Electromyography in MRI - First recordings of peripheral nerve activation caused by fast magnetic field gradients

Prior studies on the evaluation of stimulation by MRI were based on the sub jective feeling of the volunteers. A wide variety of stimulation thresholds between the subjects was observed. In order to exclude subjective percepti on levels as a cause of this variation, we developed a method to investigat e the activation of peripheral nerves after gradient switching by electromy ography (EMG) within the MR-imager. Five healthy volunteers were positioned in the MR-scanner with the bridge of the nose at isocenter. The amplitude of sinusoidal pulse trains of the anterior-posterior gradient (rise-times: 200 or 300 mu s, various numbers of oscillations) was increased stepwise. F our surface electrodes were placed on the region where a muscle-twitch was reported. Electric activity of the muscle during stimulation experiments wa s recorded with an MR-compatible electro-physiologic amplifier. Stimulation thresholds were defined by the appearance of an EMG-signal. Thresholds wer e sharp and consistent with the report of the subjects. (C) 2000 Wiley-Liss , Inc.