Cooling of Magnetic Resonance Imaging (MRI) Machines
Magnetic Resonance Imaging (MRI) is a medical imaging technique that employs the use of a magnetic field, along with radio wave energy, to produce images of the body’s internal organs and structures. MRI scans are a valuable tool across virtually all areas of health care; doctors and professionals use MRIs to diagnose conditions non-invasively. In order to operate, an MRI machine requires an extremely powerful and uniform magnetic field. Thus, superconducting magnets are necessary. Superconductors utilize coolants to function properly because to obtain superconductivity, magnet coils must be cooled below a critical temperature. Most MRI machines are cooled by liquid helium; however, helium is scarce on Earth due to lack of a strong gravitational pull on the element. Recently, cryogenic technology refrigeration has allowed for MRI magnets to be cooled with just small amount of liquid helium. Additionally, the technology uses mechanical refrigerators that run on electrical power and cooling water in a fashion similar to air conditioning units. The volume of the helium gas is fixed in a closed circuit that is allowed to compress and expand, and the magnets are cooled close to absolute zero.
References: http://www.webmd.com/a-to-z-guides/magnetic-resonance-imaging-mri#1