Usage of helium in MRIs More and more articles pop up on the shortage of helium, and on the importance of it. Its usage in MRI's spring to mind for example. I looked it up and found out that helium is used for its 'low boiling point' and 'electrical superconductivity'. So this gives me a couple of questions:


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*How can the amount of helium be depleting? When we use helium (for purposes other than balloons) it stays on earth right? Since it doesn't dissappear, can't we 'recycle' the helium previously used for certain purposes and just use it again?  

*We often hear that helium supplies are depleting at alarming rates. This makes me wonder; isn't every element we're using on earth depleting in supplies? Or are there elements which 'arrive' on earth at a faster rate than they're leaving earth? Beforehand I thought of carbon (in relation  with the emission of carbondioxide), however, then I figured that the amount of carbon in the atmosphere is increasing while the amount beneath the ground is decreasing, thus making no difference to the amount of carbon on the earth as a whole.

*Why is helium the only element suitable for usage in MRI's? In other words, why are its properties so unique or rare? And what properties are those, besides 'low boiling point' and 'electrical superconductivity' ? 

*Is there research being done towards replacing helium in its purposes by a more viable substitute? 
 A: I'll try to give a very short answer to most of the questions. Some parts are already explained in the other answers but a few important aspects are missing.


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*How can the amount of helium be depleting? 
The Helium ($^4$He) that is used in a number of applications is extracted from natural gas. All other sources are much more difficult and expensive. So less natural gas means less Helium.

*Isn't every element we're using on earth depleting in supplies? 
Helium is special in that aspect, that once evaporated and escaped into the atmosphere you can hardly get it back. In normal air only 0.0005% are Helium so extraction by condensation is not efficient.

*Why is helium the only element suitable for usage in MRI's? 
Helium is the material with the lowest boiling point, this makes it ideal to cool superconducting magnets. Hydrogen comes in second but the relatively high boiling point (20K) is above the critical temperature of Niobium-titanium (9K) so you cannot use Hydrogen to get the standard material superconducting.

