title: Controlling radiofrequency-induced currents in guidewires using parallel transmit.
abstract: OBJECTIVE
  Elongated conductors, such as pacemaker leads, neurostimulator leads, and conductive guidewires used for interventional procedures can couple to the MRI radiofrequency (RF) transmit field, potentially causing dangerous tissue heating. The purpose of this study was to demonstrate the feasibility of using parallel transmit to control induced RF currents in elongated conductors, thereby reducing the RF heating hazard.
  
  
  METHODS
  Phantom experiments were performed on a four-channel parallel transmit system at 1.5T. Parallel transmit "null mode" excitations that induce minimal wire current were designed using coupling measurements derived from axial B1 (+) maps. The resulting current reduction performance was evaluated with B1 (+) maps, current sensor measurements, and fluoroptic temperature probe measurements.
  
  
  RESULTS
  Null mode excitations reduced the maximum coupling mode current by factors ranging from 2 to 80. For the straight wire experiment, a current null imposed at a single wire location was sufficient to reduce tip heating below detectable levels. For longer insertion lengths and a curved geometry, imposing current nulls at two wire locations resulted in more distributed current reduction along the wire length.
  
  
  CONCLUSIONS
  Parallel transmit can be used to create excitations that induce minimal RF current in elongated conductors, thereby decreasing the RF heating risk, while still allowing visualization of the surrounding volume.
authors.affiliation: Magnetic Resonance Systems Research Laboratory, Department of Electrical Engineering, Stanford University, Stanford, California, USA.
authors.forename: Maryam
authors.initials: M
authors.lastname: Etezadi-Amoli
authors.affiliation: Magnetic Resonance Systems Research Laboratory, Department of Electrical Engineering, Stanford University, Stanford, California, USA.
authors.forename: Pascal
authors.initials: P
authors.lastname: Stang
authors.affiliation: Magnetic Resonance Systems Research Laboratory, Department of Electrical Engineering, Stanford University, Stanford, California, USA.
authors.forename: Adam
authors.initials: A
authors.lastname: Kerr
authors.affiliation: Magnetic Resonance Systems Research Laboratory, Department of Electrical Engineering, Stanford University, Stanford, California, USA.
authors.forename: John
authors.initials: J
authors.lastname: Pauly
authors.affiliation: Magnetic Resonance Systems Research Laboratory, Department of Electrical Engineering, Stanford University, Stanford, California, USA.
authors.forename: Greig
authors.initials: G
authors.lastname: Scott
mesh_terms: D016477:Artifacts; D004867:Equipment Design; D019544:Equipment Failure Analysis; D005240:Feasibility Studies; D005246:Feedback; D057918:Fiducial Markers; D007089:Image Enhancement / Q000295:instrumentation*; D008279:Magnetic Resonance Imaging / Q000295:instrumentation*; D019047:Phantoms, Imaging; D011846:Radio Waves; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity
keywords: MR safety; RF heating; RF safety; implanted devices; parallel transmit; transmit arrays