Numerical and Experimental Analysis of Radiofrequency-Induced Heating Versus Lead Conductivity During EEG-MRI at 3 T

Abstract

This study investigates radiofrequency (RF)-induced heating in a head model with a 256-channel electroencephalogram (EEG) cap during magnetic resonance imaging (MRI). Nine computational models were implemented each with different EEG lead electrical conductivity, ranging from 1 to 5.8 × 10⁷ S/m. The peak values of specific absorption rate (SAR) averaged over different volumes were calculated for each lead conductivity. Experimental measurements were also performed at 3-T MRI with a Gracilaria Lichenoides (GL) phantom with and without a low-conductive EEG lead cap ("InkNet"). The simulation results showed that SAR was a nonlinear function of the EEG lead conductivity. The experimental results were in line with the numerical simulations. Specifically, there was a ΔT of 1.7 °C in the GL phantom without leads compared to ΔT of 1.8 °C calculated with the simulations. Additionally, there was a ΔT of 1.5 °C in the GL phantom with the InkNet compared to a ΔT of 1.7 °C in the simulations with a cap of similar conductivity. The results showed that SAR is affected by specific location, number of electrodes, and the volume of tissue considered. As such, SAR averaged over the whole head, or even SAR averaged over volumes of 1 or 0.1 g, may conceal significant heating effects and local analysis of RF heating (in terms of peak SAR and temperature) is needed.

Keywords: Anatomical models; computational electromagnetic modeling; finite-element method (FEM); specific absorption rate (SAR).

Fig. 1.

(a) Computational model of 256-channel hdEEG cap. (b) Anatomical model (head and torso) with the hdEEG cap. (c) Anatomical model with the hdEEG cap and the RF receive array coil model. (d) Anatomical model with the hdEEG cap and the MRI RF receive and transmit coil models.

Fig. 2.

(a) GL phantom used for measurements. Temperature probes are visible (orange plastic optical fibers). (b) hdEEG cap, InkNet.

Fig. 3.

||B₁|| maps with a GL phantom without (a) and with (b) the InkNet. Arrows indicate location of the three largest peak values of ||B₁|| as three locations in the GL phantom expected to undergo the highest temperature rise. Each ||B₁|| peak was mapped to the closest electrode location of the InkNet registered to the GL phantom. (c) Schematic representation of the 256 electrode locations on the head. The bottom of the image corresponds to the back of the head. The three largest ||B₁|| peaks for the GL phantom alone were around Cz and on top of the head (electrodes #8, #44, and #81 marked with red circle), whereas the three ||B₁|| peaks for the GL phantom with the InkNet were around the neck (electrodes #111, #120, and #208 marked with blue circle) where the leads are bundled.

Fig. 4.

Values of R for nine hdEEG lead conductivities for peak single-point SAR (pSAR), peak 0.1-g SAR, peak 1-g SAR, and SAR averaged over the whole head.

Fig.5.

Results of numerical simulations for the head model with No-Cap, and with a hdEEG cap with σ₁ = 5.8 10⁷ S/m and σ2 = 40S/m. Images show: (a) Single-point SAR (pSAR)on the head surface. (b) pSAR in midsagittal plane. (c) Electric field magnitude. (d) Current density magnitude.

Fig. 6.

Temperature simulation results for the head model wearing (a) no EEG cap, (b) EEG cap with lead conductivities of 5.8 × 10⁷ S/m, and (c) EEG cap with lead conductivities of 40 S/m.

Fig. 7.

Temperature rise measured at 1 S/s rate in (a) GL phantom alone and (b) GL phantom with the InkNet. The first 10 min of recording is the baseline measurement with no MRI scanning. During the next 30 min, the high-power TSE sequences are applied then finished 10 min of no MRI scanning.

Fig.8.

Top: R ratio computed for the peak pSAR in three different conditions: (bottom left) full cap; (bottom middle) single lead (a) corresponding to the lead that is the closest to the location of the peak pSAR; (bottom right) single lead (b) corresponds to a neighboring lead.