7 ms·

An MRI machine has a very uniform field inside the magnet, and a very strong gradient outside as the field curls back on itself. If you move your head back and

An MRI machine has a very uniform field inside the magnet, and a very strong gradient outside as the field curls back on itself. If you move your head back and forth outside the end of the bore, it causes dizziness, by inducing eddy currents in the brain (I've not done it myself, I'm very risk averse, but I've seen others doing it). I'd be much more worried about moving through a very strong gradient, spatially or temporally, than about the absolute field strength. Human made sustained field strengths are generally not dangerous, if the field is uniform. For example there is human and animal imaging at 7T , although lower field strengths are more common (the limiting factor, other than making the static field, is generally the RF frequency, which gets higher with field strength and introduces other issues as it does)

goldenkey 5y ago
Yeah, this question is akin to: how strong a gravitational field would you have to be in, to kill you? Well, you could be in a gravitational field of 10^12548125971 Newtons, you wouldn't feel anything though, you would be weightless. It's the Spaghettification [1] that can hurt you, and that's due to the gradient in the field, not the actual field strength. Thank you for your comment. [1] https://en.wikipedia.org/wiki/Spaghettification https://en.wikipedia.org/wiki/Spaghettification
- vikingerik 5y ago
  It's not the same question, and that's the point of this article. Gravity acts equally on all your particles, so it doesn't cause any disruption (other than that gradient.) Magnetism affects different particles differently, particularly moving ones, as in electrons. The article says how a strong field compresses and deforms electron orbitals, to the point where covalent bonds fail. Field strength alone causes that, not gradient.
  - BizarroLand 5y ago
    I don't know if we could measure the difference, but since gravity's force decreases with distance, I bet there is a difference between the gravity on the top of your head vs the bottom of your feet. It's probably small enough to be negligible, but it should be there.
- snet0 5y ago
  Are you so sure that you wouldn't be harmed in an extraordinarily strong gravitational "field", with field lines parallel to your body? I imagine that the difference in force exerted on your feet vs. your head is significant, in raw Newton's rather than as a percentage. The r^2 is basically constant across the length of your body, compared to M, but it's not actually constant, and when M is so large, I imagine even small changes in r^2 lead to (relative to our scale) large changes in F. Is my intuition wrong?
  - baranoff 5y ago
    You are saying the same exact thing as the comment you replied to. Differences in the force at the feet and head is exactly the gradient of the field. If you have no gradient the forces are equal everywhere.
  - eximius 5y ago
    You're describing a way for a meaningful gradient to exist, so it amounts to the same thing.
- RavlaAlvar 5y ago
  So, would a very strong gravitational wave wipe out humanity?
  - Filligree 5y ago
    Certainly, but making one that strong is essentially impossible even if you have a convenient pair of black holes.
kurthr 5y ago
Yeah, it's less likely that constant high field will get you, but dB/dt will quickly induce deadly currents (in the heart/brain etc) on anything in it. That's particularly true for AC fields (or quenched magnets), because your body actually has a lower impedance at moderate (100Hz to MHz) frequencies than at DC.
heisenzombie 5y ago
Yep! At these fields, the effects of shaking your head around an MRI machine are: - Dizziness/vertigo/headache: AC: typical thresholds is 2T/s for 1 s. DC mechanism: differential susceptibility of vestibular structures (46 T2/m  perceived 1% g) - Acidic/metallic taste: provoked presumably by electrolytic reactions due to eddy currents in the tongue, effect felt by 15% of test subjects @3T. Threshold dB/dt2.3 T/s (e.g. shaking head @ 0.6 Hz in a 0.5 T fringe field). May be contrasted by opening the mouth. - Magnetophosphenes: perceived flickering lights caused by electrical stimulation of the retina/optical nerve. (0.2 mV). Threshold dB/dt 2T/s for 50 mS (max. sensitivity 20 Hz) (source in my other comment)
- mattkrause 5y ago
  Another way to think about this is via the effects achievable with transcranial magnetic stimulation. A TMS machine generates fields on the order of 1T at the magnet, and rather less in the brain. From this, you can get all kinds of interesting phenomena: phosphenes, temporary “lesions”, and even facilitation of some behaviors.
