Hacker News Reader

Comment by version_five | Hacker News Reader

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)

View on HN

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)

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.

What behaviors?

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-perm...

There are also promising studies into the use of TMS for treating substance addiction.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4206564/

Tough question!

TMS is fairly common tool for neuroscience research and its effects depend on where, when, and how it is applied, as well as the quirks of each person’s own anatomy. This paper has a table of some studies reporting “beneficial” effects, mostly on perception and memory: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4083569/ There are many more studies showing that TMS (transiently) impairs performance too.

We don’t totally understand how it works—-or what’s normally required for many of these behaviors—-so it’s hard to give a grand unified theory of how it acts, but it definitely does something to brain circuits. That said, the FDA approved TMS treatments for depression and a few other indications, and it’s also used as a tool for neurosurgical planning.

I was a test subject for someone's PHD on this. AFAIR it was about identifying the part of the brain that is responsible for aborting actions. Like if you decide to pickup a pan, and then notice the handle is hot, some people can't help but pickup the pan anyways because they lack this abort function.

They took an MRI of my brain. They found where one part of my brain resided in the MRI images. I performed reaction tests -- press this button for green circle, press that button for blue circle, abort pressing button if the icon gets crossed out (the crossing out was delayed) Then they strapped the magnet to my head (not touching but very close AFAIR) Then do all the tests again

AFAIR they showed that part of the brain did affect your ability to abort an action. I think they knew this anyway because of behavior of people with brain injuries. So I guessed they learned the magnet scrambled that part of the brain?

It was extremely boring doing these tests. I don't remember much about it except that the magnet made unreasonably loud popping sounds.

Absolutely fascinating :-)

I am assured that the human brain is so incredibly complex that it is almost impossible to understand it ... but experiments like this make me worry that Evolution just knocked most of it up in Perl over a weekend...

An inability to abort an action when facts change ? What if there was a test for that. Would we stop people standing for office? Be set free from some crimes?

I demand to see the source code for human brains ! It needs a proper security audit.

(relevant XKCD reference to be looked up later)

My PhD was (partially) on this. The inability to inhibit actions when facts change on a short timescale (sub-second) is thought to be biological. According to the most widely accepted theory, think of the brain as having a slow system and a fast system. The slow system is good for complex processing, but of course it is slow. There is also a fast system for quickly responding to things. If something changes while the slow system is working, usually the fast system is pretty good at stopping the slow system from acting, allowing the brain time to incorporate the new information into its plans. But if you are already planning on doing something and getting ready to do it, there is a limit to how quickly the fast system can interrupt the slow system. It tends to be on the order of 1/10 to 1/5 of a second.

On a longer timescale (minutes to days), there is a clinical symptom called "perseveration" whereby people can't let go of previously held beliefs in the face of changing information. It is common in, e.g., patients with schizophrenia.

https://en.wikipedia.org/wiki/Perseveration

oh - the highly repetitive response common in autism.

Are there gradients of perseveration? This is absolutely fascinating - a brain based answer to why people do many behaviours - from sitting in the corner mumbling one word over and over, to various forms of self sabotage ("always picking the wrong man")

(And I might say heartening - as the father of an ASD child, it can seem hopeless, but just being armed with some knowledge of where the behaviours come from might allow some hope)

There are ways we can measure perseveration in the lab, and so yes, it can be higher or lower in different people. If you want to look into it more, a popular way to measure it is the Wisconsin Card Sort Test (WCST), or the version for children, the Dimension Change Card Sort task (DCCS). However, like all cognitive tests, these are imperfect measurements of perseveration - they may be indicators of higher or lower perseveration, but scores may be influenced by a multitude of other factors as well. Likewise, there may be aspects of perseveration not captured by these tests - like you mention, there are different ways that one can be perseverative, and one single number can't ever represent the complexity of human behavior.

thank you

I was once using an old but high end HP DC power supply to test a repaired 12V microcontroller circuit that someone else mysteriously blew up "with a spark". Before I hooked up the fraying and grimy alligator clips coming off the supply, I had the thought to check the actual voltage with a multimeter, in case the supply was out of calibration. Before I consciously perceived what my multimeter read, my hands rapidly dropped the frayed wires. I connected up the multimeter again, and it read 120V! Someone had mistakenly missed a decimal point, and the archaic LED display made it look like 12.0V.

A low level part of my brain definitely took over control of my hands, based on interpretation of visual signals. That was 6 years ago and I still think about it every few days.

https://xkcd.com/224/

> I think they knew this anyway because of behaviour of people with brain injuries.

Tests like this are intended to prove or refine information derived from clues from damaged subjects. You can learn a lot of things about complex systems like the brain by studying their failure modes, but you have to be careful of inferring causation from correlation and other such fallacies — for instance here they could have been trying to rule out the behaviour being a secondary symptom (the correct response actually being controlled elsewhere normally, but that is blocked by the damage rather than the damage having affected it more directly), or testing to see if multiple areas are directly involved in the behaviour rather than it being as simple as that one area seeming to control the veto, or just ruling out a pre-existing condition in the initial subjects unrelated to the subsequent damage.

"Sensory Substitution Devices" by Tom Wright was an interesting talk at CodeKen 2011[1] touching on the subject of TMS. It was the first time I had heard about this and it fascinated me deeply. I remember that the talk got a lot of votes in the feedback round, so I was certainly not alone with this experience.

[1] https://m.youtube.com/watch?v=81cN6XFIPSg

Dumb question potentially, but this sounds incredibly dangerous to the layperson. Is it?

