Patients undergoing an MRI occasionally report seeing flashes of light. Some pra

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/t as large as 50 T/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?

Explanation / Answer

Given that diameter is d=25mm hence the radius is R=12.5mm                                     =12.5 x 10-3 m then the area is A=R2 the rate of change of magnetic field is given by B/t =50T/s if e is the indeced emf then e=AB/t =(R2)(50)    =((12.5 x 10-3 )2)(50)    =24.531mV the rate of change of magnetic field is given by B/t =50T/s if e is the indeced emf then e=AB/t =(R2)(50)    =((12.5 x 10-3 )2)(50)    =24.531mV

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