by
Barbara Kram, Editor | February 04, 2008
Helium is a byproduct of natural gas production so it is separated and refined in conjunction with natural gas fields; nitrogen is extracted from the atmosphere. These products are distributed by medical gas suppliers including Linde, Inc., Praxair, Air Liquide, and Air Products.
In addition to the suppliers, selected specialized independent service organizations also provide helium service, including remote monitoring of the MRI to check helium levels, topping off the helium regularly, and providing cool down service when needed.
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Cool Down Service Requires Great Care
MRI magnets are contained in sealed vacuums with a vent to allow gas to escape since the helium is always boiling. But if a leak breaks the seal on that venting, ice (frozen air) could get into the magnet, triggering the dreaded quench. Once that occurs, the magnet must be brought back up to room temperature and cooled down again. (Think of defrosting a freezer.) Because of the high cost of helium, a warm magnet is first pre-cooled with nitrogen, a process that takes several days to "soak." Nitrogen is then carefully extracted and helium introduced into the thermal vacuum of the MRI.
Specialized vacuum
equipment on a magnet
being pumped down.
(Photo courtesy of
Magnetic Resonance
Technologies)
"With a liquid that is very expensive if you were to just start pouring helium into [the MRI], it would vaporize because it's like pouring drops of water on a hot frying pan," says Thomas Freund, Director of MRI Services, Oxford Instruments. "What we do is fill it with liquid nitrogen first, which is very very cold but also very cheap compared to helium. This is a pre-cooling process-just as you would pre-heat an oven, you would pre-cool a magnet."
The cost of cooling down the magnet runs in the 10,000 to $20,000 range just for labor, not including the significant cost of about 5,000 liters of helium at about $5 per liter.
"Another important thing is the time it takes because, if it's in a hospital or clinic, you are looking at least three weeks of down time: the warming up process, pre-cool when we put nitrogen in, then pump the nitrogen out, and fill with helium," Freund says.
