title: Dependence of RF heating on SAR and implant position in a 1.5T MR system.
abstract: OBJECTIVE
  We evaluated radiofrequency (RF) heating of a humerus implant embedded in a gel phantom during magnetic resonance (MR) imaging for the specific absorption rate (SAR), angle between the implant and static magnetic field (B(0)), and position of the implant in the irradiation coil.
  
  
  METHODS
  We embedded a stainless steel humerus implant 2 cm deep in tissue-equivalent loop and mass phantoms, placed it parallel to the static magnetic field of a 1.5T MR scanner, and recorded the temperatures of the implant surface with RF-transparent fiberoptic sensors. We measured rises in temperature at the tips of the implant by varying the SAR from 0.2 to 4.0 W/kg and evaluated RF heating of the implant for its angle to B(0) and its displacement along B(0) from the center of the RF irradiation coil.
  
  
  RESULTS
  RF heating was similar for the loop and mass phantoms because the eddy current flows through the periphery of both. As the SAR increased, the temperature at the implant tip increased, and there was a linear relationship between the SAR and temperature rise. The values were 6.4 degrees C at 2.0 W/kg and 12.7 degrees C at 4.0 W/kg. Rise in temperature decreased steeply as the angle between the implant and B(0) surpassed 45 degrees . In addition, as the implant was displaced from the center of the RF coil to both ends, the rise in temperature decreased.
  
  
  CONCLUSIONS
  The rise in temperature in deep tissue was estimated to be higher than 1.0 degrees C for SAR above 0.4 W/kg. RF heating was greatest when the implant was set parallel to B(0). In MR imaging of patients with implants, there is a risk of RF heating when the loop of the eddy current is formed inside the body.
authors.affiliation: Medical Engineering and Technology, Integrated Human Sciences Studies, Hiroshima International University, Higashi-Hiroshima, Hiroshima, Japan. sg05203@mst.hirokoku-u.ac.jp
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Muranaka
authors.forename: Takayoshi
authors.initials: T
authors.lastname: Horiguchi
authors.forename: Shuji
authors.initials: S
authors.lastname: Usui
authors.forename: Yoshitake
authors.initials: Y
authors.lastname: Ueda
authors.forename: Osamu
authors.initials: O
authors.lastname: Nakamura
authors.forename: Fumiaki
authors.initials: F
authors.lastname: Ikeda
mesh_terms: D001858:Bone Nails; D004553:Electric Conductivity; D006358:Hot Temperature; D006811:Humerus / Q000528:radiation effects*; D008279:Magnetic Resonance Imaging / Q000295:instrumentation*; D010539:Permeability; D019047:Phantoms, Imaging; D011846:Radio Waves; D013193:Stainless Steel; D013814:Thermal Conductivity
chemical_list: D013193:Stainless Steel