A workflow for predicting temperature increase at the electrical contacts of deep brain stimulation electrodes undergoing MRI

Abstract

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.

Implant‐friendly MRI of deep brain stimulation electrodes at 7 T

“…On the other hand, previous studies 32,33 show that the induced current on the shaft of an electrode (near the tip) reliably controls the temperature increase (ΔT) at the tip of the electrode. Therefore, any complex excitation vector, $$$ {\boldsymbol{\uplambda}}_{\mathbf{null}} $$$, that is orthogonal to the row vector $$$ {\mathbf{I}}^{\mathbf{ind}} $$$ sets Eq.…”

“…Previously, the analytical expression that relates the current, I ind , induced by an RF coil on an oblique DBS electrode (Figure 1A) to the left-handed circularly polarized component (defined as B1 + ) of the scattered magnetic field on an arbitrary transversal plane has been calculated as follows 32 : where B1 +sca is the scattered B1 + field caused by the induced current on the electrode; r 0 is the radial distance of an arbitrary observation point from the electrode; and 𝜙 0 denotes the azimuthal angle of the observation point in relation to the conventional MRI scanner coordinate system (i.e., laboratory frame). As shown in Figure 1A, the rotation and obliqueness of the electrode with respect to the primary coordinate system are presented by a polar angle, 𝜃 r , and an azimuthal angle, 𝜙 r .…”

“…Throughout this workflow, it was assumed that the incident B1 + (around the shaft on plane P 2 , where the current is measured) is approximately equal to the total B1 + underneath the tip of the electrode on plane P 1 (Figure 1C,D). 32 This approximation is based on the rationale that the electrode current…”

“…Previously, the analytical expression that relates the current, $$$ {I}^{ind} $$$, induced by an RF coil on an oblique DBS electrode (Figure 1A) to the left‐handed circularly polarized component (defined as $$$ B{1}^{+} $$$) of the scattered magnetic field on an arbitrary transversal plane has been calculated as follows 32 : $$$$ {\displaystyle \begin{array}{cc}& B{1}^{+ sca}\left({r}_0,{\phi}_0\right)\\ {}& \kern1em =\frac{\mu_0{I}^{ind}}{4\pi {r}_0\sqrt{\cos^2{\theta}_r{\cos}^2\left({\phi}_0-{\phi}_r\right)+{\sin}^2\left({\phi}_0-{\phi}_r\right)}}\end{array}} $$$$where $$$ B{1}^{+ sca} $$$ is the scattered $$$ B{1}^{+} $$$ field caused by the induced current on the electrode; $$$ {r}_0 $$$ is the radial distance of an arbitrary observation point from the electrode; and $$$ {\phi}_0 $$$ denotes the azimuthal angle of the observation point in relation to the conventional MRI scanner coordinate system (i.e., laboratory frame). As shown in Figure 1A, the rotation and obliqueness of the electrode with respect to the primary coordinate system are presented by a polar angle, $$$ {\theta}_r $$$, and an azim...…”

“…Scattered B 1 + fields: Assuming the incident B1 + field on Plane P 2 and the total B1 + field on P 1 are approximately equal,32 per-channel complex B1 the induced current by Channel 1, I ind 1 . For this purpose, N−1 minimization routines using MATLAB's fminsearch function were performed on pairs of scattered B1 + fields ( i.e., I ind 1 and I ind…”