Week ago I went to MRI and was surprised that they gave me headphones because there’s so much noise during procedure. It sounds like some effects from older PC mainly and I’m curious why is it there.
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An MRI machine has a lot of parts that can make noise. The ones you’re most likely to hear during the operation of the machine would be the truly *massive* coils that generate the magnetic field used to make the images. It has to be turned on and generate an extremely powerful field at specific frequencies, then turned off and the “echoes” (vastly simplified) of the effects the field had on your tissues are measured to produce the image.
The amount of power going through it causes the coils to expand and contract, as well as vibrate a bit. Also, really big relays are used to switch the currents (which are huge), and they make a lot of noise.
And lastly, the MRI magnets have to be cooled down to be superconductive, which means liquid helium and the systems to pump it, which also can make some noise.
This is in some ways similar to the low hum generated by electrical distribution transformers.
In the case of transformers, the hum is caused by the effect of *Magnetostriction*. That is, most materials will change shape slightly when exposed to a strong magnetic field.
Because a typical transformer has an iron core the field inside the iron is quite strong, 0.5-2 Tesla. This causes it to change shape and vibrate slightly at the grid frequency, usually 60Hz resulting in a low humming noise.
The effect is exploited in some kinds of headphones which contain a material called Terfinol-A. This has a massive magnetostriction response, compared most other materials. Thus simply wrapping a coil of fine wire around it, then running an audio signal will cause the material to generate quite a bit of sounds, in response to the varying magnetic field produced by the coil of wire.
MRI machines use very powerful magnetic pulses to manipulate the magnetic fields of hydrogen nuclei in your body. The sudden pulses of field cause the entire machine to want to change shape slightly. This presents itself as a loud thump that can even be felt inside one’s own body because the body is subject to the magnetostriction effect as well.
This is done by pulsing large current through a series of secondary field coils inside the machine. In addition to the magnetostriction forces, this also generates magnetic attraction and repulsion forces between the coils themselves. (Such forces are exploited in electric motors.) Because the pulses involve very high, but very brief currents, a large amount of force is produced against any structures supporting those coils.
All this combined makes a lot of knocking sounds.
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