title: Parallel radiofrequency transmission at 3 tesla to improve safety in bilateral implanted wires in a heterogeneous model.
abstract: OBJECTIVE
  Elongated implanted conductors can interact with the radiofrequency (RF) transmission field during MRI, posing safety concerns of excessive heating in patients with deep brain stimulators. A technique using parallel RF transmission (pTx) is evaluated on an anthropomorphic heterogeneous model with bilateral and unilateral curved wires.
  
  
  METHODS
  Amplitude and phase were optimized by simulation to minimize heating surrounding the implanted wires and to minimize B1+ inhomogeneity for four-channel and eight-channel pTx in a heterogeneous model. MRI experiments were conducted in an equivalent test phantom created from a common digital mesh file.
  
  
  RESULTS
  In four-channel pTx, maximum local specific absorption rate (SAR) was reduced in both unilateral and bilateral wires by 47% and 59%, respectively, but with compromised B1+ homogeneity. Optimized eight-channel pTx substantially reduced local SAR compared with birdcage coil RF excitation in both unilateral and bilateral wires (reduction of maximum local SAR of 79% and 87%, respectively). B1+ inhomogeneity was limited to 17% and 26%, respectively. Experimental validation with four-channel pTx showed 80% and 92% temperature reduction at the tips of wire 1 and wire 2, respectively.
  
  
  CONCLUSIONS
  This pTx approach yields promising reductions in local SAR at the tips of unilateral and bilateral implanted wires while maintaining image quality in simulation and experiment. Magn Reson Med 78:2408-2415, 2017. © 2017 International Society for Magnetic Resonance in Medicine.
authors.affiliation: Physical Sciences Platform, Sunnybrook Health Sciences Institute, Toronto, Canada.|Department of Medical Biophysics, University of Toronto, Toronto, Canada.
authors.forename: Clare E
authors.initials: CE
authors.lastname: McElcheran
authors.affiliation: Physical Sciences Platform, Sunnybrook Health Sciences Institute, Toronto, Canada.
authors.forename: Benson
authors.initials: B
authors.lastname: Yang
authors.affiliation: Physical Sciences Platform, Sunnybrook Health Sciences Institute, Toronto, Canada.
authors.forename: Kevan J T
authors.initials: KJT
authors.lastname: Anderson
authors.affiliation: Massachusetts General Hospital, Harvard Medical School, Charlestown, Massachusetts, USA.
authors.forename: Laleh
authors.initials: L
authors.lastname: Golestanirad
authors.affiliation: Physical Sciences Platform, Sunnybrook Health Sciences Institute, Toronto, Canada.|Department of Medical Biophysics, University of Toronto, Toronto, Canada.
authors.forename: Simon J
authors.initials: SJ
authors.lastname: Graham
mesh_terms: D000465:Algorithms; D000886:Anthropometry; D001921:Brain / Q000000981:diagnostic imaging*; D003198:Computer Simulation; D046690:Deep Brain Stimulation; D004867:Equipment Design; D006257:Head / Q000000981:diagnostic imaging; D006358:Hot Temperature; D006801:Humans; D008279:Magnetic Resonance Imaging; D008962:Models, Theoretical; D061214:Patient Safety; D019047:Phantoms, Imaging; D019736:Prostheses and Implants; D011846:Radio Waves
keywords: RF shimming; deep brain stimulation; optimization; parallel RF transmission