In transcranial magnetic stimulation (TMS) an abrupt decrease in the electric current in a small coil placed on the scalp produces an abrupt decrease in the magnetic field inside the brain

In transcranial magnetic stimulation (TMS) an abrupt decrease in the electric current in a small coil placed on the scalp produces an abrupt decrease in the magnetic field inside the brain. Suppose the magnitude of the B(arrow on top of the b) field changes from 0.64 T to 0 Tin 7.3×10−2 s.
Determine the induced emf around a small circle of brain tissue of radius 1.2 ×10 ^ −3 m. The B (the arrow on top ) field is perpendicular to the surface area of the circle of brain tissue.  
units - V