title: Comparison of simulated parallel transmit body arrays at 3 T using excitation uniformity, global SAR, local SAR, and power efficiency metrics.
abstract: OBJECTIVE
  We compare the performance of eight parallel transmit (pTx) body arrays with up to 32 channels and a standard birdcage design. Excitation uniformity, local specific absorption rate (SAR), global SAR, and power metrics are analyzed in the torso at 3 T for radiofrequency (RF)-shimming and 2-spoke excitations.
  
  
  METHODS
  We used a fast cosimulation strategy for field calculation in the presence of coupling between transmit channels. We designed spoke pulses using magnitude least squares optimization with explicit constraint of SAR and power and compared the performance of the different pTx coils using the L-curve method.
  
  
  RESULTS
  PTx arrays outperformed the conventional birdcage coil in all metrics except peak and average power efficiency. The presence of coupling exacerbated this power efficiency problem. At constant excitation fidelity, the pTx array with 24 channels arranged in three z-rows could decrease local SAR more than 4-fold (2-fold) for RF-shimming (2-spoke) compared to the birdcage coil for pulses of equal duration. Multi-row pTx coils had a marked performance advantage compared to single row designs, especially for coronal imaging.
  
  
  CONCLUSIONS
  PTx coils can simultaneously improve the excitation uniformity and reduce SAR compared to a birdcage coil when SAR metrics are explicitly constrained in the pulse design.
authors.affiliation: Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Charlestown, Massachusetts, USA.
authors.forename: Bastien
authors.initials: B
authors.lastname: Guérin
authors.forename: Matthias
authors.initials: M
authors.lastname: Gebhardt
authors.forename: Peter
authors.initials: P
authors.lastname: Serano
authors.forename: Elfar
authors.initials: E
authors.lastname: Adalsteinsson
authors.forename: Michael
authors.initials: M
authors.lastname: Hamm
authors.forename: Josef
authors.initials: J
authors.lastname: Pfeuffer
authors.forename: Juergen
authors.initials: J
authors.lastname: Nistler
authors.forename: Lawrence L
authors.initials: LL
authors.lastname: Wald
mesh_terms: D065638:Absorption, Radiation; D003198:Computer Simulation; D017076:Computer-Aided Design; D004735:Energy Transfer; D004867:Equipment Design; D019544:Equipment Failure Analysis; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000295:instrumentation*; D008280:Magnetics / Q000295:instrumentation*; D008954:Models, Biological; D011829:Radiation Dosage; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity
keywords: Local SAR; excitation fidelity; global SAR; pTx coils; parallel transmission; power; spokes
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