Why do MRI machines cost so much to shut off?


Why do MRI machines cost so much to shut off?

In: Other

You need liquid helium cooling to keep the magnets working. The only way to shut the MRI machine off is to flush the helium. And then you need to pump in new helium to be able to start it again. That’s not particularly cheap.

In addition to the helium issue, there is also the expense of re-calibrating it which requires some very special nerds and is a fairly extensive process.

Because you’re talking about high-field or superconducting MRI — the biggest cost isn’t the helium, the turning off / on, but the lost productivity with people twiddling their thumbs for the 5 hours it takes to turn the thing off and on again. Also to charge up the magnet requires more energy than to maintain the superconductive state.

This could be made much easier and cheaper if we used low-field or non-superconducting magnets, which can be turned off within 30 minutes, require no liquid coolant, use far less power and overall are just much more user friendly.

So basically the magnetic field from the MRI comes from a huge current in the superconducting wires inside a liquid helium bath. The superconducting wires are special in that below a certain temperature, they can flow the current without resistance. Normally, because the current is so large, you want to slowly put it in or take it out, because if you do it too fast, it’s too “turbulent” for the current to flow without resistance. This heats up the wires and causes a runaway effect, which will boil off the liquid helium. In an emergency, you can’t do it slowly, so a lot of the liquid helium is blown off. For regular maintenance, you can take the current out slowly and the liquid helium is not wasted.

Normally, you can collect the helium flowing out and recycle it, but in an emergency, a lot of liquid is turned into gas and takes up a lot of room, so the pumps can’t collect all of it and most of the gas is relieved to atmosphere.

By the way, just to give an idea of the cost of helium. I work in lab settings not hospital so maybe the prices are slightly off. But 100L liquid helium costs about $1500 plus shipping and handling. Typical MRI machines holds 1700L liquid helium. So each emergency quench will cost $25K in just liquid helium, plus shipping and machine maintenance.