  - xwdv 5y ago
    What behaviors?
    - kyshoc 5y ago
      Not sure this is what GP was referencing, but TMS is approved[0] for treatment-resistant OCD, depression, and migraine headaches. [0]: https://www.fda.gov/news-events/press-announcements/fda-permits-marketing-transcranial-magnetic-stimulation-treatment-obsessive-compulsive-disorder https://www.fda.gov/news-events/press-announcements/fda-perm...
      - amelius 5y ago
        There are also promising studies into the use of TMS for treating substance addiction. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4206564/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4206564/
dheera 5y ago
At 16 T you can levitate a frog purely through diamagnetism. I imagine you could do the same for a human as well if you could create a chamber that large with a uniform field of that magnitude. https://www.youtube.com/watch?v=KlJsVqc0ywM https://www.youtube.com/watch?v=KlJsVqc0ywM That said, assuming 16 T cancels gravity, I imagine being imaged in a MRI machine at 7 T horizontal magnetic field would probably feel like a net gravity vector at an angle, i.e. lying on a steep slope of arcsin(7/16) = 0.452 radians, which is pretty steep.
- pfdietz 5y ago
  That levitation is based on their being a large field gradient (diamagnetic levitation is based on the field gradient), so you could not do the same for a human.
  - lend000 5y ago
    Do you have a source for this? I don't believe it is true. Diamagnetism is a fundamental quantum mechanical property related to the quantum mechanical analog to the Lorentz force. Spin 1/2 electrons have exactly 2 states: opposing or attracting a static magnetic field, regardless of direction of the field or orientation of the particle. When all electrons are paired (and the spin fields cancel), all that is left is that Lorentz force and you get diamagnetism. Otherwise, you generally get paramagnetism (attractive because of the net dipole created by net spin). See the Stern Gerlach experiment: https://en.wikipedia.org/wiki/Stern%E2%80%93Gerlach_experiment https://en.wikipedia.org/wiki/Stern%E2%80%93Gerlach_experime...
    - pfdietz 5y ago
      The energy of a diamagnetic material is lower in a lower magnetic field, so there's a force pushing it away from higher fields. This force is proportional to the gradient of the field. In a constant magnetic field, there will be no change in energy and so no force on the diamagnetic material.
      - 5y ago
didntknowya 5y ago
yeh one of the common injuries around MRI are not the magnetic field affecting your body but the hidden little metal jewelries in people pockets flying around the room. once we had a metal chair get stucked in and had to shut everything down to remove it.
- lostlogin 5y ago
  Projectile hazards get all the attention, but burns are by far the most common injury in MRI, and account for about 60-70% of adverse events. https://www.auntminnie.com/index.aspx?sec=ser&sub=def&pag=dis&ItemID=129887 https://www.auntminnie.com/index.aspx?sec=ser&sub=def&pag=di... https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6734033/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6734033/
  - shapefrog 5y ago
    > Among the 133 projectile events were 35 reports (26%) involving patient walkers, wheelchairs, stretchers, and chairs, 13 incidents (10%) with gas cylinders, 13 cases (10%) of magnet components exposed during servicing, 10 events (8%) with workmen's tools. Yeah, but a gas cylinder or a hammer flying across the room is way cooler.
    - lostlogin 5y ago
      Oh for sure. When a crisis occurs in the MRI department (a collapse, arrest, drug reaction etc) someone better man the door, or even better, lock it. Every emergency seemingly has someone helpfully attempt to take something into the magnet. A wheelchair, oxygen cylinder, defibrillator, stethoscope, or most often, scissors.
    - hermitcrab 5y ago
      Somebody told me that they knew of a case where a hospital porter decided to push a gas cyclinder through the MRI room while the machine was on (shortcut?). The gas cyclinder went through a wall. It says at: https://www.hospitalmanagement.net/features/feature51496/ https://www.hospitalmanagement.net/features/feature51496/ "Gas cylinders are a particular risk in the MRI environment. Such cylinders can weigh from 30lb to 150lb when full. Ferromagnetic gas cylinders are especially dangerous in a magnetic environment, where they can be uncontrollably accelerated. Potential hazards include gas-propulsive missile impaction, explosion and fire. If the cylinder regulator valve is damaged on initial impact, the cylinder may propel away from the magnet, only to return for a second impact."

