Artifact-Suppressing Amplifier for EMG Registration during Repetitive Peripheral Magnetic Stimulation (RPMS)

Abstract

To induce a most physiological proprioceptive inflow to the CNS, the intensity and the repetition frequency of the RPMS are adapted by a closed loop control to induce coordinated movements. For the closed loop control and the quantification of the therapeutic outcome it is necessary to measure the (remaining) voluntary muscle activity, while the RPMS is applied. This is essential, since the patient is advised to assist the induced movements as far as possible. Therefore an RPMS artefact-suppressing EMG amplifier has to be developed to measure the activity of the stimulated muscle during the stimulation. Hereby the stimulation coil is located directly above the two EMG electrodes. Such amplifiers are well-known and established for functional electrostimulation, while artefact-suppressing amplifiers for RPMS are fairly unknown. Because of RPMS impulses, a high (compared to the EMG) voltage is induced into the EMG electrodes. This voltage changes the charge of the skin capacity and therefore the DC offset of the EMG. The induced voltage also elicits an impulse response of the filters in the EMG amplifier. Hence an artefact suppression is necessary, otherwise the amplifier is permanently in saturation since the RPMS impulses are repeated with 20Hz. The trigger signal for the stimulation device is also used for the artefact suppression. The artefact suppression itself runs on three levels: Voltages higher than 1.4 V at the EMG electrodes are filtered by antiparallel diodes independent of the stimulation impulse. If the suppression is triggered, the input to the difference amplifier is shorted by two bipolar transistors and the input to the DC offset compensation is grounded. With this suppression unit its is possible to reduce the gap in the measured EMG down to 3 ms. In addition to the artefact suppression, the following requirements (excerpt) are fulfilled in the developed EMG amplifier: selectable amplification gain, bandwidth of 3Hz to 15 kHz, an anti-aliasing filter with a selectable edge frequency, a 50Hz band-stop filter and an active shielding (no external pre-amplifier module). Due to a difference amplifier and the artefact suppression the input resistance is at least 500 MOhm and the patient is protected form dangerous high voltages by an isolation barrier (3 kVrms). Also a conventional speaker module is integrated into the designed amplifier.