title: Offline impedance measurements for detection and mitigation of dangerous implant interactions: an RF safety prescreen.
abstract: OBJECTIVE
  The concept of a "radiofrequency safety prescreen" is investigated, wherein dangerous interactions between radiofrequency fields used in MRI, and conductive implants in patients are detected through impedance changes in the radiofrequency coil.
  
  
  BACKGROUND
  The behavior of coupled oscillators is reviewed, and the resulting, observable impedance changes are discussed.
  
  
  METHODS
  A birdcage coil is loaded with a static head phantom and a wire phantom with a wire close to its resonant length, the shape, position, and orientation of which can be changed. Interactions are probed with a current sensor and network analyzer.
  
  
  RESULTS
  Impedance spectra show dramatic, unmistakable splitting in cases of strong coupling, and strong correlation is observed between induced current and scattering parameters.
  
  
  CONCLUSIONS
  The feasibility of a new, low-power prescreening technique has been demonstrated in a simple phantom experiment, which can unambiguously detect resonant interactions between an implanted wire and an imaging coil. A new technique has also been presented which can detect parallel transmit null modes for the wire.
authors.affiliation: Department of Electrical Engineering, Stanford University, Stanford, California, USA.
authors.forename: Christopher W
authors.initials: CW
authors.lastname: Ellenor
authors.forename: Pascal P
authors.initials: PP
authors.lastname: Stang
authors.forename: Maryam
authors.initials: M
authors.lastname: Etezadi-Amoli
authors.forename: John M
authors.initials: JM
authors.lastname: Pauly
authors.forename: Greig C
authors.initials: GC
authors.lastname: Scott
mesh_terms: D002058:Burns, Electric / Q000209:etiology* / Q000517:prevention & control*; D000075202:Contraindications; D004867:Equipment Design; D019544:Equipment Failure Analysis; D004869:Equipment Safety; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000295:instrumentation*; D019047:Phantoms, Imaging; D019736:Prostheses and Implants; D011846:Radio Waves / Q000009:adverse effects; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity
keywords: RF safety; implant safety; interventional MRI; pacemakers and MRI; parallel radiofrequency transmission