People immersed in strong unchanging magnetic fields occasionally report sensing

Question

People immersed in strong unchanging magnetic fields occasionally report sensing a metallic taste. Some investigators suspect that motion in the constant field could produce a changing flux and a resulting emf that could stimulate nerves in the tongue. We can make a simple model to see if this is reasonable by imagining a somewhat extreme case. Suppose a patient having an MRI is immersed in a 3.0T field along the axis of his body. He then quickly tips his head to the side, toward his right shoulder, tipping his head by 30degrees in the rather short time of 0.15s. Calculate the emf that could be induced in a loop around the outside of the tongue by this motion of the head. (A typical tongue area is 25*10^-4 m^2 .) How does this emf compare to the approximately 15mV necessary to trigger an action potential? Does it seem reasonable to suppose that an induced emf is responsible for the noted effect?

Explanation / Answer

a. Induced EMF e = - df/dt = - d/dt (B * A * cos ?) = B * A * sin ? * d? / dt here ? = 300 = p/6 rad e = 3 * 25 * 10-4 * sin 300 * (p/6) / 0.15 = 1.308 * 10-2 V = 13.08 mV b. since e = 13.08 mV < 15 mV This amount of emf is inadequate to trigger an action potential. c. Since induced emf is inadequate to trigger an action potential, correct option is no.

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