eli5: how do MRI machines actually work?
In: 4
In a very strong magnetic field, the hydrogen atoms in the soft (i.e. watery) parts of the body spin with their axis of rotation aligned along the magnetic field. When a pulse of radio energy at a particular frequency is applied, these hydrogen atoms get a kick and wobble out of alignment. As they settle back into alignment with the magnetic field, they give off their own radio signal, which is detected by the machine.
Different sequences of radio pulses can be applied to target different types of soft tissue. The strength of the detected radio signal can indicate the density of hydrogen atoms in the targeted tissue.
The strength of the signal varies with the strength of the magnetic field, so the magnetic field is rapidly varied in all three dimensions within the region being scanned; the received radio signal can then be located to a point source.
This information is combined to work out the density of the targeted tissue type in three dimensions and the results are converted into images.
(Edit: grammar.)
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