Patients undergoing an MRI occasionally report seeing flashes of light. Some pra
ID: 1263804 • Letter: P
Question
Patients undergoing an MRI occasionally report seeing flashes of light. Some practitioners assume that this results from electric stimulation of the eye by the emf induced by the rapidly changing fields of an MRI solenoid. We can do a quick calculation to see if this is a reasonable assumption. The human eyeball has a diameter of approximately 25 mm. Rapid changes in current in an MRI solenoid can produce rapid changes in field, with ?B/?tas large as 44T/s .
What emf would this induce in a loop circling the eyeball?
How does this compare to the 15 mV necessary to trigger an action potential?
How does this compare to the 15 necessary to trigger an action potential?
This amount of emf is more than adequate to trigger an action potential. This amount of emf is inadequate to trigger an action potential. This amount of emf is absolutely inadequate to trigger an action potential. This amount of emf is almost adequate to trigger an action potential.Explanation / Answer
The radius of the eyeball is 25 mm. Thus, r = 12.5 mm = 0.0125 m.
Thus,
A = pi * r^2
= 4.908E-4 m^2
As
emf = A dB/dt
and dB/dt = 44 T/s
Then
emf = 0.0216 V
or
emf = 21.6 mV [ANSWER]
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As 21.6 mV > 15 mV, the amount of emf is more than adequate. [ANSWER, OPTION A]