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Both TMS and MRI have strong magnetic field. Why we often assume MRI being very safe because it does not affect the brain while TMS is used to influence the brain?

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Relevant for the reaction of a physical system to an excitation is most of all the frequency spectrum of the signal. Most signals have a center frequency and a bandwidth, i.e. you can specify contained frequencies in the form "from Hz - to Hz".

TMS is typically an exponential pulse excitation with 200 to 600 microseconds half-width duration (the numbers I found on german Wikipedia). This results in a spectrum that is weighted around 0 Hz with a bandwidth between 1700 and 5000 Hz. In short, the TMS signal almost only contains frequencies between 0 Hz and 5000 Hz at most.

MRI is typically a combination of a DC (0 Hz) magnetic field (which causes nuclear spins to precess around the magnetic field axis) and an oscillating, narrow-band signal in the radio frequency range (10's of MHz, see also german Wikipedia for a quick number), which injects energy into the spin precession, simply speaking. The DC component is likely to be irrelevant for transient "stimulation" (even though there could be circular currents in the brain, whatever the result is, if at all). The oscillating component is not only a factor of 1000-10000 faster in frequency, but due to the narrow-band nature of this component, it is also very unlikely that it contains any significant amount of frequencies that dominate TMS.

This makes it about as unlikely that MRI stimulates the same states of the brain that TMS does, as you can receive a radio programme with a loudspeaker only (i.e. without a receiver).

This is not surprising, because MRI is deliberately designed to excite the nuclear spins of hydrogen and nothing else, not even the spins of other atoms. If it could excite the neural functions of the brain, the corresponding energy absorption from the original signal would disturb the incoming signal so heavily, that useful imaging of hydrogen matter distribution would not be possible.

Note, however, that this does not necessarily mean that MRI does not somehow (minutely) influence the brain's function, and that these effects could somehow be amplified by the brain to detectable levels. It only means that what function is influenced cannot be the same.

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  • $\begingroup$ Excellent answer, though you could improve it by adding references or directions for further reading. $\endgroup$
    – Bryan Krause
    Apr 1, 2022 at 20:52
  • $\begingroup$ Phew, the numbers I found on Wikipedia, I will add them. The rest is physics and signal processing 101, that can be found in about any textbook. $\endgroup$
    – oliver
    Apr 1, 2022 at 20:54
  • $\begingroup$ Good answer, but it only covers one of several differences between TMS and MRI: Though of comparable strengths, TMS is focused, while MRI is unfocused (think of a laser vs a flashlight); TMS uses a magnet against the scalp, while the MRI's magnet is far from the scalp (magnetic field strength falls sharply with distance); and TMS may use multiple magnets, where MRI uses only one; also, TMS uses a changing magnetic field, while MRI uses a static pulsed field (this is why patients are advised to hold still inside the machine, or their brain will be affected). $\endgroup$
    – Arnon Weinberg
    Apr 2, 2022 at 1:53
  • $\begingroup$ @ArnonWeinberg: "but it only covers one of several differences between TMS and MRI", the question was not about all differences between TMS and MRI, but rather about at least one difference that explains why MRI does not affect the brain like TMS. If you have something interesting to add, feel free to post your own answer, though, so that others can rate it. $\endgroup$
    – oliver
    Apr 2, 2022 at 14:48

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