Deep Brain Stimulation (DBS) and Implant-friendly (IF) Mode Calculator

The purpose of this study is to present a strategy to calculate the implant-friendly (IF) excitation modes—which mitigate the radiofrequency (RF) heating at the contacts of deep brain stimulation (DBS) electrodes—of multi-channel RF coils at 7T.
Methods:
An induced RF current on an implantable electrode generates a scattered magnetic field whose left-handed circularly polarizing component (B1+) is approximated using a -mapping technique and subsequently utilized as a gauge for the electrode’s induced current. Using this approach, the relative induced currents due to each channel of a multi-channel RF coil on the DBS electrode were calculated. The IF modes of the corresponding multi-channel coil were determined by calculating the null space of the relative induced currents. The proposed strategy was tested and validated for unilateral and bilateral commercial DBS electrodes (directional lead, Infinity DBS system, Abbott Laboratories) placed inside a uniform phantom by performing heating and imaging studies on a 7T MRI scanner using a 16-channel transceive RF coil.
Results:
Individual IF modes nor shim solutions obtained from IF modes did not induce significant temperature increase when used for a high-power Turbo Spin Echo sequence. In contrast, shimming with the scanner’s toolbox (i.e., based on per-channel B1+ fields) resulted in a more than 2°C temperature increase for the same amount of input power.
Conclusion:
A strategy for calculating the IF modes of a multi-channel RF coil is presented. This strategy was validated using a 16-channel RF coil at 7T for unilateral and bilateral commercial DBS electrodes inside a uniform phantom.