goldenkey 5y ago

Yeah, this question is akin to: how strong a gravitational field would you have to be in, to kill you? Well, you could be in a gravitational field of 10^12548125971 Newtons, you wouldn't feel anything though, you would be weightless. It's the Spaghettification [1] that can hurt you, and that's due to the gradient in the field, not the actual field strength. Thank you for your comment. [1] https://en.wikipedia.org/wiki/Spaghettification https://en.wikipedia.org/wiki/Spaghettification

vikingerik 5y ago
It's not the same question, and that's the point of this article. Gravity acts equally on all your particles, so it doesn't cause any disruption (other than that gradient.) Magnetism affects different particles differently, particularly moving ones, as in electrons. The article says how a strong field compresses and deforms electron orbitals, to the point where covalent bonds fail. Field strength alone causes that, not gradient.
- BizarroLand 5y ago
  I don't know if we could measure the difference, but since gravity's force decreases with distance, I bet there is a difference between the gravity on the top of your head vs the bottom of your feet. It's probably small enough to be negligible, but it should be there.
snet0 5y ago
Are you so sure that you wouldn't be harmed in an extraordinarily strong gravitational "field", with field lines parallel to your body? I imagine that the difference in force exerted on your feet vs. your head is significant, in raw Newton's rather than as a percentage. The r^2 is basically constant across the length of your body, compared to M, but it's not actually constant, and when M is so large, I imagine even small changes in r^2 lead to (relative to our scale) large changes in F. Is my intuition wrong?
- baranoff 5y ago
  You are saying the same exact thing as the comment you replied to. Differences in the force at the feet and head is exactly the gradient of the field. If you have no gradient the forces are equal everywhere.
- eximius 5y ago
  You're describing a way for a meaningful gradient to exist, so it amounts to the same thing.
RavlaAlvar 5y ago
So, would a very strong gravitational wave wipe out humanity?
- Filligree 5y ago
  Certainly, but making one that strong is essentially impossible even if you have a convenient pair of black holes.

kurthr 5y ago

Yeah, it's less likely that constant high field will get you, but dB/dt will quickly induce deadly currents (in the heart/brain etc) on anything in it. That's particularly true for AC fields (or quenched magnets), because your body actually has a lower impedance at moderate (100Hz to MHz) frequencies than at DC.

heisenzombie 5y ago

Yep! At these fields, the effects of shaking your head around an MRI machine are: - Dizziness/vertigo/headache: AC: typical thresholds is 2T/s for 1 s. DC mechanism: differential susceptibility of vestibular structures (46 T2/m  perceived 1% g) - Acidic/metallic taste: provoked presumably by electrolytic reactions due to eddy currents in the tongue, effect felt by 15% of test subjects @3T. Threshold dB/dt2.3 T/s (e.g. shaking head @ 0.6 Hz in a 0.5 T fringe field). May be contrasted by opening the mouth. - Magnetophosphenes: perceived flickering lights caused by electrical stimulation of the retina/optical nerve. (0.2 mV). Threshold dB/dt 2T/s for 50 mS (max. sensitivity 20 Hz) (source in my other comment)

mattkrause 5y ago
Another way to think about this is via the effects achievable with transcranial magnetic stimulation. A TMS machine generates fields on the order of 1T at the magnet, and rather less in the brain. From this, you can get all kinds of interesting phenomena: phosphenes, temporary “lesions”, and even facilitation of some behaviors.
- xwdv 5y ago
  What behaviors?
  - kyshoc 5y ago
    Not sure this is what GP was referencing, but TMS is approved[0] for treatment-resistant OCD, depression, and migraine headaches. [0]: https://www.fda.gov/news-events/press-announcements/fda-permits-marketing-transcranial-magnetic-stimulation-treatment-obsessive-compulsive-disorder https://www.fda.gov/news-events/press-announcements/fda-perm...
    - amelius 5y ago
      There are also promising studies into the use of TMS for treating substance addiction. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4206564/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4206564/

