Feasibility of using linearly polarized rotating birdcage transmitters and close-fitting receive arrays in MRI to reduce SAR in the vicinity of deep brain simulation implants

Abstract

Purpose: MRI of patients with deep brain stimulation (DBS) implants is strictly limited due to safety concerns, including high levels of local specific absorption rate (SAR) of radiofrequency (RF) fields near the implant and related RF-induced heating. This study demonstrates the feasibility of using a rotating linearly polarized birdcage transmitter and a 32-channel close-fit receive array to significantly reduce local SAR in MRI of DBS patients.

Methods: Electromagnetic simulations and phantom experiments were performed with generic DBS lead geometries and implantation paths. The technique was based on mechanically rotating a linear birdcage transmitter to align its zero electric-field region with the implant while using a close-fit receive array to significantly increase signal to noise ratio of the images.

Results: It was found that the zero electric-field region of the transmitter is thick enough at 1.5 Tesla to encompass DBS lead trajectories with wire segments that were up to 30 degrees out of plane, as well as leads with looped segments. Moreover, SAR reduction was not sensitive to tissue properties, and insertion of a close-fit 32-channel receive array did not degrade the SAR reduction performance.

Conclusion: The ensemble of rotating linear birdcage and 32-channel close-fit receive array introduces a promising technology for future improvement of imaging in patients with DBS implants. Magn Reson Med 77:1701-1712, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Keywords: MRI; computational modeling; deep brain stimulation (DBS); electrode artifact; finite element modeling; medical implants; safety; specific absorption rate.

FIG. 1

Anatomical structures segmented for the head model with related biophysical properties assigned for this study. ^(a) Ref .

FIG. 2

Left: the linearly polarized rotating birdcage and the anthropomorphic head phantom. The coil can be easily rotated by hand and locked in a marked position. Right: Finite element method simulation setup. Linear birdcage was fed with a 46-cm coaxial cable (inner conductor radius Rin = 0.245 mm, shield radius Rout = 0.856 mm, dielectric relative permittivity ε_r = 2:25). Positions of the head model, scanner shield, and tuning/matching ports are illustrated. RF = radiofrequency.

FIG. 3

Left: Typical DBS implantation paths modeled based on conventional subthalamic nucleus DBS surgical approach as laid out in Ref . Right: Generic DBS lead model used in the simulations, with spacing of 0.5 mm based on lead model 3389 (Medtronic Inc., Minneapolis, MN). DBS = deep brain stimulation.

FIG. 4

(a) MaxSAR and RefSAR values for 1g-averaged SAR in the heterogeneous head model in presence of DBS implant models with different entry angles. (b)&(c) Electric field of the linear birdcage transmitter with feed in the default position (φ = 0°) and optimum position for SAR reduction. The path of DBS lead is indicated. DBS = deep brain MaxSAR = maximum SAR; RefSAR = reference SAR; SAR = specific absorption rate.

FIG. 5

Simulations results versus measurements. Integrated computation pipeline combines finite element method simulations with radiofrequency circuit analysis and includes details of feed model. Simulation show excellent agreement between scattering parameters (S) measured both outside and inside of magnet bore. ${\mathrm{B}}_1^{+}$ maps are simulated and measured on an axial plane at 8 cm from the top of the head.

FIG. 6

MaxSAR and RefSAR values for 1 g-averaged SAR in the heterogeneous head model in presence of a deep brain stimulation implant with 15-degree off-sagittal entry angel. Simulations are performed with and without loops around the surgical burr hole. DBS = deep brain stimulation; MaxSAR = maximum SAR; RefSAR = reference SAR; SAR = specific absorption rate.

FIG. 7

MaxSAR values for 1 g-averaged SAR in the heterogeneous head model in the presence of a deep brain stimulation implant with 15-degree off-sagittal entry angel with and without 32-channel receive array. MaxSAR = maximum specific absorption rate; SAR = specific absorption rate.

FIG. 8

MaxSAR and RefSAR for 1 g averaged SAR inside the inhomogeneous head model for a range of gray matter conductivities. Left: coil in default position (feed up), Right: coil with optimum rotation angle for SAR reduction. MaxSAR=maximum SAR; RefSAR=reference SAR; SAR=specific absorption rate.

FIG. 9

(a) Simulation results showing visual artifact in ${\mathrm{B}}_1^{+}$ field maps vs. 1g averaged SAR for different coil rotation angles. As predicted, the location of minimum ${\mathrm{B}}_1^{+}$ artifact coincides with the location of minimum SAR amplification. (b) Anthropomorphic head phantom with implants used in artifact measurement. (c)&(d) Several implantation paths with typical subthalamic nucleus deep brain stimulation surgical approach were studied. In all studies, we were able to find an optimum coil rotating angle that significantly reduced the electrode artifact in ${\mathrm{B}}_1^{+}$ field maps. SAR = specific absorption rate.