A Reproducible Safety Assessment Model for Concurrent Transcranial Magnetic Stimulation with Implanted Intracranial...

Concurrent transcranial magnetic stimulation and intracranial EEG (TMS-sEEG) offers a powerful approach for interrogating human brain circuits with high spatiotemporal resolution. However, standardized safety testing procedures for TMS in the presence of implanted electrodes are lacking.

To establish and validate a reproducible methodological framework for assessing TMS safety with sEEG hardware, addressing potential risks of electrode displacement, heating, and secondary current induction.

A conductive agarose phantom implanted with clinical-grade platinum sEEG electrodes was exposed to single-pulse TMS (0.4 Hz), repetitive TMS (1-50 Hz), and intermittent theta burst stimulation. Electrode displacement was quantified via high-speed video and motion tracking. Temperature changes were measured with fiber-optic thermal probes positioned at stimulated electrode contacts. Induced voltages were recorded using an oscilloscope and intracranial EEG hardware, and secondary current and charge density were calculated from oscilloscope outputs and impedance measurements.

sEEG lead displacement remained micrometer-scale across stimulation conditions, with a maximum transient displacement of 10.99 μm and a maximum 1-s mean displacement from the pre-TMS position of 16.27 μm. Temperature changes remained well within the 2°C safety threshold across all tested conditions; in the primary temperature series, the maximum observed increase was 0.073°C. Induced voltages and charge densities remained below established safety limits, exhibiting consistent exponential decay with increasing distance from the coil. All stimulation conditions, including those exceeding typical clinical ranges, remained well within safety margins.

Under the tested conditions in a controlled phantom environment, TMS exposure produced micrometer-scale transient and longer-timescale displacement, temperature changes within established safety thresholds, and charge densities within established safety thresholds. This validated methodological pipeline provides a standardized framework for future safety experiments involving TMS and implanted devices and supports continued development of TMS-sEEG as a tool for studying human brain circuitry and neuromodulation.