Novel test field diversity method for demonstrating magnetic resonance imaging safety of active implantable medical devices.

ABSTRACT

Electromagnetic (EM) radiofrequency (RF) safety testing of elongated active implantable medical devices (AIMD) during magnetic resonance imaging (MRI) requires an RF response model of the implant to assess a wide range of exposure conditions. The model must be validated using a sufficiently large set of incident tangential electric field ([Formula see text]) conditions that provide diversified exposure. Until now, this procedure was very time consuming and often resulted in poorly defined [Formula see text] 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 [Formula see text] 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 a > 10 dB dynamic range of RF-heating enhancement factors (for identical trajectory-averaged incident field strength). The new TFD method yields an improved and reliable validation of the AIMD RF response model with low uncertainty, i.e. < 1.5 dB, for both 1.5 and 3.0 T evaluations.