RF-induced heating in tissue near bilateral DBS implants during MRI at 1.5 T and 3T: The role of surgical lead management

Abstract

Access to MRI is limited for patients with deep brain stimulation (DBS) implants due to safety hazards, including radiofrequency (RF) heating of tissue surrounding the leads. Computational models provide an exquisite tool to explore the multi-variate problem of RF heating and help better understand the interaction of electromagnetic fields and biological tissues. This paper presents a computational approach to assess RF-induced heating, in terms of specific absorption rate (SAR) in the tissue, around the tip of bilateral DBS leads during MRI at 64MHz/1.5 T and 127 MHz/3T. Patient-specific realistic lead models were constructed from post-operative CT images of nine patients operated for sub-thalamic nucleus DBS. Finite element method was applied to calculate the SAR at the tip of left and right DBS contact electrodes. Both transmit head coils and transmit body coils were analyzed. We found a substantial difference between the SAR and temperature rise at the tip of right and left DBS leads, with the lead contralateral to the implanted pulse generator (IPG) exhibiting up to 7 times higher SAR in simulations, and up to 10 times higher temperature rise during measurements. The orientation of incident electric field with respect to lead trajectories was explored and a metric to predict local SAR amplification was introduced. Modification of the lead trajectory was shown to substantially reduce the heating in phantom experiments using both conductive wires and commercially available DBS leads. Finally, the surgical feasibility of implementing the modified trajectories was demonstrated in a patient operated for bilateral DBS.

Keywords: Computational modeling and simulations; Deep brain stimulation (DBS); Finite element method (FEM); MRI safety; Magnetic resonance imaging (MRI); Medical implants; Neuromodulation; Neurostimulation; Specific absorption rate (SAR).

Figure 1:

Post-operative CT images of patients with bilateral DBS leads. The ipsilateral and contralateral labels are with respect to the system IPG.

Figure 2:

Steps of image segmentation and lead model construction. (A) 3D view of the CT image of a patient (B) Threshold mask covering the center of hyper dense lead artifact (C) Preliminary 3D surfaces of patient’s head and lad trajectories constructed in Amira. (D) Lead trajectories reconstructed in Rhino3D. Adjustments were made to assure there was at lead 1.27 mm gap between overlapping segments. (E) Patient’s head aligned with the homogeneous MIDA model. (F) Details of lead structure and mesh in HFSS.

Figure 3:

(A) position of paitent body in head and body coils. (B) the B₁+ and (C) 1gSAR calculated on an axial plane passing through electrode contacts.

Figure 4:

(A) Trajectories of ipsilateral and contralateral leads superimposed in one head model. (B) Incident electric field (green arrows) and E_tan (color field) along the trajectory of ipsilateral and contralateral leads in Patient 2. Points P and Q show the limits of the initial segment over which the induced voltage V_8cm in equation [2] was calculated. The evolution of E_tan at different time points through the cycle is given in Supplementary Figure S1.

Figure 5:

Time evolution of the induced voltage over the first 8cm segment of contralateral and ipsilateral leads in Patient 2.

Figure 6:

MaxSAR1g for contralateral and ipsilateral leads in patients 1–9 for RF exposure at 64 MHz and 127 MHz with transmit head coil and transmit body coil.

Figure 7:

Scattering plots and correlation coeffecients of V_8cm and MaxSAR1g for different body coils and at different resonant frequencies

Figure 8:

(A) 3D printed DBS lead phantoms used as a guide to shape wires in the form of patient-derived trajectories. (B-E) Generic metallic wires and commercially available leads (Models 3387 and 3389, Medtronic Inc., Minneapolis) implanted into semi-solid anthropomorphic head phantoms for MRI at 1.5 T and 3T. (F) Modified contralateral trajectory of Patient 2 with an extracranial loop added at the surgical burr-hole.

Figure 9:

(A) Postoperative CT images of a patient (Patient 10) operated using the modified lead trajectories. (B-E) Calculated MaxSAR1g at the tip of ipsilateral and contralateral leads of a computational model derived from Patient-10 data compared to mean MaxSAR1g values calculated with models derived from patients 1–9.