Synopsis

Keywords: Safety, Safety

A prototype of an adjustable DBS-friendly Tx/Rx 3T coil system was built and evaluated with measurements and numerical simulations characterizing its image quality and SAR profile. The work envisions the use of novel MRI hardware that will make 3T MRI fully accessible to patients with DBS implants.

Introduction

Deep brain stimulation (DBS) of the human brain is a remarkable technique that can treat the symptoms of several major debilitating neurological and psychiatric disorders [1-4]. While the clinical utilities of DBS have grown exponentially, its underlying therapeutic mechanisms of action remain controversial. Due to its superb soft tissue contrast and high-resolution visualization of the brain’s anatomy, MRI is excellently poised to address certain problems about targeting and mechanisms of DBS. Unfortunately, the interactions between the radiofrequency fields of MRI scanners and DBS leads can result in restrictive safety hazards that limit the accessibility of MRI for patients with DBS implants. In our previous study, the feasibility of a rotating birdcage transmitter has been shown to greatly reduce the specific absorption rate (SAR) during imaging of patients with deep brain stimulation implants at 1.5 T [5]. Here we present the first prototype of a DBS-friendly Tx/Rx coil system at 3T MRI, along with the results of measurements and comprehensive numerical simulations that characterize its image quality and SAR profile. The presented work anticipates deploying novel MRI hardware that will enable 3T MRI accessible to DBS patients in its full capacity.

Methods

Hardware design and construction: The 3T coil system was composed of a linearly-polarized rotating high-pass birdcage transmitter, and an anatomical conformal 32-channel receive array (see Fig.1). Uniquely, the transmitter has a slab-like region of low electric field that can be co-aligned with the patient’s individual lead trajectory to encompass the DBS implant. This technique highly reduces the local SAR at the implant tip. An array of 32-channel surface receiver coils was designed for employing highly accelerated brain imaging, such as GRAPPA [6] or simultaneous multislice (SMS) [7,8] methodologies. The coil ensemble went through a comprehensive battery of safety tests to assess the quality of active detuning and SNR maps, as well as evaluating the potential temperature increases due to eddy-currents and RF absorption.
Numerical models: A simulation-based finite element method (FEM) approach with the software package ANSYS Electronic Desktop (HFSS 19.2, ANSYS Inc., Canonsburg, PA.), was used to assess the transmission characteristics of the rotating LP birdcage coil. Tuning and matching of the birdcage coil at the 3T Larmor frequency was achieved by linking the embedded circuit designer with the HFSS model. Patient-specific simulations were performed on 2 realistic DBS lead trajectories, extracted from post-operative CT images of patients. The simplified models consisted of human head like tissue (ε_r= 78; σ = 0.47 S/m), the electrodes consisted of Pt-Ir (ε_r= 1; σ = 4*10^6 S/m), and their Insulation of Urethane (ε_r =3.5; σ = 0 S/m). The meshing all numerical models was automatically adjusted by the FEM solver until it achieved the set accuracy threshold. All simulations converged within 6 Passes and a convergence criteria of ΔS = 0,02. In total 32 simulations were performed on two different patient models with different trajectory paths of the DBS leads and unequal number of implanted pulse generators (IPG). Two patient simulations with a conventional CP birdcage body coil were performed for comparison, one for each patient model. The RF heating was measured in an anthropomorphic 3D printed phantom model (Figure 3). The skull was filled with NaCl-doped agar gel and the torso was filled with saline, both having conductivity of 0.47 S/m. We measured the heating during T1-TSE sequence (B1+_rms = 1.8 µT) in the constructed DBS-friendly coil system and in the regular CP body coil (Figure 3).

Results

The 32-channel DBS-friendly coil showed slight SNR improvement when compared with the standard 32-channel head coil (Figure 5) and is well-suited for modern neuro-MR imaging sequences. The simulation of the LP birdcage coil has shown that the max SAR (0.1g) can be reduced significantly by a factor of 14-15x for both simulated patient models, when compared with regular CP body coil. Temperature measurements in the anthropomorphic phantom setup confirmed this characteristic. However, depending on the rotation angle of the LP birdcage coil, the max SAR could be amplified up to 180% of the CP mode of the body coil. Measurements in the DBS phantom setup showed a temperature increase of only 0.5°C for the optimum angle of the transmit coil.

Conclusion

A first prototype of a DBS-friendly Tx/Rx coil system at 3T MRI was designed, constructed, and validated. The coil showed substantial SAR reduction at the optimum rotation angle, suggesting that it is well suited for imaging patients with DBS implants.

Acknowledgements

No acknowledgement found.