Vertigo Explanation
It is caused by the transition zone, which has a steep spatial gradient dB/dz, which is if you have a small change in distance moved, the magnetic strength B goes up immensely. The field goes from almost zero outside the scanner to its full strength over just a few centimeters. When you insert the head quickly, you create a rapid change in magnetic flux through the semicircular canals of the inner ear.
If you hold a loop of wire in a uniform magnetic field, the flux is essentially the field strength multiplied by the area of the loop. In the inner ear, the relevant “loop” is not a wire but the circulating conductive fluid—called endolymph—that flows through the semicircular canals. As that fluid moves from a weaker-field region into a stronger one, the amount of magnetic field “going through” that canal changes.
If you slide that loop (or the fluid) slowly into a stronger magnetic zone, the amount of field going through it increases gradually. But if you push it in quickly, the field passing through it jumps up in a very short time.
By faradays law, the speed at which the loop/fluid goes into the magnetic zone corresponds to a faster increase of magnetic field through a loop, and a larger electrical voltage (emf) is generated in that loop. Simple: A sudden increase produces a large, induced voltage, whereas a gradual increase produces a small one.
Flux is simply how much field goes through the loop
Faster motion -> bigger d(PHI)/dt -> larger induced emf
Lenz’s law tells us the direction of the current: it always flows in a way that opposes the change in flux. In the inner ear canals, ionic fluid (endolymph) carries this current. If you’re entering a stronger field, the induced current loops around so that it generates its own tiny magnetic field “pushing back” against the scanner’s field. The faster you enter, the greater that opposing current becomes.
Picture each ion like a little boat in a fast-moving river of magnetic field: the magnetic field applies a sideways push on any charge moving through it. In the semicircular canals, this sideways push on the endolymph forces the fluid to shift as if you were turning your head. In reality, your head is just being carried straight in, but your brain senses the fluid movement the same way it would sense a real rotation.
When your inner ear tells your brain, “We’re spinning!” but your eyes and muscles register linear motion into the scanner, you get a sensory mismatch. That conflict triggers the brain’s motion-sickness pathways.