dheera 5y ago

At 16 T you can levitate a frog purely through diamagnetism. I imagine you could do the same for a human as well if you could create a chamber that large with a uniform field of that magnitude. https://www.youtube.com/watch?v=KlJsVqc0ywM https://www.youtube.com/watch?v=KlJsVqc0ywM That said, assuming 16 T cancels gravity, I imagine being imaged in a MRI machine at 7 T horizontal magnetic field would probably feel like a net gravity vector at an angle, i.e. lying on a steep slope of arcsin(7/16) = 0.452 radians, which is pretty steep.

pfdietz 5y ago
That levitation is based on their being a large field gradient (diamagnetic levitation is based on the field gradient), so you could not do the same for a human.
- lend000 5y ago
  Do you have a source for this? I don't believe it is true. Diamagnetism is a fundamental quantum mechanical property related to the quantum mechanical analog to the Lorentz force. Spin 1/2 electrons have exactly 2 states: opposing or attracting a static magnetic field, regardless of direction of the field or orientation of the particle. When all electrons are paired (and the spin fields cancel), all that is left is that Lorentz force and you get diamagnetism. Otherwise, you generally get paramagnetism (attractive because of the net dipole created by net spin). See the Stern Gerlach experiment: https://en.wikipedia.org/wiki/Stern%E2%80%93Gerlach_experiment https://en.wikipedia.org/wiki/Stern%E2%80%93Gerlach_experime...
  - pfdietz 5y ago
    The energy of a diamagnetic material is lower in a lower magnetic field, so there's a force pushing it away from higher fields. This force is proportional to the gradient of the field. In a constant magnetic field, there will be no change in energy and so no force on the diamagnetic material.
    - 5y ago

didntknowya 5y ago

yeh one of the common injuries around MRI are not the magnetic field affecting your body but the hidden little metal jewelries in people pockets flying around the room. once we had a metal chair get stucked in and had to shut everything down to remove it.

lostlogin 5y ago
Projectile hazards get all the attention, but burns are by far the most common injury in MRI, and account for about 60-70% of adverse events. https://www.auntminnie.com/index.aspx?sec=ser&sub=def&pag=dis&ItemID=129887 https://www.auntminnie.com/index.aspx?sec=ser&sub=def&pag=di... https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6734033/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6734033/
- shapefrog 5y ago
  > Among the 133 projectile events were 35 reports (26%) involving patient walkers, wheelchairs, stretchers, and chairs, 13 incidents (10%) with gas cylinders, 13 cases (10%) of magnet components exposed during servicing, 10 events (8%) with workmen's tools. Yeah, but a gas cylinder or a hammer flying across the room is way cooler.
  - lostlogin 5y ago
    Oh for sure. When a crisis occurs in the MRI department (a collapse, arrest, drug reaction etc) someone better man the door, or even better, lock it. Every emergency seemingly has someone helpfully attempt to take something into the magnet. A wheelchair, oxygen cylinder, defibrillator, stethoscope, or most often, scissors.
  - hermitcrab 5y ago
    Somebody told me that they knew of a case where a hospital porter decided to push a gas cyclinder through the MRI room while the machine was on (shortcut?). The gas cyclinder went through a wall. It says at: https://www.hospitalmanagement.net/features/feature51496/ https://www.hospitalmanagement.net/features/feature51496/ "Gas cylinders are a particular risk in the MRI environment. Such cylinders can weigh from 30lb to 150lb when full. Ferromagnetic gas cylinders are especially dangerous in a magnetic environment, where they can be uncontrollably accelerated. Potential hazards include gas-propulsive missile impaction, explosion and fire. If the cylinder regulator valve is damaged on initial impact, the cylinder may propel away from the magnet, only to return for a second impact."