eli5 If MRI machines a super powerful magnets, how come they do not disrupt or damage the electrical impulses in our bodies when we are scanned?


eli5 If MRI machines a super powerful magnets, how come they do not disrupt or damage the electrical impulses in our bodies when we are scanned?

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We don’t have “electrical impulses”. We have electrochemical reactions (which is basically the same thing batteries have) which release chemicals. The nervous system communicates through these chemicals, not electrical impulses.

While you may describe our neural impulses as electrical, they are not electrical in the traditional sense. There are no wires. Chains of cells communicate via chain reactions, not by electrons flowing freely down the line.

Voltages build up on long wires. The voltage across individual cells in your body from an MRI machine is tiny, and since those cells don’t pass that voltage along very well (again, they don’t work like wires) no real significant voltages are formed.

it’s not the same kind of electricity that runs through wires.

it’s more like electrically charged atoms (ions) moving in and out of cells. There are little protein pumps and channels that allow the ions to move in and out.

If you pump a lot of positively charge ions out of a cell, then the inside becomes more negatively charged (relative to the outside). you can actually measure this tiny amount of voltage.

If you then open up channels for those positive ions, they’ll leak back in, and the voltage will change, and you can set up a chain reaction that causes other channels to open, and this opening and closing of channels will ripple down the length of the cell like a wave.

I’m seeing some correct comments about how our bodies are different from wires, etc. Those are correct, but I would like to add that MRI machines *can* interfere with the electrical signals in our body. Particularly for larger people, it is possible to have involuntary muscle spasms during an MRI. However, the big magnet is a static field: it is always pointing in the same direction with the same strength. An MRI also involves radio frequency pulses (kind of like a microwave), and sometimes those RF pulses will trigger muscle contractions, sort of like those things that shock your abs from the 2000s.

They do, but it’s not quite what you think.

The main magnet is, indeed, extremely strong – however, it doesn’t change. Maxwell’s laws of electromagnetism say that a changing magnetic field creates an electric field. From which it follows that a magnetic field which doesn’t change doesn’t create an electric field.

However, if you were to walk past an MRI scanner, then as you move through the magnetic field, from a weak area into a strong area, then your body experiences a changing magnetic field, and a tiny amount of electricity is generated in the body. This isn’t enough to be a problem – you wouldn’t feel it – but as a precaution, there are legal limits on how much electricity can be generated, and so MRI workers are advised to stick to “speed limits” when walking past an MRI scanner (no running close by).

However, although an MRI scanner has a strong unchanging magnet, it actually has a number of weaker magnets which can be changed. The scanner uses these to focus on individual pixels in the image, so in order to get a detailed image reasonably quickly, these magnets have to be changed very quickly (10s or hundreds of times per second) These magnets create rapidly changing magnetic fields during a scan, and this does create electrical currents in the body. These usually aren’t much of a problem, and you usually don’t feel them.

However, sometimes, if the scanner is working very hard and fast – or if you are very tall, then enough electrical current can be produced to cause muscles to twitch. Scanners have legal limits as to how much muscle twitching they are allowed to cause. Actually getting electrical shocks or affecting the heart requires a ton more electrical current, so the legal limit on the design of scanners means that there is absolutely no chance of causing heart disturbances or painful shocks.

Finally, there is a 3rd magnetic field produced by an MRI scanner – this changes 10s or 100s of millions of times per second – this also produces a lot of of electrical current. However, the electrical currents it produces change so quickly, that no biological process can respond that fast – so even though there is quite a lot of current produced, it can’t affect muscles, nerves, brain or the heart. However, electrical currents do produce heat, and this current produces heat in the body. If the scanner is working very fast and hard, then this heat production can be considerable – literally as much heat as going to the gym for some light aerobics.