Realistic modeling of deep brain stimulation implants for electromagnetic MRI safety studies

Abstract

We propose a framework for electromagnetic (EM) simulation of deep brain stimulation (DBS) patients in radiofrequency (RF) coils. We generated a model of a DBS patient using post-operative head and neck computed tomography (CT) images stitched together into a “virtual CT” image covering the entire length of the implant. The body was modeled as homogeneous. The implant path extracted from the CT data contained self-intersections, which we corrected automatically using an optimization procedure. Using the CT-derived DBS path, we built a model of the implant including electrodes, helicoidal internal conductor wires, loops, extension cables, and the implanted pulse generator. We also built four simplified models with straight wires, no extension cables and no loops to assess the impact of these simplifications on safety predictions. We simulated EM fields induced by the RF birdcage body coil in the body model, including at the DBS lead tip at both 1.5 Tesla (64 MHz) and 3 Tesla (123 MHz). We also assessed the robustness of our simulation results by systematically varying the EM properties of the body model and the position and length of the DBS implant (sensitivity analysis). The topology correction algorithm corrected all self-intersection and curvature violations of the initial path while introducing minimal deformations (open-source code available at http://ptx.martinos.org/index.php/Main_Page). The unaveraged lead-tip peak SAR predicted by the five DBS models (0.1 mm resolution grid) ranged from 12.8 kW/kg (full model, helicoidal conductors) to 43.6 kW/kg (no loops, straight conductors)at 1.5 T (3.4-fold variation) and 18.6 kW/kg (full model, straight conductors) to 73.8 kW/kg (no loops, straight conductors)at 3 T (4.0-fold variation). At 1.5 T and 3 T, the variability of lead-tip peak SAR with respect to the conductivity ranged between 18% and 30%. Variability with respect to the position and length of the DBS implant ranged between 9.5% and 27.6%.

Application of Surgical Lead Management and Reconfigurable Coil Technology to Reduce RF Heating of DBS Implants during MRI at 3T Under Variant Body Compositions

“…In the past few years, numerical simulations have been increasingly used to assess safety of medical devices and imaging instruments [36], [54]- [58]. An important aspect of such practice is to validate simulations against measurements whenever possible, in order to provide confidence in future predictions of such models.…”

Parallel transmission to reduce absorbed power around deep brain stimulation devices in MRI: Impact of number and arrangement of transmit channels

“…As shown in Refs. and , this likely has a major impact on the predicted APAE. Ideally, the internal components of the DBS implant should be modeled as accurately as possible and include helicoidal internal wires; however, this is exceedingly difficult even using state‐of‐the‐art electromagnetic solvers such as Ansys Electronics.…”

“…In addition, E‐fields were exported on a 0.1 mm isotropic voxel grid covering the tip of the left and right electrodes (and, for Angus, the 2 abandoned lead tips). We selected such high spatial resolution because the E‐field is known to vary extremely rapidly around the DBS electrodes; therefore, accurate prediction of RF safety metrics in this region requires a much higher resolution than typically used when evaluating specific absorption rate (SAR) in patients without implants . The magnitude and phase values of the B‐field were used to compute the maps, and the magnitude values of the E‐field field were used to compute global SAR and the APAE matrix (see details below).…”

“…We selected such high spatial resolution because the E-field is known to vary extremely rapidly around the DBS electrodes; therefore, accurate prediction of RF safety metrics in this region requires a much higher resolution than typically used when evaluating specific absorption rate (SAR) in patients without implants. 35 The magnitude and phase values of the B-field were used to compute the B + 1 maps, and the magnitude values of the E-field field were used to compute global SAR and the APAE matrix (see details below). The E-fields and B + 1 maps of the individual channels were then combined by linear superposition using the multichannel RF pulse as weight factors (see next section).…”

“…20,21 This effect has been characterized in multiple experimental and simulation studies. [22][23][24][25][26][27][28][29][30][31][32][33][34][35] Parallel transmission (pTx) has been proposed to mitigate this problem because pTx coils and pulses offer additional transmit degrees of freedom that can be used to shape the B + 1 distribution, which controls the flip angle (FA); as well as electric fields (E-field) induced in the patient, which cause heating. For example, Etezadi-Amoli et al built a 4-channel planar pTx system and showed that it is possible to generate transmit waveform combinations minimizing the current induced in a guidewire equipped with a current sensor.…”