Not really, these are huge fields and quite mild effects. The vertigo one is the most dangerous because you could fall and hit your head on something!

> AC: typical thresholds is 2T/s for 1 s.

Does this depend on frequency?

Yeah, 2, tesla per second is a ramp rate — equivalent to roughly half a Hz for a 1T magnitude field.

I've done a few MRI scans around the head (both 3T and 1.5T machines) and I tasted this metallic taste every time, despite staying still as much as possible (I'm not so bad at it).

Interesting - if it happened during the scan it would be due to the gradient coils, not the main solenoid field. The gradient coils are much weaker but turn on and off during the scan (the main solenoidal field is static).

https://radiopaedia.org/articles/slew-rate

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

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.

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.

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_experime...

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.

Okay, there are a few misunderstandings here.

That is the 1800's classical explanation of electric charge, which is not consistent with experiments, nor does it line up with magnetism, which is inherently quantum.

1. Even in your classical description, what you just described (following field lines) does not require there to be a gradient. If there is a gradient, yes, the forces will follow it. But there is still a force in a static infinite applied field, and potential energies still change as things move.

2. Diamagnetism is a quantum property, not a classical one. It exists due to the polarity of spins in an electron pair (although frankly, this explanation is weak and magnetism is one of the less understood subjects in physics). Even if a macroscopic field has a gradient, it will look like a static field at the scale of an atom, at which scale diamagnetic forces exist.

Sigh.

1. Yes, a gradient is required. If the energy of the diamagnetic material does not change, there cannot be a force, as this will violate conservation of energy. You are proposing a perpetual motion machine of the first kind.

2. The quantum nature of diamagnetism doesn't matter in the slightest to the argument being made, so why are you bringing this up?

It appears that you are conflating a field gradient with a potential energy gradient.

Potential energy changes when moving parallel to any field force, regardless of whether that field is static or has some gradient.

Let us consider an experiment. Take a long cylindrical coil, so that in most of the coil the magnetic field is constant. In the center, place a diamagnetic material. Does it experience a force toward either end of the coil? If not, why not, according to your reasoning? If so, why does it experience a force toward that end of the coil and not the other?

I misinterpreted your point due to my own misunderstanding of quantum spin, a subject I'm currently studying. I see your point now, due to the dipole nature of magnetism, and the confusion came from the original "so you could not do the same for a human," in which I think you meant "you could not do the same in a perfectly uniform field."

This back and forth was fantastic to read, though I understand basically none of it.

You could not do it to a human because you'd need a nonuniform field over a much larger distance, which means the maximum of that field would have to be enormous, far higher than in the frog case, and far higher than could be generated by any practical electromagnet, superconducting or otherwise.

I do believe I was mistaken in my original post and that a locally constant magnetic field will not produce an appreciable diamagnetic force.

Diamagnetism works by inducing dipoles in the medium, and a dipole placed in a constant field experiences no net force. It may experience a torque if it is not aligned with the field, but in the case of diamagnetism the induced dipoles are naturally aligned, and so there is neither force nor torque.

Correction: proportional to the gradient of the square of the field.

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.

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.

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=di...

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6734033/

> 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.

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.

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/

"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."

> 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 trap is that they are pretty much never ‘off’, they are magnetic when not scanning.

This issue may be reduced with technology such as the Philips Blue Seal thing where you can remove (reduce?) the field at the flick of a switch. However things like this where the danger goes from being ‘always’ to ‘sometimes’ can actually increase accidents. It’ll be an interesting space to watch.

https://www.medgadget.com/2018/09/philips-helium-free-mri-sy...

I assumed it was an electromagnet. Do MRIs all have permanent magnets?

Most are superconducting electromagnets, so.. sort of? They have shipped full of liquid helium then get ramped up (or at least, this is how Siemens do it). With any luck they stay up for many years.

Yeah, undetected metal can certainly be an issue.

I was with my wife in the MRI room, when it was done I handed her her shoes--and felt the magnet pulling on them. I have no idea how there was metal in there, she specifically picked them because they were soft at all points. The pull wasn't strong (had I let go they would have gone to the floor, not to the magnet) but it definitely was there.

From that second article it sounds like manufacturers should be required to put on the label whether a garment was MRI safe or not.

All patients/volunteers going into the scanner should change into a gown and remove their shoes. Places that don’t do this have a vastly greater accident rate in my experience.

It’s surprising what is conductive or magnetic. Many garments now include silver or other anti-microbial materials. I’ve encountered clothing with electronics inside or with metal in the labelling.

Burns are the most common accident in MRI and in someone sedated or under anaesthesia you will cause massive harm. It’s all scary, so I change everyone.

‘MRI safe’ is a risky claim to make, as something safe on one scanner is unsafe on another.

In an experimental setting, I was once given a screwdriver to feel the field around an MRI machine. They asked me to be very careful of course, because loosing the grip would indded have ment the machine would need to be shut down...

I have a meteorite wedding band and we were passing it around outside the bore to feel how strong the pull was. A grad student lost their grip and the ring shot into the bore like a bullet. No damage luckily but it really drove home how dangerous these can be.

>we had a metal chair get stucked in

You mean like this? https://youtu.be/6BBx8BwLhqg

Do you still have to quench the magnet to shut it down? I read back in the day, older magnets could suffer irreversible damage so it would often total the whole machine.

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

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.

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.

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?

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.

You're describing a way for a meaningful gradient to exist, so it amounts to the same thing.

So, would a very strong gravitational wave wipe out humanity?

Certainly, but making one that strong is essentially impossible even if you have a convenient pair of black holes.

Reply on news.ycombinator.com