Cryostats are devices for maintaining a constant low temperature (as by means of liquid helium used in MRI). They require vacuum chambers with thermal isolation.

See also Dewar, Cryoshielding and Boil off Rate.

Magnetic shielding through the use of secondary shielding coils designed to produce a magnetic field that cancels the field from primary coils in regions where it is not desired. These coils may be inside the magnet cryostat. Active shielding can be applied to the main magnet or to the gradient magnetic fields.

See also Magnetic Shielding, Self Shielding and Room Shielding.

A large vacuum jacketed double walled container generally made out of stainless steel that is used to transport cryogens. Cryogenic liquids are shipped and used non-pressurized in this thermally insulated containers. Dewar flasks are specifically designed to withstand rapid temperature changes and extreme differences in temperature. Flasks containing helium (used in MRI for cooling of superconducting magnets), hydrogen and other low-boiling liquid cryogens have an outer vessel of liquid nitrogen for insulation.
Dewars are also called cryostats.

Electric currents induced in a conductor by a changing magnetic field or by motion of the conductor through a magnetic field.
One of the sources of concern about a potential hazard to subjects in very high magnetic fields or rapidly varying gradient or main magnetic fields. Can be a practical problem in the cryostat of superconducting magnets. Eddy currents can cause artifacts in images and may seriously degrade overall magnet performance. Common means to reduce the influence of eddy currents on gradient fields are eddy current compensation and shielded gradient coils (active or passive).

See also Eddy Current Compensation.

See also the related poll result: 'Most outages of your scanning system are caused by failure of'

A quench is the rapid helium evaporation and the loss of superconductivity of the current-carrying coil that may occur unexpectedly, or from pressing the emergency button in a superconducting magnet. As the superconductive magnet becomes resistive, heat will be released that can result in boiling of liquid helium in the cryostat. This may present a hazard if not properly planned for.
The evaporated coolant requires emergency venting systems to protect patients and operators. Quenching can cause total magnet failure and cannot be stopped. MRI systems are designed such that all of the escaping cryogenic gas is directed out of the building (quench pipe through the roof or the wall). In the event of a burst of the tank (possible in the case of an accident) or a blockage of the pipes, the helium gas will be forced into the scanner room, giving rise to a large white cloud of chilled gas. Under such circumstances it is essential that the scanner room is evacuated, also caused by the displacement of oxygen, which under extreme conditions could lead to asphyxiation. The force of quenching can be strong enough to destroy the walls of the scanner room or the MRI equipment.