Permanent Magnets in Magnetic Resonance Equipment

Nuclear magnetic Resonance (NMR) is a kind of nuclear physics phenomenon. Block and Purcell reported this phenomenon as early as 1946 and applied it to spectroscopy. Lauter Burr published MR Imaging in 1973, making NMR useful for more than just physics and chemistry. It is also used in clinical medicine.

In recent years, magnetic resonance imaging technology has developed rapidly and become more and more mature. The scope of inspection basically covers the whole system, and has been promoted and applied worldwide. In order to accurately reflect the imaging basis and avoid confusion with nuclide imaging, it is now called magnetic resonance imaging.

Magnetic resonance imaging requires a strong uniform magnetic field, which is generated by a magnet. Magnets are the most important and expensive part of MR Equipment. Currently, two types of magnets are commonly used: permanent magnets and electromagnets, which are divided into two categories: permanent conductivity and superconductivity.

The constant conduction electromagnet uses strong direct current flowing through the coil to produce a magnetic field. The power required to maintain a main magnet field is about 100kW. Generally, it takes several hours of electricity before the magnetic field reaches a stable state. Excessive current in the coil will generate a lot of heat, heat exchanger to cooling water heat dissipation.

Superconducting magnets are widely used at present. In the superconducting state, the current flows through the conductor without loss of resistance and thus does not heat the conductor. A wire of the same diameter can pass through a larger current in the superconducting state without damage. A coil made of superconducting material can generate a strong magnetic field with a strong current, and after the external current is cut off, the current in the superconducting coil remains unchanged, so the superconducting magnetic field is extremely stable.

Permanent magnet materials can maintain magnetism for a long time after magnetization, and the magnetic field strength is stable, so the maintenance of the magnet is simple and the maintenance cost is minimal. Permanent magnets used in magnetic resonance equipment include AlNiCo magnets, permanent ferrite magnets and NdFeB magnets, etc. Among them, NdFeB magnets have the highest magnetic energy product and can reach the maximum field intensity with a small amount (up to 0.2T field intensity requires 23 tons of aluminum nickel-cobalt, if the NdFeB is used, only 4 tons). The disadvantage of permanent magnet as the main magnet is that it is difficult to reach 1T field intensity. Currently, the field intensity is generally below 0.5T, which can only be used in low-frequency magnetic resonance equipment.

When a permanent magnet is used as the main magnet, the magnetic resonance equipment can be designed in a ring or yoke shape, and the instrument is semi-open. This structure is a great boon for children or people with claustrophobia.