Purpose The purpose of this study is to present a workflow for predicting the radiofrequency (RF) heating around the contacts of a deep brain stimulation (DBS) lead during an MRI scan. Methods The induced RF current on the DBS lead accumulates electric charge on the metallic contacts, which may cause a high local specific absorption rate (SAR), and therefore, heating. The accumulated charge was modeled by imposing a voltage boundary condition on the contacts in a quasi‐static electromagnetic (EM) simulation allowing thermal simulations to be performed with the resulting SAR distributions. Estimating SAR and temperature increases from a lead in vivo through EM simulation is not practical given anatomic differences and variations in lead geometry. To overcome this limitation, a new parameter, transimpedance, was defined to characterize a given lead. By combining the transimpedance, which can be measured in a single calibration scan, along with MR‐based current measurements of the lead in a unique orientation and anatomy, local heating can be estimated. Heating determined with this approach was compared with results from heating studies of a commercial DBS electrode in a gel phantom with different lead configurations to validate the proposed method. Results Using data from a single calibration experiment, the transimpedance of a commercial DBS electrode (directional lead, Infinity DBS system, Abbott Laboratories, Chicago, IL) was determined to be 88 Ω. Heating predictions using the DBS transimpedance and rapidly acquired MR‐based current measurements in 26 different lead configurations resulted in a <23% (on average 11.3%) normalized root‐mean‐square error compared to experimental heating measurements during RF scans. Conclusion In this study, a workflow consisting of an MR‐based current measurement on the DBS lead and simple quasi‐static EM/thermal simulations to predict the temperature increase around a DBS electrode undergoing an MRI scan is proposed and validated using a commercial DBS electrode.
PurposeThe purpose of this study is to present a strategy to calculate the implant‐friendly (IF) excitation modes—which mitigate the RF heating at the contacts of deep brain stimulation (DBS) electrodes—of multichannel RF coils at 7 T.MethodsAn induced RF current on an implantable electrode generates a scattered magnetic field whose left‐handed circularly polarizing component () is approximated using a ‐mapping technique and subsequently used as a gauge for the electrode's induced current. Using this approach, the relative induced currents resulting from each channel of a multichannel RF coil on the DBS electrode were calculated. The IF modes of the corresponding multichannel 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.ResultsNeither individual IF modes nor shim solutions obtained from IF modes induced 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 fields) resulted in a more than 2°C temperature increase for the same amount of input power.ConclusionA strategy for calculating the IF modes of a multichannel RF coil is presented. This strategy was validated using a 16‐channel RF coil at 7 T for unilateral and bilateral commercial DBS electrodes inside a uniform phantom.
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