title: Magnetic resonance imaging investigation of the bone conduction implant - a pilot study at 1.5 Tesla.
abstract: OBJECTIVE
  The objective of this pilot study was to investigate if an active bone conduction implant (BCI) used in an ongoing clinical study withstands magnetic resonance imaging (MRI) of 1.5 Tesla. In particular, the MRI effects on maximum power output (MPO), total harmonic distortion (THD), and demagnetization were investigated. Implant activation and image artifacts were also evaluated.
  
  
  METHODS
  One implant was placed on the head of a test person at the position corresponding to the normal position of an implanted BCI and applied with a static pressure using a bandage and scanned in a 1.5 Tesla MRI camera. Scanning was performed both with and without the implant, in three orthogonal planes, and for one spin-echo and one gradient-echo pulse sequence. Implant functionality was verified in-between the scans using an audio processor programmed to generate a sequence of tones when attached to the implant. Objective verification was also carried out by measuring MPO and THD on a skull simulator as well as retention force, before and after MRI.
  
  
  RESULTS
  It was found that the exposure of 1.5 Tesla MRI only had a minor effect on the MPO, ie, it decreased over all frequencies with an average of 1.1±2.1 dB. The THD remained unchanged above 300 Hz and was increased only at lower frequencies. The retention magnet was demagnetized by 5%. The maximum image artifacts reached a distance of 9 and 10 cm from the implant in the coronal plane for the spin-echo and the gradient-echo sequence, respectively. The test person reported no MRI induced sound from the implant.
  
  
  CONCLUSIONS
  This pilot study indicates that the present BCI may withstand 1.5 Tesla MRI with only minor effects on its performance. No MRI induced sound was reported, but the head image was highly distorted near the implant.
authors.affiliation: Department of Signals and Systems, Chalmers University of Technology, Gothenburg, Sweden.
authors.forename: Karl-Johan Fredén
authors.initials: KJ
authors.lastname: Jansson
authors.affiliation: Department of Signals and Systems, Chalmers University of Technology, Gothenburg, Sweden.
authors.forename: Bo
authors.initials: B
authors.lastname: Håkansson
authors.affiliation: Department of Signals and Systems, Chalmers University of Technology, Gothenburg, Sweden.
authors.forename: Sabine
authors.initials: S
authors.lastname: Reinfeldt
authors.affiliation: Department of Signals and Systems, Chalmers University of Technology, Gothenburg, Sweden.
authors.forename: Cristina
authors.initials: C
authors.lastname: Rigato
authors.affiliation: Department of Otorhinolaryngology Head and Neck Surgery, Sahlgrenska University Hospital, The Sahlgrenska Academy, University of Gothenburg, Gothenburg, Sweden.
authors.forename: Måns
authors.initials: M
authors.lastname: Eeg-Olofsson
keywords: bone conduction implant (BCI); demagnetization; image artifacts; magnetic resonance imaging (MRI); magnetic torque