Читать книгу Essentials of MRI Safety - Donald W. McRobbie - Страница 24

Superconducting magnets are capable of generating magnetic flux densities, colloquially referred to as “magnetic field strength”, up to 7 T (tesla) in currently available systems. 1.5 and 3 T systems are most common. B₀ is orientated horizontally along the scanner bore or patient aperture, by convention denoted as the z‐axis.

Figure 1.15 shows a schematic of a modern MRI magnet in cross‐section. There are two sets of windings: the field coils and the shield coils. The shield coils are wound in opposition to the field coils in order to reduce the extent of the fringe field. The windings are held in a large vessel or cryostat and bathed in liquid helium. Surrounding this is a vacuum to prevent conduction and convection heating, and layers of highly reflective sheeting to prevent radiative heating.

Figure 1.15 Schematic of a self‐shielded superconducting MRI system.

During operation some helium will evaporate or “boil off”. In older magnets this was wasted as exhaust, but modern magnets have a refrigeration system, the cold head or cryo‐cooler which re‐condenses the gas as liquid. Such “zero boil‐off” systems generally do not require helium replenishment. If electrical power is lost, the reliquification will not occur, but the magnet can stay cold for several days. This allows new systems to be transported cold.

To generate the field initially, a power supply is required. The process of energizing the magnet or “ramping up” involves a gradual increase of electrical current. A superconducting switch or shunt is maintained in a non‐superconductive state by a small heater whilst the current grows. At the desired current the heater is turned off and the switch becomes superconducting, completing the electrical circuit. The power supply can then be disconnected and removed from site. The reverse process, ramping down, can be used to reduce or remove the field when required, e.g. for a major hardware upgrade or after a non‐injurious ferromagnetic incident to remove the offending object.

The Nb‐Ti wires are 50–150 μm in diameter, embedded in a copper matrix. This provides additional mechanical strength – they are subject to significant magnetic forces – and provides a means for conducting excess heat and current in the event of a magnet failure or quench to prevent damage to the more delicate Nb‐Ti filaments. In the superconducting state, the copper matrix acts like an insulator, providing isolation between the Nb‐Ti strands.