The design of MRI is essentially determined by the type and format of the main magnet, i.e. closed, tunnel-type MRI or open MRI.
The most commonly used magnets are superconducting electromagnets (figure 2.1). These consist of a coil that has been made superconductive by helium liquid cooling, and immersed in liquid nitrogen. They produce strong, homogeneous magnetic fields, but are expensive and require regular upkeep (namely topping up the helium tank).
In the event of loss of superconductivity, electrical energy is dissipated as heat. This heating causes a rapid boiling-off of the liquid Helium which is transformed into a very high volume of gaseous Helium (quench). In order to prevent thermal burns and asphyxia, superconducting magnets have safety systems: gas evacuation pipes, monitoring of the percentage of oxygen and temperature inside the MRI room, door opening outwards (overpressure inside the room).
Superconducting magnets function continuously. To limit magnet installation constraints, the device has a shielding system that is either passive (metallic) or active (an outer superconducting coil whose field opposes that of the inner coil) to reduce the stray field strength.
Low field MRI also uses:
To obtain the most homogeneous magnetic field, the magnet must be finely tuned (“shimming”), either passively, using movable pieces of metal, or actively, using small electromagnetic coils distributed within the magnet.
The main characteristics of a magnet are: