2.1. Establishing optimised SNR for sEMG in MR-environments
When considering SNR optimisation, we assume that a crosstalk free sEMG is the signal of interest and the noise is caused by the scanner gradient system.
The first point we should consider is impedance at the skin-electrode interface. Due to the higher frequency band of interest, skin electrode impedances for sEMG are more important than for scalp EEG. We advise that impedances are approximately 5 kOhm for sEMG (Fridlund & Caccioppo, 1986). Preparation of the measurement site is also important and there are some considerations specific to sEMG. For example, limb sEMG is often carried out in regions with a thicker and drier epidermis than scalp EEG, so careful preparation using alcohol and high chloride abrasive gel is important and we recommend using Abralyt HiCl. Achieving the recommended impedance level is facilitated by the BrainAmp ExG MR impedance measurement circuit which gives precise impedance values for the positive (+) and negative (–) lead wire of each sEMG channel separately. Separate measurement of the + and – channels is important because they must be balanced to facilitate BrainAmp ExG MR common mode suppression in the best possible way.
In addition, the impedance of the ground (GND) electrode must be as low as possible and the lead wire paths of the + and – wires must be as close to each other as possible. Our pairs of Multitrode MR electrodes are bound together with a spiral tubing to make sure they stay close together.
The position of the recording electrodes is critical to optimising the SNR in the MR environment because the inter electrode distance affects not only the sEMG amplitude and possible crosstalk, but also the gradient artifact amplitude, thus signal AND noise. Consequently, when measuring sEMG in the scanner there is trade-off between maximised EMG amplitude and minimised gradient artifact amplitude. The further apart the electrodes are, the larger the gradient artifact, so a classical placement that works in the lab doesn’t generally work in the scanner, the electrodes need to be closer together. While EEG electrode positioning is given by the cap layout, sEMG electrode positioning has more degrees of freedom and therefore a greater impact on the overall signal quality. A good place to start for meaningful electrode positions and electrode distance is the SENIAM project: http://www.seniam.org. Once you have a good starting point, the fine tuning must be done in pilot sessions until the gradient correction results and EMG signal properties match the requirements of the study.