Types of magnets

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).

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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:

  • Resistive electromagnets, which are cheaper and easier to maintain than superconducting magnets. These are far less powerful, use more energy and require a cooling system.
  • Permanent magnets, of different formats, composed of ferromagnetic metallic components. Although they have the advantage of being inexpensive and easy to maintain, they are very heavy and weak in intensity.

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.

Characteristics of the main magnet

The main characteristics of a magnet are:

  • Type (superconducting or resistive electromagnets, permanent magnets)
  • Strength of the field produced, measured in Tesla (T). In current clinical practice, this varies from 0.2 to 3.0 T. In research, magnets with strengths of 7 T or even 11 T and over are used.
  • Homogeneity