MRI scanners rely on extremely powerful magnetic fields. This is why you can't take metal objects into the scanner room, as they'd be pulled into it. Yet the fields can also exert other kinds of effects on the body.
I'd always been told that static, unchanging magnetic fields are biologically inert. But moving through the field too quickly can cause side effects. When an object moves through a magnetic field, induction happens - electrical currents are produced.
In the case of the human body, these small currents can activate nerve cells. Depending on which cells they hit this can cause you to feel dizzy, see flashes of light, experience tingling sensations, and so on. Or so I thought.
However, a new paper from Dale Roberts et al of Johns Hopkins shows that just being in a powerful magnetic field can cause dizziness and vertigo - with no movement required. They noticed that lying still in or near an MRI scanner causes nystagmus, abnormal horizontal eye movements, and that the amount of eye movement is directly correlated with the angle at which the head is positioned relative to the field.
The nystagmus was caused by an automatic reflex in response to effects in the vestibular ("balance") system of the ear. Roberts et al realized that the static magnetic field causes electrical currents that activate vestibular cells, even when the head is perfectly still. It happens because there's a natural flow of electrically charged ions into these cells in a part of the ear called the semicircular canal. The magnetic field interacts with this ion current, in what's called a Lorentz force.
The semicircular canals normally allow us to sense when our head is moving. Our eyes automatically compensate for head movement to keep us looking in the same direction. The MRI magnet fooled the ear into thinking the head was rotating, and the eyes produced nystagmus as a result.
Two patients who had suffered damage to their semicircular canals were immune to the effect.
This has important implications for functional MRI studies of brain function. Many people are interesting in measuring eye movements during MRI scans. This finding suggests that these movements may be unusual, compared to normal eye movements outside the scanner. Worst, the vestibular stimulation could alter brain activity:
Vestibular stimulation induced by the magnetic field in healthy subjects simply lying in the bore could activate many brain areas related to vision, eye movements, and the perception of the position and motion of the body.
Roberts, D., Marcelli, V., Gillen, J., Carey, J., Della Santina, C., & Zee, D. (2011). MRI Magnetic Field Stimulates Rotational Sensors of the Brain Current Biology DOI: 10.1016/j.cub.2011.08.029
8 comments:
the body is energy first, only "later" is it meat.
too bad most all scientists don't get this.
Did they study control subjects just laying down? There's flow in the semicircular canals just from changing position. (Sorry, the article is behind a paywall or I'd look for myself.)
I do high field fMRI and this is a very important study. Thanks for leading my attention to it!
I just gave this study a quick read. The study was meant to account for nystagmus (rapid side-to-side movement of the eyes) when the subject is within the bore. The authors claim that none of the previously postulated but never tested mechanisms could explain the nystagmus when at the center of the bore where the field is relatively homogeneous and where ostensibly the subject is not moving. This study is not meant to explain the feelings of dizziness when one enters the magnetic (passing through a homogeneous field with respect to time). I would bet big bucks that the effect seen while entering the magnet is due to magnetic susceptibility effects (what the authors call MS effects) and I would not rule out MS as the cause for the effect once in the magnet as the authors have done. My reason for hesitating to accept the authors suggested cause is that the head was free to move. Once a magnetic moment is induced in the cupula any movement, whether the field is homogeneous or not, can cause a motion of the cupula which is then modified by the presence of the field (the moment tends to align with the field).
Huh. I've had a dozen or so MRI scans (brain and occasionally spine) and the only ones in which I noticed much dizziness were the two in which my eyes were covered. I figured it was because I was lying there without the benefit of vision to orient me while loud noises were going on at angles I couldn't predict and the table was moving now and then. I would think that could be fairly disorienting to anyone. Surprises me there's more of a reason than that.
Beth
The nystagmus is seen when the imaging gradients - the source of auditory noise - are not operating.
On fMRIs, evil and neuroscience--references neuroskeptic.
evilhttp://www.slate.com/articles/health_and_
science/
the_spectator/2011/09/does_evil_exist_
neuroscientists_say_
no_.html
Sorry can seem to make url show up in box or fit go to www.slate.com
"Does Evil Exist"
Ron: Oooh, many thanks!
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