Metallic electrodes and leads in simultaneous EEG‐MRI: Specific absorption rate (SAR) simulation studies

Abstract

The purpose of this study was to investigate the changes in specific absorption rate (SAR) in humanhead tissues while using nonmagnetic metallic electroencephalography (EEG) electrodes and leads during magnetic resonance imaging (MRI). A realistic, high resolution (1 mm 3 ) head model from individual MRI data was adopted to describe accurately thin tissues, such as bone marrow and skin. The RF power dissipated in the human head was evaluated using the FDTD algorithm. Both surface and bird cage coils were used. The following numbers of EEG electrodes/leads were considered: 16, 31, 62, and 124. Simulations were performed at 128 and 300 MHz. The difference in SAR between the electrodes/leads and no-electrodes conditions was greater with the bird cage coil than with the surface coil. The peak 1 g averaged SAR values were highest at 124 electrodes, increasing to as much as two orders of magnitude (Â172.3) at 300 MHz compared to the original value. At 300 MHz, there was a fourfold (Â3.6) increase of SAR averaged over the bone marrow, and a sevenfold (Â7.4) increase in the skin. At 128 MHz, there was a fivefold (Â5.6) increase of whole head SAR. Head models were obtained from two different subjects, with an inter-subject whole head SAR variability of 3%.

Simultaneous EEG–fMRI at ultra-high field: Artifact prevention and safety assessment

“…While useful and practical, these tests are limited in spatial coverage, and cannot assess local SAR variations occurring in vivo within the brain. As a valuable complement, EM simulations using realistic head models allow the estimation of high-resolution SAR distributions across the head, but only a small number of studies have presented results from such approaches (Angelone et al, 2004.…”

“…Safety concerns arise from the possible generation of electric currents along the EEG wires and through biological tissues, induced by the fast-switching MRI gradients or radio-frequency (RF) pulses ). Additionally, the presence of the conductive EEG materials may alter the transmit B 1 field (B 1 + ) distribution across the head, introducing unpredicted local changes in specific absorption rate (SAR) (Angelone et al, 2004). At 7 T, RF pulse wavelengths become smaller than the typical sample size, greatly increasing the risk of resonant antenna effects along the EEG leads ) and creating more heterogeneous B 1 distributions (Eggenschwiler et al, 2012).…”

An EEG (electroencephalogram) recording system with carbon wire electrodes for simultaneous EEG-fMRI (functional magnetic resonance imaging) recording

“…Widely used materials for EEG electrodes used in EEG-fMRI recordings are Ag (sliver)/AgCl (silver-chloride), metal coated solid carbon or plastic, or solid carbon. Metals in the MRI environment cause RF (radio-frequency) heating (Angelone et al 2004;Bonmassar 2004) and susceptibility artifacts (Krakow et al 2000). Moreover, EEG electrodes that are made of solid materials can be uncomfortable for subjects to wear during long EEG-fMRI sessions.…”

On the feasibility of concurrent human TMS-EEG-fMRI measurements

“…Safety and quality assurance should also be cautiously examined for any hardware deviations from the current setup such as the use of different RF coils (a body coil might, for example, induce higher temperature increases; Nöth et al 2012) or use with higher static magnetic field strengths (which can lead to higher temperature increases; Mullinger et al 2008a). Furthermore, resistive rather than (nonmagnetic) metallic electrode leads should be used to avoid temperature increases (Angelone et al 2004). In addition, low-SAR imaging sequences should be used to avoid potential heating of biological tissue.…”