Investigation of Parallel Radiofrequency Transmission for the Reduction of Heating in Long Conductive Leads in 3 Tesla Magnetic Resonance Imaging

Abstract

Deep Brain Stimulation (DBS) is increasingly used to treat a variety of brain diseases by sending electrical impulses to deep brain nuclei through long, electrically conductive leads. Magnetic resonance imaging (MRI) of patients pre- and post-implantation is desirable to target and position the implant, to evaluate possible side-effects and to examine DBS patients who have other health conditions. Although MRI is the preferred modality for pre-operative planning, MRI post-implantation is limited due to the risk of high local power deposition, and therefore tissue heating, at the tip of the lead. The localized power deposition arises from currents induced in the leads caused by coupling with the radiofrequency (RF) transmission field during imaging. In the present work, parallel RF transmission (pTx) is used to tailor the RF electric field to suppress coupling effects. Electromagnetic simulations were performed for three pTx coil configurations with 2, 4, and 8-elements, respectively. Optimal input voltages to minimize coupling, while maintaining RF magnetic field homogeneity, were determined for all configurations using a Nelder-Mead optimization algorithm. Resulting electric and magnetic fields were compared to that of a 16-rung birdcage coil. Experimental validation was performed with a custom-built 4-element pTx coil. In simulation, 95-99% reduction of the electric field at the tip of the lead was observed between the various pTx coil configurations and the birdcage coil. Maximal reduction in E-field was obtained with the 8-element pTx coil. Magnetic field homogeneity was comparable to the birdcage coil for the 4- and 8-element pTx configurations. In experiment, a temperature increase of 2±0.15°C was observed at the tip of the wire using the birdcage coil, whereas negligible increase (0.2±0.15°C) was observed with the optimized pTx system. Although further research is required, these initial results suggest that the concept of optimizing pTx to reduce DBS heating effects holds considerable promise.

Fig 1. Electric field coupling with wire.

Coupling of the electric field ($\stackrel{\rightharpoonup }{E}$) with the wire (shown in grey). Charge distribution is represented by “+”. (a) The tangential component of $\stackrel{\rightharpoonup }{E}$, E _t, induces a current (I) along the wire. (b) The current produces a charge build-up at the tip of the wire creating a concentrated electric field (${\stackrel{\rightharpoonup }{E}}^{final}$), and enhancing the overall power deposition at the tip of the wire according to Ohm’s law.

Fig 2. Geometry of test medium with optimization regions.

Uniform finite cylindrical medium with straight, perfectly conducting wire used in numerical optimization. The plane of interest (POI) for monitoring B₁-field homogeneity is shown shaded. The region of minimization (ROM) is shown in (a) for minimizing the E-field at the tip of the wire, and (b) along the length of the wire.

Fig 3. Coil configurations.

Uniform cylindrical phantom with coil configurations for (a) 16-rung birdcage coil, (b) 2-element pTx, (c) 4-element pTx and (d) 8-element pTx systems.

Fig 4. Electric field enhancement as function of wire position.

(a) Uniform cylindrical medium with implanted wire. Radial position (ρ), position of wire tip (z) and angular location (θ) are indicated. (b) Preliminary simulation results showing the percentage E-field increase between wire tip and background as a function of ρ with z = 0 cm, θ = 90°. Both pTx configurations exhibited E-field increases that grew monotonically from zero as the wire was moved toward the edge of the medium, with 8-element pTx showing smaller effect than 4-element pTx. In contrast, the E-field increase for the birdcage coil grew from zero to a plateau at approximately ρ = 7 cm. (c) Percentage E-field increase between wire tip and background plotted as a function of z with ρ = 7 cm, θ = 90°. Maximal increase occurred between z = -1 cm and z = 4 cm for 4-element and 8-element pTx, respectively, whereas the birdcage coil produced maximal effect between z = -2 cm and z = 0 cm. (d) Percentage E-field increase between wire tip and background plotted as a function of θ with ρ = 7 cm, z = 0 cm. Maximal heating occurs at θ = 90° with 4-element pTx showing the largest angular variation.

Fig 5. Percent reduction in E-field magnitude between pTx and birdcage coil for various pTx channel counts.

Data for four wire positions are shown: ρ = 2.5 cm, z = 0 cm (blue squares); ρ = 7.5 cm, z = 0 cm (purple circles); ρ = 8 cm, z = 2 cm (green triangles); and ρ = 6 cm, z = 0 cm (red triangles).

Fig 6. Contour plots of E-field magnitude.

Contour plots of E-field magnitude on a log scale in the plane parallel to the wire for (a) 16-rung birdcage coil operating in quadrature mode, and optimized (b) 2-element pTx, (c) 4-element pTx and (d) 8-element pTx. All pTx configurations are operating with amplitudes and phase shifts determined for optimal E-field suppression (Table 1). Wire is located at x = 7.5 cm, z = -12 cm to z = 0 cm.

Fig 7. Magnetic field in the imaging plane (z = 0 cm).

(a) Birdcage coil, signal variation = 24.3%, (b) 2-element pTx, signal variation = 46.0%, (c) 4-element pTx, signal variation = 17.3% and (d) 8-element pTx, signal variation = 9.0%. Wire is located at x = 7.5 cm, y = 0 cm, indicated by white arrow. Signal variation is the standard deviation divided by the mean calculated over circular region of interest with radius = 8 cm (dashed line).

Fig 8. Contour plots of magnetic field magnitude.

Contour plots of magnetic field magnitude on a log scale in the plane parallel to the wire for (a) 16-rung birdcage coil, signal variation = 17.1%, (b) 2-element pTx, signal variation = 42.1%, (c) 4-element pTx, signal variation = 13.9%, and (d) 8-element pTx, signal variation = 15.3%. Signal variation is the standard deviation divided by the mean calculated over a 14 cm x 8 cm rectangular region of interest (dashed line). The wire is located at x = 7.5 cm, z = -12 cm to z = 0 cm, and is readily identifiable in (a).

Fig 9. Change in temperature during FRFSE imaging of a uniform phantom containing an inserted copper wire.

Results are shown for the 4-element pTx system operating in quadrature mode, in suppression mode, and the standard birdcage coil available with the MRI system.

Fig 10. Axial slice in center of phantom obtained using 4-element pTx coil in suppression mode compared with simulation.

(a) H-field obtained from simulation with optimal phase shimming (φ₁ = 40.2°, φ₂ = 15.7°, φ₃ = 200.0°, φ₄ = 301.1°). Wire position indicated by white cross. (b) Axial slice in center of phantom obtained with GRE sequence (φ₁ = 40°, φ₂ = 16°, φ₃ = 200°, and φ₄ = 301°). Wire position indicated by white arrow. Signal voids created by fiber optic temperature probe indicated by black arrows.