ELI5 MRIs - how do they work and why are they so noisy?

How do MRIs work? Why are they so expensive? Why are they so noisy? Are MRIs related to other scans like x-rays, CT scans and PET scans? Why aren’t full body MRIs more part of routine checkups? Why are they so expensive? If there is iron in our blood, is this affected by the magnetic fields in an MRI? Why was there controversy with the nobel prize awarded for MRI technology?

... we explain like I'm five

Thank you to the r/explainlikeimfive community and in particular the following users whose questions and comments formed the basis of this discussion: danielaumbrella, superdad0206, duferino, 8daysuntiltheweekend, war-ear, glittering-ocean1, let_me-out, one2manythrees and truguido

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