Cooling of MRI Magnets

MRI scanners use superconducting magnets cooled to 4K in order to take high-definition pictures of a patient’s brain, vital organs or soft tissue. These superconducting magnets need to be cooled down and need to be kept cold which is a cryogenic challenge. This is usually done by using liquid helium.

For a first cool-down, a superconducting magnet is often cooled with LN₂ to first reach 77K. The LN₂ is then pumped out, after which LHe is used to cool down further to the required 4K.

This process is complicated and costly because the LN₂ must be removed completely while the next step, further cool-down from 77K with 4K liquid with liquid helium, is even less efficient. Helium is a critical component in MRI systems that has gone through two potential shortage crises, impacting hospitals and patients around the globe. But the helium supply is finite and demand has been rising over the past decades.

Stirling Cryogenics has designed an alternative pre-cooling system to make the total system more efficient. Using VJ lines, this system is connected to the in- and outlets that connect to the LHe vessel. The system will start producing cooling power which is circulated to the vessel. At start-up the loop will be still warm, then slowly dropping in temperature as more and more energy is removed from the magnets thermal mass. Depending this, cool down from ambient temperature to 20K takes 1-3 days.

Once pre-cooled to 20K, the system is disconnected and the magnet is connected directly to the LHe supply as usual. Advantage is that the pre-cooling has been done with helium, so no cleaning step is required. Since most of the energy of the thermal mass is already removed at 20K, the remainder consumes only a limited amount of LHe.

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