Most of us are familiar with helium: it makes party balloons float, and makes your voice sound funny if you breathe it in. Where does it come from? Most of it is found in natural gas reserves, so when natural gas is pumped out of the ground, helium is also collected. In the 1920s, the US government banned the export of helium and created the National Helium Reserve to store helium so that it could be used in military blimps. Thirty percent of the world’s stored helium remains in the reserve, but other countries have now created their own reserves. The United States and Qatar together produce about 25% of the world’s helium.
What does this have to do with radiology? Well, the single largest use of helium is as a cryogen in MRI machines. Cryogens are things used to maintain very low temperatures, like ice in a cooler. When helium is in liquid form, its temperature is more than 450 degrees below zero! The coils used in MRI machines have to be kept near this temperature in order to create the powerful magnetic field used to create images, so MRI machines have liquid helium surrounding the coils. If the helium goes from a liquid to a gas, the temperature quickly rises and the magnetic field goes away. Heating the helium so that it boils off like this is called “quenching” the MRI scanner, and it is done only if there is an emergency that requires turning off the magnetic field very quickly.
When helium goes from a liquid to a gas, it takes up 750 times the space that it did as a liquid. If the helium boils off into the MRI scanner room, it will push all the oxygen out of the room and anybody inside the room will quickly lose consciousness. In order to prevent that from happening, MRI machines have vents that allow the helium to be expelled outside the building if a quench occurs.
Recently, MRI designers have built MR scanners that use only a tiny amount of helium. If a quench happens, the helium gas stays inside the machine and doesn’t escape into the room at all, so there is no danger to the people in the room. Our newest MRI scanners, located in the new critical care building and at our outpatient Kenwood site, have this design, using only 7 liters of helium as compared to the 1500 liters used in standard MR scanners. As our other MRI scanners get replaced, they will also use this new design, reducing the need for (and potential risk from) large amounts of helium. And leaving more for the rest of us to use in balloons.
Dr. Blaise Jones, author; Glenn Minano, BFA, editor; Meredith Towbin, copy editor
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