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

Area of Science:

• Medical Physics
• Biomedical Engineering
• Electromagnetic Compatibility

Background:

• Magnetic Resonance Imaging (MRI) safety is paramount, with radiofrequency (RF) power deposition being a key concern.
• Electroencephalography (EEG) is increasingly used during MRI, necessitating an understanding of its impact on RF safety.
• Non-magnetic metallic EEG electrodes and leads can potentially alter RF field distribution and tissue heating.

Purpose of the Study:

• To quantify the changes in specific absorption rate (SAR) in human head tissues when using non-magnetic metallic EEG electrodes and leads during MRI.
• To evaluate the influence of different numbers of EEG electrodes/leads and coil types (surface vs. bird cage) on SAR.
• To assess SAR variations at different operating frequencies (128 MHz and 300 MHz).

Main Methods:

• A realistic, high-resolution (1 mm³) head model derived from individual MRI data was utilized.
• The Finite-Difference Time-Domain (FDTD) algorithm was employed to calculate RF power deposition.
• Simulations were conducted using both surface and bird cage coils with varying numbers of EEG electrodes (16, 31, 62, 124) at 128 MHz and 300 MHz.

Main Results:

• The presence of EEG electrodes/leads increased SAR, with a more pronounced effect observed with the bird cage coil compared to the surface coil.
• At 300 MHz, using 124 electrodes led to a significant SAR increase (up to x172.3) compared to the no-electrode condition.
• Specific tissues showed notable SAR increases: bone marrow (x3.6) and skin (x7.4) at 300 MHz. Whole head SAR increased by x5.6 at 128 MHz.
• Inter-subject variability in whole head SAR was found to be 3%.

Conclusions:

• Non-magnetic metallic EEG electrodes and leads can substantially elevate SAR in human head tissues during MRI procedures.
• The increase in SAR is dependent on the number of electrodes, operating frequency, and coil type.
• These findings highlight the importance of considering EEG hardware in MRI safety assessments to prevent potential thermal risks.