title: SAR reduction in 7T C-spine imaging using a "dark modes" transmit array strategy.
abstract: OBJECTIVE
  Local specific absorption rate (SAR) limits many applications of parallel transmit (pTx) in ultra high-field imaging. In this Note, we introduce the use of an array element, which is intentionally inefficient at generating spin excitation (a "dark mode") to attempt a partial cancellation of the electric field from those elements that do generate excitation. We show that adding dipole elements oriented orthogonal to their conventional orientation to a linear array of conventional loop elements can lower the local SAR hotspot in a C-spine array at 7 T.
  
  
  METHODS
  We model electromagnetic fields in a head/torso model to calculate SAR and excitation B1 (+) patterns generated by conventional loop arrays and loop arrays with added electric dipole elements. We utilize the dark modes that are generated by the intentional and inefficient orientation of dipole elements in order to reduce peak 10g local SAR while maintaining excitation fidelity.
  
  
  RESULTS
  For B1 (+) shimming in the spine, the addition of dipole elements did not significantly alter the B1 (+) spatial pattern but reduced local SAR by 36%.
  
  
  CONCLUSIONS
  The dipole elements provide a sufficiently complimentary B1 (+) and electric field pattern to the loop array that can be exploited by the radiofrequency shimming algorithm to reduce local SAR.
authors.affiliation: Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA; A. A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Charlestown, Massachusetts, USA; Madrid-MIT M+ Vision Consortium, Madrid, Spain.
authors.forename: Yigitcan
authors.initials: Y
authors.lastname: Eryaman
authors.forename: Bastien
authors.initials: B
authors.lastname: Guerin
authors.forename: Boris
authors.initials: B
authors.lastname: Keil
authors.forename: Azma
authors.initials: A
authors.lastname: Mareyam
authors.forename: Joaquin L
authors.initials: JL
authors.lastname: Herraiz
authors.forename: Robert K
authors.initials: RK
authors.lastname: Kosior
authors.forename: Adrian
authors.initials: A
authors.lastname: Martin
authors.forename: Angel
authors.initials: A
authors.lastname: Torrado-Carvajal
authors.forename: Norberto
authors.initials: N
authors.lastname: Malpica
authors.forename: Juan A
authors.initials: JA
authors.lastname: Hernandez-Tamames
authors.forename: Emanuele
authors.initials: E
authors.lastname: Schiavi
authors.forename: Elfar
authors.initials: E
authors.lastname: Adalsteinsson
authors.forename: Lawrence L
authors.initials: LL
authors.lastname: Wald
mesh_terms: D016477:Artifacts; D017076:Computer-Aided Design; D004867:Equipment Design; D019544:Equipment Failure Analysis; D006801:Humans; D007089:Image Enhancement / Q000295:instrumentation*; D008279:Magnetic Resonance Imaging / Q000295:instrumentation*; D008280:Magnetics / Q000295:instrumentation*; D019047:Phantoms, Imaging; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D013131:Spine / Q000033:anatomy & histology*; D014159:Transducers
keywords: excitation fidelity; global SAR; local SAR; loop-dipole arrays; parallel transmit; radiative dipole
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