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Brain_Hawk t1_jbfrv88 wrote

Aside from the other answers better than I could say, it's cheap abd plentiful!

MRI used liquid helium for supercooling. It's very expensive. Some helium loss is natural and replacement is an ongoing expense.

Likewise if you need to turn thr magnet off you need to dump the helium (MRI magnet is always on), and replacement is $15-25,000.

If water works, it's basically or nearly free! This is a very non trivial concern for.most uses. Obviously for super cooling you need something more.

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luckyluke193 t1_jbgsgxe wrote

> if you need to turn thr magnet off you need to dump the helium

I've never worked with an MRI magnet, but in general, superconducting magnet systems can be ramped down to virtually zero without evaporating all the helium. You just need a power supply for the magnet.

I don't know if the magnet power supply comes with the MRI system though. I think in NMR spectroscopy, magnet systems are often sold without power supply. The technician who installs the system charges it with their power supply, and takes it back with them. As long as the users don't somehow empty out the helium or damage the magnet, it will hold its field indefinitely.

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mesouschrist t1_jbko5ad wrote

IDK what hospitals are doing, but I work in a physics lab, and nobody is ever letting their liquid helium just boil into the atmosphere unless something has gone catastrophically wrong. There's a whole infrastructure for recovering boiled off helium and sending it back to the liquefaction plant.

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luckyluke193 t1_jblh3o3 wrote

Sure, but usually the liquefication plant fills it into dewars, and the magnet system needs to be refilled manually.

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CocktailChemist t1_jby03y8 wrote

Can’t say I’ve ever seen that for an NMR. A standard 400 MHz instrument has to be topped up with 70-100 L of liquid helium every 6-12 months.

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