Читать книгу Essentials of MRI Safety - Donald W. McRobbie - Страница 23

Superconductivity was discovered by Dutch scientist Heike Kamerlingh Onnes in 1911, but it took until 1957 for Bardeen, Cooper, and Schrieffer to formulate a quantum mechanical theory (BCS theory) to account for the phenomenon. The electrical resistance of a non‐superconducting metal, such as copper, depends upon its temperature, decreasing with lower temperatures, but possessing a finite resistance even at absolute zero (−273.15 °C). Below T_c the electrons in a superconductor pair up into “Cooper pairs” acting as a superfluid resulting in zero resistance (FIGURE 1.14a).

Figure 1.14 Superconductivity: (a) resistance v temperature for a superconductor and a non‐superconductor; (b) Superconducting phase diagram: each of temperature, current density and magnetic field must be below a critical value T_c, J_c, B_c to maintain the superconductive state.

Niobium titanium (Nb‐Ti) alloy used in MR magnets is a type 2 superconductor.¹ It has a second, higher critical temperature at which some magnetic flux may exist within the material. Superconductivity behaves as a thermodynamic phase with a relationship between temperature, field, and current density (Figure 1.14b). There is a critical field B_c and current density J_c above which the superconductive state cannot exist. This puts an ultimate limit on the field strength that can be achieved. Nb‐Ti has a T_c of 9.5 K and B_c of 15 T. Niobium‐tin (Nb‐Sn) alloy can sustain higher fields.

High temperature superconductors can have T_c above 90K and can be cooled using liquid nitrogen (N) with a boiling point of 77 K (−196 °C) or with cooled helium gas. These have been used to produce 0.5 T MRI magnets, but not operating in a persistent current mode. Research is ongoing with the prospect of simplifying the cooling system and reduced dependence upon helium. Helium is a by‐product of natural gas extraction, a limited resource. Nitrogen can be produced from the atmosphere.