, Nashville, Tennessee, USA.
authors.forename: Zhipeng
authors.initials: Z
authors.lastname: Cao
authors.affiliation: Vanderbilt University Institute of Imaging Science, Nashville, Tennessee, USA.|Department of Radiology, Vanderbilt University, Nashville, Tennessee, USA.|Department of Biomedical Engineering, Vanderbilt University, Nashville, Tennessee, USA.|Department of Electrical Engineering, Vanderbilt University, Nashville, Tennessee, USA.
authors.forename: William A
authors.initials: WA
authors.lastname: Grissom
mesh_terms: D004867:Equipment Design; D008279:Magnetic Resonance Imaging / Q000295:instrumentation / Q000379:methods*; D019047:Phantoms, Imaging; D012815:Signal Processing, Computer-Assisted / Q000295:instrumentation*
keywords: MR engineering; RF coils; RF pulse design; array compression; optimization; parallel transmission; selective excitation; ultrahigh field MRI