RF heating of deep brain stimulation implants during MRI in 1.2 T vertical scanners versus 1.5 T horizontal systems: A simulation study with realistic lead configurations

Abstract

Patients with deep brain stimulation (DBS) implants are often denied access to magnetic resonance imaging (MRI) due to safety concerns associated with RF heating of implants. Although MR-conditional DBS devices are available, complying with manufacturer guidelines has proved to be difficult as pulse sequences that optimally visualize DBS target structures tend to have much higher specific absorption rate (SAR) of radiofrequency energy than current guidelines allow. The MR-labeling of DBS devices, as well as the majority of studies on RF heating of conductive implants have been limited to horizontal close-bore MRI scanners. Vertical MRI scanners, originally introduced as open low-field MRI systems, are now available at 1.2 T field strength, capable of high-resolution structural and functional imaging. No literature exists on DBS SAR in this class of scanners which have a 90° rotated transmit coil and thus, generate a fundamentally different electric and magnetic field distributions. Here we present a simulation study of RF heating in a cohort of forty patient-derived DBS lead models during MRI in a commercially available vertical openbore MRI system (1.2 T OASIS, Hitachi) and a standard horizontal 1.5 T birdcage coil. Simulations were performed at two major imaging landmarks representing head and chest imaging. We calculated the maximum of 0.1g-averaged SAR (0.1g-SAR_Max) around DBS lead tips when a B₁⁺ = 4 µT was generated on an axial plane passing through patients body. For head landmark, 0.1g-SAR_Max reached 220±188 W/kg in the 1.5 T birdcage coil, but only 14±11 W/kg in the OASIS coil. For chest landmark, 0.1g-SAR_Max was 24±17 W/kg in the 1.5 T birdcage coil and 3±2 W/kg in the OASIS coil. A paired two-tail t-test revealed a significant reduction in SAR with a large effect-size during head MRI (p < 1.5×10^-8, Cohen's d = 1.5) as well as chest MRI (p < 6.5×10^-10, Cohen's d = 1.7) in 1.2 T Hitachi OASIS coil compared to a standard 1.5 T birdcage transmitter. Our findings suggest that open-bore vertical scanners may offer an untapped opportunity for MRI of patients with DBS implants.

Fig. 1 -

Coil configuration of 1.2 T vertical and 1.5 T horizontal coils loaded with a human body model placed at two different positions corresponding to head and chest imaging.

Fig. 2 -

Geometry configuration of (A) 1.2 T high-pass radial planar birdcage coil and (B) 1.5 T high-pass birdcage coil. (C-D) B₁⁺ field maps on the central coronal and axial planes passing through the human body model with no implants. The input power of coils is adjusted to generate a mean B₁⁺ = 4 μT over a circular plane placed on an axial plane passing through coils’ iso-center.

Fig. 3 –

(A) Examples of post-operative CT images of three patients (patient numbers ID1-ID3). (B) Reconstructed models of isolated DBS leads. Lead trajectories were extracted using CT images of 20 patients with bilateral DBS implantation (patient numbers ID1-ID20) and were registered in a homogenous body phantom for electromagnetic simulations.

Fig. 4 -

Local SAR distributions in patient 11 (ID11) for the 1.2 T vertical and 1.5 T horizontal coils both with head and chest landmarks on an axial plane that passes through the tips of implants. The input power of coils is adjusted to generate a mean B₁⁺ = 4μT over a circular plane placed on an axial plane passing through coils’ iso-center.

Fig. 5 -

Local 0.1g-SAR_max over 40 leads shown for the 1.2 T vertical and 1.5 T horizontal coils for head and chest landmarks. The outliers were plotted individually using a red ‘+’ symbol.