"Earth only has a limited supply of helium, which is released as a by-product of the petrochemical industry. Essentially, pockets of the gas are disturbed during gas and oil drilling and rise to the surface. In the 1920s the US decided helium would be a strategic resource. It realised that air power would be crucial in future wars, and assumed that these would be fought by airships that would use helium to float. "The US created a vast stockpile of billions of litres of helium in the 1920s and kept it until the late 1990s, when it decided to sell it off," said Jonathan Flint, the CEO of Oxford Instruments, whose scanners and other devices use helium for cooling.
For the past decade that vast stockpile has been sold off, causing prices to plummet. "Helium was cheap and we learned to be wasteful with it," said Kirichek. "Now the stockpile is used up, prices are rising and we are realising how stupid we have been." ...
"Helium is particularly important for running super-conducting magnets. These have to be cooled to -270C to operate, and liquid helium does that perfectly. These magnets are now widespread and found in machines that range from the Large Hadron Collider in Geneva to MRI scanners in hospitals," said Professor Jim Wild, of Sheffield University. "Without helium, none of these machines would work. Unfortunately that threatens to be a real prospect in the near future."...
Researchers are now warning that the use of scanners and other machines may be increasingly disrupted, an unpleasant prospect, said David Ward of the Culham Centre for Fusion Energy.
"I will not be happy if I cannot have a medical scan in my 70s, because we wasted helium on party balloons while I was in my 30s."" http://www.guardian.co.uk/science/2012/mar/18/helium-party-balloons-squandered
With more discussion and detail
(NOTE: I moved the paragraph beginning "Helium is particularly ..." from its place in the article.)