Low impedance electrodes improve detection of high frequency oscillations in the intracranial EEG

Abstract

Objective: Epileptic fast ripple oscillations (FR, 250-500 Hz) indicate epileptogenic tissue with high specificity. However, their low amplitude makes detection demanding against noise. Since thermal noise is reduced by low impedance electrodes (LoZ), we investigate here whether this noise reduction is relevant in the FR frequency range.

Methods: We analyzed intracranial electrocorticography during neurosurgery of 10 patients where a low impedance electrode was compared to a standard electrode (HiZ) with equal surface area during stimulation of the somatosensory evoked potential, which evokes a robust response in the FR frequency range. To estimate the noise level, we computed the difference between sweep 2n and sweep 2n + 1 for all sweeps.

Results: The power spectral density of the noise spectrum improved for the LoZ over all frequencies. In the FR range, the median noise level improved from HiZ (0.153 µV) to LoZ (0.089 µV). For evoked FR, the detection rate improved (91% for HiZ vs. 100% for LoZ).

Conclusions: Low impedance electrodes for intracranial EEG reduce noise in the FR frequency range and may thereby improve FR detection.

Significance: Improving the measurement chain may enhance the diagnostic value of FR as biomarkers for epileptogenic tissue.

Keywords: Fast ripple; Intraoperative neuromonitoring; Neurosurgery; Noise level; Sensory evoked potentials; Signal-to-noise ratio.