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Influence of the Electrode Diameter on Radio Frequency-induced Heating in MRI

Johannes B Erhardt¹, Thomas Lottner², Jessica Martinez³, Ali C Özen², Martin Schuettler⁴, Thomas Stieglitz⁵, Daniel B Ennis⁶, and Michael Bock²

¹University of Freiburg, Freiburg, Germany, ²University Medical Center Freiburg, Freiburg, Germany, ³University of California Los Angles, Los Angeles, CA, United States, ⁴CorTec, Freiburg, Germany, ⁵Dep. of Microsystems Engineering, University of Freiburg, Freiburg, Germany, ⁶Stanford University, Stanford, CA, United States

Synopsis

Patients with implanted electrodes often require an MRI exam. This exposes them to the risk of radio-frequency induced heating. In this study heating of implantable electrodes of diameters between 0.3-4mm were evaluated in a 1.5T MRI system. In situ temperature measurements were compared to simulations of the specific absorption rate to assess local and total dissipated power. Measurements showed temperature increases between 0.8-53K. Compared to large electrodes, small electrodes are subject to less dissipated power, but more localized power density. Thus, smaller electrodes might be classified as safe in current certification procedures but may be more likely to burn tissue.

Introduction

Planar multi-electrode arrays are commonly implanted onto the brain surface to record the electro-corticogram of epilepsy patients. Accurate localization of the implanted electrodes with respect to the brain anatomy is often done using MRI. However, the radio frequency (RF) excitation during MRI can cause severe temperature increase (ΔT) in tissue surrounding the electrodes¹. In the recent years there was a trend towards smaller electrode diameters, but their RF-induced heating properties in MRI have not been investigated. Therefore, in this work 4 electrode strips with different electrode diameters were investigated to identify geometrical factors influencing RF-induced heating.

Methods

Four different electrode diameters (0.3, 1, 2.7 and 4mm) were analyzed using four electrode strips. Each strip had four electrodes of the same diameter connected by meandering interconnection lines (MIL), which are connected to 34cm-long PtIr(90/10) wires that were dip-coated in silicone rubber for insulation. The sample holder was placed into an MRI phantom as described in the ASTM-F2182 standard² and filled with 30L of hydroxyethyl cellulose gel with a conductivity of 0.49S/m. Reproducible sample placement was enabled by a LEGO (LEGO, Denmark) plate glued to the bottom of the phantom and consistent placement of the sample holder. Temperature measurements were performed using a fibre-optic thermometer (Optocon, Germany) with 0.1K resolution. The distance of the probe tip to the electrode was varied between 0 and 3mm. Figure1-AB shows the experimental setup for phantom experiments and the fiber optic temperature probes (FOTP). Heating experiments were performed in a 1.5T MRI system (Tim-Symphony, Siemens). A patient weight of 75kg was assumed and the energy deposition adjusted to 1W/kg. The most distal electrode was placed at the center of the bore with 11,3cm lateral offset. Finite difference time domain simulations of the E-field were performed using Sim4Life (Version3.4, ZMT-AG, Switzerland) to investigate the local SAR distribution around the electrodes with and without FOTP. RF excitation was simulated by a harmonic voltage source (f=64MHz) at the MIL end. The electrode material was modeled as a perfect electric conductor covered by a 60µm isolating silicone layer (ε_r=3, σ=0S/m) except for the electrode contact, all surrounded by a 10x10x17cm³ volume of gel (εr=81, σ=0.47S/m) as in the measurements. The smallest voxel size was 0.04mm³ and the calculation time was 15-20hours. Figure1-C shows the drawing upon which simulations of the specific absorption rate (SAR) have been performed on.

Results

Figure2 and Figure3 show measurement results of the most distal electrodes as an example. SAR maps and maximum local SAR obtain from simulations are shown in Figure4 and Figure5 respectively.