*Is there research being done towards replacing helium in its purposes by a more viable substitute?
Oh yes. There is a lot of research going on. Previously MRI just blew of the Helium to the air, everything else was not considered economically sound. This has changed to some degree, you can collect the Helium in high pressure vessels and recondense it. Alternatively using a pulse tube cooler you can keep the whole system cool enough that no Helium evaporates but this is not widely used but already commercially available. 
In principle you could cool to very low temperatures completely without Helium using adiabatic demagnetization but this is much more involved than 'simply' using liquid Helium.
A: Helium is a valuable resource because of its relative availability, fantastic chemical stability, and exceptionally low boiling point. This means it can be used to efficiently cool things to 4K and below where other gases are prohibitively hard to work with.
Helium is running out because the only viable stores of it we have are underground. When we use helium carelessly - in a lab or a balloon - it escapes into the atmosphere and therefore dilutes into the $\sim 10^{18}$ kilograms of oxygen and nitrogen that make it up. In principle it is possible to purify helium out of the atmosphere. In practice it is only present in trace amounts and no such scheme will ever be realistic. 
To make things even more difficult, only physical separation schemes are viable since helium is chemically inert, and those are very, very expensive in terms of money and energy. (Think how hard it is to extract nitrogen!)
To be clear, then: the only supplies of helium we are depleting are usable, underground ones. I am unaware of any other viable alternative to helium for sub-4K cooling.
A: MRI machines use liquid helium to cool down the superconducting magnets that are needed to create the high magnetic field necessary for magnetic resonance imaging. Every high-field magnetic resonance machine, MRI or NMR, has an inner dewar filled with helium and an outer one filled with liquid nitrogen.
The insulation is of course not perfect, so a certain amount of helium will evaporate over time. You can catch the evaporating helium, cool it down and reuse it, but that isn't done everywhere. Until recently it just wasn't economical to do so, you always lose some amount of helium in the process and you don't get the whole machinery for free. I know of at least two NMR facilities that recycle their helium, so this is certainly feasible. But both are rather large, and I suspect that the financial aspects are worse for smaller sites.
As for replacing helium, one way would be to invent high-temperature superconductors suitable for building MRI machines. If liquid nitrogen would be enough to cool them down, this would eliminate the need for liquid helium.
A: Helium is relatively rare on Earth, 0.00052% of the atoms or molecules in the atmosphere (or the same fraction of the volume; much lower fraction of the mass). The concentration of helium in the atmosphere is low. Moreover, it's dropping because of atmospheric escape. About 4 tons of helium escape from the atmosphere every day because there's a significant probability that the helium atoms' speed exceeds the escape velocity, so there's no return anymore.
Because the amount of helium in the atmosphere is so low, that's not where we are getting it from. We are getting it from natural gas at places where it's created from alpha-decay of uranium and other elements – alpha-particles are helium nuclei. And such natural gas has up to 7% concentrations of helium so it's convenient to get it from there via fractional distillation. The depletion of helium from the "realistic sources" therefore occurs at a similar relative rate as the depletion of the "conventional" natural gas. If the helium escapes to the atmosphere, it's effectively lost. No one is going to catch the rare molecules from the atmosphere: you would have to grab huge volumes of the air to find the required amount of helium.
Different elements or compounds are being "depleted" or "accumulated" at very different rates. You must understand that for practical reasons, only the elements and/or compounds contained in materials where their relative concentration is high enough may be counted as accessible. So once they're lost, e.g. the helium in the atmosphere, they're lost and they can't be recycled.
The Earth's soil and crust and water contains some elements and/or compounds whose amount is effectively infinite relatively to the human consumption, so it makes no sense to talk about their depletion. The Earth will almost certainly be burned when the Sun goes red giant in 7.5 billion years before we would be able to deplete nitrogen from the atmosphere or silicon oxides from the rocks etc. The whole upper layers of the Earth are largely composed of such things.
We won't deplete carbon dioxide anytime soon; as long as we have any fossil fuels etc., the concentration of CO2 in the air will be kept elevated which is a good thing. However, it's true that a few centuries after we run of fossil fuels and similar things to be burned, CO2 in the atmosphere will converge back towards the equilibrium concentration dictated by the temperature (around 280 ppm for today's temperature). If this happened abruptly (it will take a century or more for the drop to occur), the plant growth rate would drop by about 20% and about 1 billion people in the world would have to starve to death rather quickly. Most plants stop growing below 150 ppm of CO2; during the coldest ice ages in the recent 1 million year, the concentration never went below 180 ppm or so and the plant species that wouldn't be able to survive this drop have gone extinct.
Some other elements or compounds are rare, e.g. gold and platinum. If you don't want to search for them 30 km beneath the surface (or try to bring them from other celestial bodies which is still prohibitively expensive – the price to get X kilograms of matter to the orbit is comparable to the price of X kilograms of gold and you would need even higher expenses to launch spaceships from Mars etc. to get the gold here), the total amount of these precious metals that can be "mined" isn't too much larger than what we have already gotten.
Concerning your "why helium question", let me just quote Wikipedia.

Multinuclear imaging: Hydrogen is the most frequently imaged nucleus in MRI because it is present in biological tissues in great abundance, and because its high gyromagnetic ratio gives a strong signal. However, any nucleus with a net nuclear spin could potentially be imaged with MRI. Such nuclei include helium-3, lithium-7, carbon-13, fluorine-19, oxygen-17, sodium-23, phosphorus-31 and xenon-129. 23Na and 31P are naturally abundant in the body, so can be imaged directly. Gaseous isotopes such as 3He or 129Xe must be hyperpolarized and then inhaled as their nuclear density is too low to yield a useful signal under normal conditions. 17O and 19F can be administered in sufficient quantities in liquid form (e.g. 17O-water) that hyperpolarization is not a necessity.

So the helium is the best one but it doesn't quite have a monopoly. Clearly, if we ran out of helium, it wouldn't be the end of MRI. But the price of the helium is finite, a particular number dictated by the balance between supply and demand, and it's simply still better for many users to use helium even though we will probably deplete it well before others. As the reserves decrease, the price will increase and the proportion of other isotopes used in MRI will go up.
