Patients undergoing an MRI occasionally report seeing flashes of light. Some pra
ID: 1373522 • 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/?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:
diameter of human eyeball = d = 25 mm
Radious of the human eye ball = r = 12.5 mm = 12.5 x10-3 m
thus, Area of the eye -ball is
A = ? r2 = (3.14)(12.5x10-3)2 (since eye -ball assumed as circle)
= 490.625 x 10-6 m2
Rapid changes in current in an MRI solenoid
can produce rapid changes in field,
with ?B/?t as large as 50 T/s.
Part: A )
emf would this induce in a loop circling the eyeball = e = ?
It is known by the formula of e.m.f
e = - A ?B / ?t
= -(490.625 x 10-6 )(50)
= 24531.25 x 10-6 volt
thus, e.m.f = e = 24.53125 m V
Part: B )
induce in a loop circling the eyeball is
e= 24.53125 m V
is more than that of adequate to trigger an action potential of 15 m V