Discussion

As expected the temperature decreases with increasing FOTP distance, except for the two smaller diameters with the FOTP in contact. The highly localized SAR hot spots predicted by the simulations could not be detected in the measurements possibly due to the displacement of gel by the FOTP - which is the same size or larger than the electrode diameter. It is interesting to note that ΔT_max of the 4mm electrodes is lower than for the 2.7mm electrodes. A possible explanation is a larger volume in which the power is dissipated. This relationship between the input energy and the distribution volume is also seen in the simulations (Figure5). This suggests that a significantly larger volume is heated in the vicinity of the larger electrodes while the local SAR is reduced, whereas SAR is locally concentrated and increased for the smaller electrodes. The total deposited power is three times higher for the two larger diameters than for the smallest. The simulations also suggest that for implanted electrodes most of the energy deposition occurs in a volume equivalent to 1g tissue, and that this value or even less should be used for safety assessment. Furthermore, for clinical applications it is necessary to decide whether a large electrode diameter with an associated higher total energy deposition over a larger area is preferable over a smaller electrode with a reduced total energy deposition but a much higher energy density in a small volume.

Conclusion

Heating measurements of electrodes with different diameters (0.3mm-4mm) showed maximum temperature increases between 0.8K and 53K. Comparing simulations of 0.3mm to 4mm electrode contacts showed 92-fold higher local energy density, but one third of the total energy dissipation. Accordingly, the diameter of planar electrodes has a strong influence on the local distribution and the total amount of dissipated RF-induced heating around the electrode contact. Small electrodes may thus be more prone to exceed safe tissue temperature thresholds, while being more likely to stay within regulatory safety limits that average over a volume.

Acknowledgements

This work was financially supported by the BrainLinks-BrainTools Cluster of Excellence funded by the German Research Foundation (DFG, grant number EXC 1086) and NIH R21 HL127433 to DBE.

References

¹ Erhardt, J.B., Fuhrer, E., Gruschke, O.G., Leupold, J., Wapler, M.C., Hennig, J., Stieglitz, T., Korvink, J.G., 2018. Should patients with brain implants undergo MRI? J. Neural Eng. 15, 041002. https://doi.org/10.1088/1741-2552/aab4e4

² ASTM standard F 2182-2011a, 2011. ASTM 2182 Standard test method for measurement of radio frequency induced heating near passive implants during magnetic resonance imaging. ASTM Int. 1–14. https://doi.org/10.1520/F2182-11A.1.7

Figures

Electrode strips mounted on identical, laser cut plexiglass holders. The electrode centre corresponds with the vertical axis in which the red FOTP tip is placed. The stripe holder is plugged into LEGO bricks which allows for reproducible placement in the phantom (not shown). The distance between the FOTP tip and the electrodes was varied by replacing all LEGO bricks in one plane (i.e. yellow piece) by shortened ones. The right inset shows drawings used for SAR simulations of the most distal electrodes and their diameters on the strips. The electrodes are connected by an identical meandering interconnection line.

Characteristic recording curves of the ΔT at the electrodes with the longest MIL. On the left with the FOTP in contact, on the right with a FOTP distance of 2mm. RF-induced heating was applied for 300s. The preceding 50s measurements (t<0) served as baseline for the displayed DT. The cool down period starts at t=300s. The 2.7mm contact heats up the most (53K). The two smaller contact diameters show less heating in the left diagram compared to the right one.

Maximum temperature difference ΔT_max in K for each FOTP distance (contact - 3mm), contact diameter, at the most distal electrodes of each strip. ΔT_max for the same electrode diameter drops with increasing FOTP distance, expect for the 0.3mm and 1mm electrode with the FOTP in contact. More heating occurs at the larger electrode diameters where the 2.7mm electrode shows most heating except for the 3mm FOTP distance.

SAR maps for each electrode in frontal and lateral views. The frontal view (left) displays SAR values in a parallel plane 1mm above the electrodes (dashed line in right panels) in the range of 0 to 0.005W/kg. With increasing electrode diameter, the SAR is distributed over an increasing area while the local SAR intensity is decreasing. The lateral view (right) shows SAR values perpendicular to the electrode plane along the central axis (dashed line in left panels) using the same SAR scale. The difference in the SAR distribution into the tissue adjacent to the different electrode diameters becomes evident.

Maximum local SAR and mean SAR in a 1x1x1.5cm³ volume above the electrodes with and without modelled FOTP calculated from the simulated E-field data. While the local SAR decreases with increasing contact diameter by a factor of 92, the mean SAR increases 3-fold where the maximum is found at 2.7mm, which is in agreement with the measured data. Introduction of a FOTP in 1mm distance to the electrode had little impact, in contact however, the mean SAR of the 0.3mm and 1mm dropped by ~90%.

Proc. Intl. Soc. Mag. Reson. Med. 27 (2019)

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