Peripheral Nerve Stimulation
The stimulation threshold is greatest for cardiac muscle, less for skeletal muscle and smallest for peripheral nerve stimulation. The current densities achieved operating standard imaging systems are well below these values. EPI however uses increased amplitudes and decreased rise times and can cause peripheral nerve stimulation, although cardiac and respiratory function are not at risk. Research with dogs has shown that respiratory stimulation occurs at exposure levels of 3 times that which is required for peripheral nerve stimulation, and for cardiac stimulation, 80 times the exposure.
Insight into the health risks of exposure to extremely low frequency magnetic fields is hindered by the lack of literature on the subject, yet it is possible to discern some level of risk due to the extensive literature on the effect of electric currents applied directly to the body via external electrodes. It is known that peripheral nerve and cardiac muscle stimulation can be adequately avoided by restricting exposure to sinusoidal or pulsed magnetic fields in any orientation to less than 20 Ts-1 (corresponding current density 400 mAm-1). It is well understood that electrical stimulation thresholds rise as the current pulse width decreases below the nerve or muscle cell time-constant (RC). This is due to the progressively shorter time available for the accumulation of the electric charge necessary for depolarisation of the cell membrane. Cardiac muscle fibres have a longer membrane time-constant (3 ms) than peripheral nerve fibres (120 μs). Thus, for pulses shorter than 3 ms, cardiac muscle fibre stimulation can still be avoided if the restriction on the rate of change of magnetic flux density (dB/dt) of 20 Ts-1 is relaxed in proportion to the frequency of (gradient) magnetic field change. For periods of magnetic field change of less than 120 μs, peripheral nerve stimulation can still be avoided if the restriction on dB/dt is similarly relaxed.