Prototype coil of an MRI (magnetic resonance imaging) machine, 1990s.

The coil is made from YBCO, a superconductor made from yttrium, barium, copper and oxygen, and is used to improve the MRI signal. Superconductors are materials which offer zero resistance to the pasage of an electric current when cooled to below a certain critical temperature. With early superconductors this was only a few degrees Kelvin (K), meaning that they had to be cooled using liquid helium, which is expensive and difficult to handle. In the 1980s, so-called high-temperature superconductors were discovered, including YBCO, which has a critical temperature of 92 K (-181 C), allowing the use of liquid nitrogen instead of helium. Today, superconductors with critical temperatures as 'high' as 150 K (-123 C) have been developed.