Novel Test Field Diversity Method for Demonstrating Magnetic Resonance Imaging Safety of Active Implantable Medical Devices

Aiping Yao, Earl Zastrow, Esra Neufeld, Maria Cabanes-Sempere, Theodoros Samaras, and Niels Kuster, Physics in Medicine & Biology 2020, Volume 65, 075004, online 07 April 2019; doi: 10.1088/1361-6560/ab7507

Electromagnetic (EM) radiofrequency (RF) safety testing of elongated active implantable medical devices (AIMD) to assess a wide range of exposure conditions during magnetic resonance imaging (MRI) requires an RF response model of the implant. A sufficiently large set of incident tangential electric field (Etan) conditions that provide diversified exposure must be used to validate the model. Until now, the validation procedure was very time consuming and often resulted in poorly defined Etan conditions. In this paper, we propose a test field diversity (TFD) validation method that provides more diverse exposure conditions of high fidelity, thereby decreasing the number of implant routings to be tested. The TFD method is based on the finding that the amplitude and phase of Etan along a single lead path in a cylindrical phantom can be sufficiently varied by changing the polarization of the incident 64 and 128 MHz magnetic fields inside standard birdcage test coils. The method is validated, its benefits are demonstrated, and an uncertainty budget is developed. First, the numerically determined field conditions were experimentally verified. The RF transfer function of a 90-cm-long spinal cord stimulator was successfully validated with the TFD approach and excitation conditions that cover RF-heating enhancement factors (for identical trajectory-averaged incident field strength) of dynamic range >10 dB. The new TFD method yields improved and reliable validation of the AIMD RF response model with low uncertainty, i.e., <1.5 dB for evaluations at 1.5 and 3.0 T

The scientific and technical impact of the study can be summarized as:

  • A novel test field diversity (TFD) validation method that reduces the required number of different routings and improves the exposure fidelity for a comprehensive transfer function validation of AIMDs with elongated leads has been proposed
  • The method has been successfully validated and demonstrated with a 90-cm-long spinal cord stimulator, and a >10 dB dynamic range for exposure has been achieved
  • An uncertainty budget for the method has been developed
  • A mathematical framework that could be employed to systematically select exposure conditions to maximize sensitivity and validation information content – potentially allowing further reduction of the required number of test conditions – has been proposed