Measurement of Peripheral Nerve Stimulation caused by Fast Magnetic Field Gradients by means of Electromyography

Introduction: The need for fast pulse sequences in magnetic resonance imaging is high. Imaging time depends on the amplitude and rise time of the switched magnetic field gradients. Advances in scanner hardware resulted in gradient systems with high dB/dt values, i.e. high gradient amplitudes with short rise times. dB/dt induces electrical fields in the human body and thus leads to electrical currents whose amount depends on the conductivity of the tissue. If the current is directed along a nerve fiber the latter might be excited. The stimulation thresholds are lowest for peripheral nerves, increase for respiratory nerves and are highest for the cardiac conduction system (7). The regulatory institutions of several countries have limited the dB/dt exposure during an MRl examination. However, only few data is available (l-8) on the exact stimulation threshold of peripheral nerves and its dependence on the gradient parameters, patient position or patient geometry. Furthermore thresholds determined so far rely only on the sensations reported by the volunteers but could not be measured in an objective way. The aim of this study therefore was to develop a method which allows to non-invasively measure the stimulation of peripheral nerves caused by magnetic field gradients used in MRI. The stimulation of an afferent nerve results in an action potential propagating to the central nervous system, where it is perceived as a sensation. If an efferent nerve is stimulated, the action potential propagates to a muscle, where it results in a twitch (7). The action potential of the nerve itself is hard to measure, but measurement of the electric activity of a muscle is well known as an electromyogram (EMG). As the stimulation threshold for muscle fibers is much higher than the threshold for nerves (7), at low stimulation currents a muscle twitch will not be generated by direct muscle excitation but via an efferent nerve leading to that muscle.