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I had an MRI scan today, and I started looking into how much power it uses, how it works, etc. and it got me wondering; how does an MRI scanner compare to a standard microwave oven? Which one can heat up objects faster? Are they using similar frequencies? Do they heat things up in the same way?

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  • $\begingroup$ why do you think that MRI machines heat things up? Also, microwave ovens don't heat everything either - they primarily target water molecules. $\endgroup$ – wcc Mar 17 '19 at 21:40
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    $\begingroup$ @IamAStudent If you ever have an MRI scan, you will go through a long checklist of things that may cause problems, in particular any metal objects - which may be medical implants for broken bones, etc, as well as cosmetic things like piercings. Even wearing makeup which contains metallic pigments is off limits. The reason is that the strong alternating magnetic fields (1000 times stronger than a fridge magnet) induce electric currents in metal objects, which generate heat. There have been fatalities through ignoring the consequences of this. $\endgroup$ – alephzero Mar 17 '19 at 23:48
  • $\begingroup$ … Ferrous metals are even more dangerous, because of the large mechanical forces caused by the magnetic fields. There have been accidents where a person carrying something like a steel oxygen cylinder into the same room as an MRI scanner has been physically crushed to death between the cylinder and the scanner. $\endgroup$ – alephzero Mar 17 '19 at 23:51
  • $\begingroup$ If you don't know the difference between a microwave oven and an mri tool, then you should look into both more closelely. They both do electromagnetic stuff, otherwise they are quite different. $\endgroup$ – my2cts Mar 18 '19 at 0:11
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    $\begingroup$ That could be from the RF pulse used to flip the nuclear spins. See the comment on radiofrequency energy in this FDA webpage (fda.gov/Radiation-EmittingProducts/…) $\endgroup$ – wcc Mar 18 '19 at 22:05
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To give a not too scientific answer that hopefully catches your question: Basically, the difference is not that big. But in this view, the difference between a mobile phone and a microwave is also not that big.

Similarities and differences

  • Both devices use some form of electro-magnetic radiation (ie "light" of different wave lengths; radio-frequency pulses/radiation). The difference is:

    • The MRI uses a pulsed pattern with very specific timing, intensity, and phases of the different RF pulses
    • The microwave uses a more or less constant irradiation where the energy is not controlled in that detail
    • The MRI uses a much lower frequency (and therefor also energy): MRI is in the order of 100 MHz where microwaves are in the order of around 2 GHz, ie an order of magnitude is the difference.
  • Both devices do actually heat their content

    • Microwaves do heat up meals, however the heating is very often not homogeneous. Everyone has had the case with a boiling-hot layer on the outside and a cold core in the inside of their meal, I guess.
    • MRI RF fields are designed to give the most homogeneous excitation possible. If the RF field in the patient would be different from place to place, severe image artifacts would appear, and diagnosis might be hindered.
    • MRI machines do quite a lot of things to ensure that the heating is within the allowed limits of the specific absorption rate (SAR - this is the same threshold that also applies to mobile phones that also irradiate radio frequency into your body, which also causes a temperature chance). You can actually feel hot during an MRI examination: In MRI there are plenty of different sequences (ie timings and intensities of the RF pulses) that generate images with very genuine contrasts. Those contrasts are medically relevant for diagnosis. Some of the timings are more energy-rich than others (eg a TRUFI or HASTE sequence can be quite energy-rich and the MRI machine then forces the operator to stick to certain pauses in between measurements or take other appropriate measures to ensure that SAR is not too high).
  • Both devices have a similar environment for different reasons:

    • The microwave is shielded so that no microwave radiation leaves the machine.
    • The MRI room is also shielded for the same reason. However, the rf radiation would not be a real problem for the people outside, but it is a massive source of radio frequency and could possibly cause trouble in other devices that are RF sensitive.
    • The MRI shielding also serves another purpose: To keep all RF outside. The MRI signal itself is extremely small and of low intensity. So ANY RF from the outside would cause image artifacts. And I mean any. A digital watch inside the room could potentially be an artifact source due to its electronics.

Bottom line

The physics of heating up the "content" of the device is the same for both devices, ie dielectric losses in the "sample". However, the degree of heating is different due to the different frequencies.

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how does an MRI scanner compare to a standard microwave oven?

A microwave oven has one primary field, which operates in the microwave region of the EM spectrum (about 2.5 GHz), and are typically powered by a 1 kW supply.

A MRI system has three important EM fields. One is the main magnetic field which is a DC field and requires no power due to the superconducting magnet. The second are the gradient fields which operate in the acoustic range (<20 kHz) and can be powered by amplifiers with >1 MW total power. The third is the RF field which operates at around 64 or 128 MHz and is typically powered by an amplifier in the 20-50 kW range.

Of those, the RF field is the most similar to a microwave oven. The primary differences are that the MRI has a much lower frequency (64 MHz vs 2.5 GHz) and higher power (20 kW vs 1 kW).

Which one can heat up objects faster?

If the objects could completely absorb the radiation from both fields, then the MRI would heat up much faster. However, biological tissues absorb much stronger at 2.5 GHz than at 64 MHz. So the microwave will heat food or other similar materials much faster than MRI, despite the lower power.

Do they heat things up in the same way?

Yes, the mechanism is the same. The microwave is more efficient simply because the most efficient frequency is used. The MRI uses a frequency that is not efficient for heating (which is good) and which is chosen for other reasons.

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