MRIs and NMR machines require incredibly uniform magnetic fields, not varying in strength by more than a fraction of a part-per-million (equivalent to looking at the thickness of a piece of paper from a mile away!)

How can you possibly build magnets that accomplish this?? The answer is shimming!

We iteratively vary the different components of the field until we cancel out all the inhomogeneities

I'll explain why and how this works in this thread 👇

How magnetic resonance works, explained with only analogies. Disclaimer: this will be a purely classical description of nuclear magnetic resonance… I can’t think of any good analogies for quantum! (1/n)