Numerical Simulations of Realistic Lead Trajectories and an Experimental Verification Support the Efficacy of Parallel Radiofrequency Transmission to Reduce Heating of Deep Brain Stimulation Implants during MRI

C E Mcelcheran,G Bonmassar,K J T Anderson,P.-S Wei,B Yang,M I Iacono,L Golestanirad,S J Graham

doi:10.1038/s41598-018-38099-w

Abstract

Patients with deep brain stimulation (DBS) implants may be subject to heating during MRI due to interaction with excitatory radiofrequency (RF) fields. Parallel RF transmit (pTx) has been proposed to minimize such RF-induced heating in preliminary proof-of-concept studies. The present work evaluates the efficacy of pTx technique on realistic lead trajectories obtained from nine DBS patients. Electromagnetic simulations were performed using 4- and 8-element pTx coils compared with a standard birdcage coil excitation using patient models and lead trajectories obtained by segmentation of computed tomography data. Numerical optimization was performed to minimize local specific absorption rate (SAR) surrounding the implant tip while maintaining spatial homogeneity of the transmitted RF magnetic field (B1+), by varying the input amplitude and phase for each coil element. Local SAR was significantly reduced at the lead tip with both 4-element and 8-element pTx (median decrease of 94% and 97%, respectively), whereas the median coefficient of spatial variation of B1+ inhomogeneity was moderately increased (30% for 4-element pTx and 20% for 8-element pTx) compared to that of the birdcage coil (17%). Furthermore, the efficacy of optimized 4-element pTx was verified experimentally by imaging a head phantom that included a wire implanted to approximate the worst-case lead trajectory for localized heating, based on the simulations. Negligible temperature elevation was observed at the lead tip, with reasonable image uniformity in the surrounding region. From this experiment and the simulations based on nine DBS patient models, optimized pTx provides a robust approach to minimizing local SAR with respect to lead trajectory.

Highlights

Patients with deep brain stimulation (DBS) implants may be subject to heating during magnetic resonance imaging (MRI) due to interaction with excitatory radiofrequency (RF) fields
Despite the fact that this value is substantially lower than the maximum raw specific absorption rate (SAR) produced by the birdcage coil for this patient model, namely 24.5 W/kg (Fig. 4d), this solution produces a temperature increase of 1.2 °C. eBs1t+SBii1nm+hiiolnamhrlooy,mgFeonigge.ei5tnysehsitooylwu(CstiOtohnVesBB1w+1 =+er- ef3i5e0l%.d1)3foi Wnr tF/hkiegg.loa5nwbd.eTs0th.B1e51c+ Woirn/rhkeosgpm, oroensgdpeienncegtiit1vy eg(lCyS.OABVRoBtv1+ha =lcua 2ess0e%fsoo)rcitnchuFerilgion.w5aeassatinnadgnldtehhpeiahgthiigeehns-tt model, patient VIII, with the best case occurring when optimization was completed with 8-channel pTx, and the worst case occurring when optimization was completed with 4-channel pTx
This study provides important evidence that pTx RF shimming can be optimized to minimize heating at DBS lead tips when different realistic lead trajectories are taken into account

Summary

Introduction

Patients with deep brain stimulation (DBS) implants may be subject to heating during MRI due to interaction with excitatory radiofrequency (RF) fields. Numerical optimization was performed to minimize local specific absorption rate (SAR) surrounding the implant tip while maintaining spatial homogeneity of the transmitted RF magnetic field (B1+), by varying the input amplitude and phase for each coil element. The efficacy of optimized 4-element pTx was verified experimentally by imaging a head phantom that included a wire implanted to approximate the worst-case lead trajectory for localized heating, based on the simulations. Negligible temperature elevation was observed at the lead tip, with reasonable image uniformity in the surrounding region From this experiment and the simulations based on nine DBS patient models, optimized pTx provides a robust approach to minimizing local SAR with respect to lead trajectory. Because of its non-invasive nature and unparalleled soft-tissue contrast, magnetic resonance imaging (MRI) with high-field systems (3 T or higher) has great potential for DBS target verification and electrode localization, as well as evaluation of co-morbidities. Temperature increases ranging from 1°16–46°17 have been measured depending on a variety of factors such as magnetic field strength, pulse sequence, DBS lead geometry and the type of RF coil used

Methods

Results

Conclusion