title: Radio frequency versus susceptibility effects of small conductive implants--a systematic MRI study on aneurysm clips at 1.5 and 3 T.
abstract: OBJECTIVE
  Metallic implants cause enlarged artifacts in magnetic resonance (MR) images at higher magnetic fields, B0, due to their magnetic susceptibility. Interactions of conductive material with radio frequency (RF) pulses also change for higher field strengths, B0, due to the frequency dependence of resonance conditions. Systematic measurements on commercial aneurysm clips and simplified copper models were performed in order to investigate both phenomena at 1.5 and 3 T.
  
  
  METHODS
  Six different commercial aneurysm clips made of titanium, straight copper wires and bent copper models were examined in Gd-DTPA-doped water. RF-related effects were measured by adapted 2D and 3D spin-echo sequences. For reliable differentiation from susceptibility-related effects, variable transmitter voltages were applied. In addition, RF-induced heating was controlled by an infrared (IR) camera.
  
  
  RESULTS
  At 3 T, a significant RF-induced electric response could be demonstrated for the copper samples and more moderate for one of the commercial clips, dependent on the geometrical structure determining possible resonant RF coupling. Related RF effects could be distinguished from susceptibility artifacts: a signal enhancement at reduced transmitter voltages indicated locally amplified B1-field amplitudes. No significant heating effect could be measured by IR measurements.
  
  
  CONCLUSIONS
  MR imaging was used to analyze possible RF-induced effects. At 3 T, resonant RF coupling even of small metallic implants has to be considered carefully. Despite a local enhancement of the RF amplitude, no significant RF-induced heating inside the surrounding fluid was found. A direct thermal endangering of patients seems to be unlikely, but extremely high B1-field amplitudes might occur adjacent to the metallic surface with potential nonthermal affection of tissue.
authors.affiliation: Section on Experimental Radiology, University Hospital Tubingen, Tubingen, Germany. ulrike.lauer@med.uni-tuebingen.de
authors.forename: Ulrike A
authors.initials: UA
authors.lastname: Lauer
authors.forename: Hansjorg
authors.initials: H
authors.lastname: Graf
authors.forename: Alexander
authors.initials: A
authors.lastname: Berger
authors.forename: Claus D
authors.initials: CD
authors.lastname: Claussen
authors.forename: Fritz
authors.initials: F
authors.lastname: Schick
mesh_terms: D016477:Artifacts; D003287:Contrast Media; D003300:Copper; D004867:Equipment Design; D019786:Gadolinium DTPA; D006801:Humans; D002532:Intracranial Aneurysm / Q000601:surgery; D008279:Magnetic Resonance Imaging / Q000379:methods*; D019047:Phantoms, Imaging; D019736:Prostheses and Implants; D011846:Radio Waves; D013525:Surgical Instruments; D014025:Titanium
chemical_list: D003287:Contrast Media; D003300:Copper; D014025:Titanium; D019786:Gadolinium DTPA