Cryogens and Refrigeration
Don't superconducting scanners consume liquid helium? How often does it need to be replenished?
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The first superconducting scanners in the 1980's had two cryogen chambers, an inner one containing liquid helium and an outer one containing liquid nitrogen. The nitrogen had to be topped off weekly and the helium monthly. In the 1990's helium-only systems were developed and by the early 2000's refrigeration systems had become so sophisticated and efficient that liquid helium needed to be added only every 2-3 years.
Over the last decade zero boil off (ZBO) refrigeration systems have become standard, allowing essentially unlimited normal operation without need for helium refill. However, the cold head and refrigeration compressors still need regular filter changes and other servicing, so some cryogen replacement at these times is unavoidable.
Over the last decade zero boil off (ZBO) refrigeration systems have become standard, allowing essentially unlimited normal operation without need for helium refill. However, the cold head and refrigeration compressors still need regular filter changes and other servicing, so some cryogen replacement at these times is unavoidable.
The most recent developments in commercially available MR magnet designs are sealed systems which require no refilling or venting.
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You can expect the next generation of superconducting MR scanners to contain no cryogens at all. This is largely due to the development of efficient pulse tube and 2-stage Gifford-McMahon (G-M) cryocoolers that are able to maintain temperatures below the 9.4°K required for NbTi superconductivity without liquid helium. Currently, the 0.5T superconducting Paramed OpenMRI is cryogen-free using a G-M system (the wire used is MgB2 that becomes superconductive below 39°K).
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Cold head and refrigeration system using Gifford-McMahon technology. The cold head, which condenses helium gas into a liquid, is inserted into the top of the MR magnet cryostat.
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References
Cosmos TC, Parizh M. Advances in whole-body MRI magnets (pdf). IEEE Trans ApplIed Superconductivity 2011: 21;2014-2019.
Janis Cryogen Free Products. (Website of a company that produces industrial cryocoolers including those used for MRI with a description and comparison of types.)
Cosmos TC, Parizh M. Advances in whole-body MRI magnets (pdf). IEEE Trans ApplIed Superconductivity 2011: 21;2014-2019.
Janis Cryogen Free Products. (Website of a company that produces industrial cryocoolers including those used for MRI with a description and comparison of types.)