Magnet Cooling

MRI scanners rely on superconducting magnets that are cooled to 4 Kelvin (K) to capture high-definition images of the brain, vital organs, and soft tissues. Maintaining these magnets at such extremely low temperatures presents a significant cryogenic challenge, typically addressed with liquid helium (LHe).
Traditionally, the initial cool-down of a superconducting magnet involves using liquid nitrogen (LN2) to reach 77 K. After this, the LN2 is completely removed, and LHe is introduced to cool the magnet further to its operational temperature of 4 K. This multi-step process is both complex and costly. The need to completely purge the LN2 is cumbersome, and the subsequent cool-down from 77 K with LHe is inefficient.
The reliance on helium is particularly problematic. Helium is a critical, finite resource essential for MRI systems, and its supply has faced potential shortage crises that have impacted healthcare globally. Demand for helium has also been steadily increasing over recent decades.

Magnet Cooling using a Stirling Cryocooler

Stirling Cryogenics has developed an alternative pre-cooling system designed to enhance the overall efficiency of the MRI cool-down process. This system utilizes vacuum-jacketed (VJ) lines to connect to the liquid helium vessel’s inlet and outlet ports.
Upon activation, the system generates cooling power, which is then circulated to the vessel. The loop begins at ambient temperature and gradually cools as more energy is drawn from the magnet’s thermal mass. Depending on the specific system, this pre-cooling process, from ambient temperature to 20 K, typically takes one to three days.

Helium Cooling to Final Temperature

Once the magnet is pre-cooled to 20 K, the Stirling system is disconnected, and the magnet is connected directly to the liquid helium supply as usual. A key advantage of this method is that the pre-cooling is performed using helium, eliminating the need for a separate cleaning step to remove contaminants like LN2. Furthermore, since the majority of the magnet’s thermal energy is already removed at 20 K, the final cool-down to 4 K requires significantly less liquid helium.

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