Magnetic Resonance Imaging scanners (MRI) are composed of three basic magnetic field sources (the polarizing magnet, the gradient coils and the excitation coil) which are used to manipulate the magnetic moments of the hydrogen atoms in the body. This article will focus on the main component which is the polarizing magnet.
There are three basic types of magnets used in MRI systems:
Resistive magnets consist of many windings or coils of wire, wrapped around a cylinder, through which an electric current is passed. This causes a magnetic field to be generated. If the electricity is turned off, the magnetic field dies out. These magnets tend to be relatively inexpensive to construct, but require significant amounts of electricity (up to 50 kilowatts) to operate because of the natural resistance in the wire.
Superconducting solenoid electromagnets (made of alloys such as niobium/titanium or niobium/tin surrounded by copper) are the most commonly used. These alloys have the property of zero resistance to electrical current when cooled down to about 10 K so they are cooled with liquid helium. The power supply is connected on either side of a short heated segment of the coil and the current to the coil is gradually increased over several hours until the desired magnetic field is reached. The heated segment is allowed to cool to superconducting temperature and the power supply removed and taken away. The current continues in the closed loop of the coil for years without significant decline. A resulting property is that the magnetic field (typically 1 ‚Äď 3 T fields) is always present. This makes these systems more economical to operate, but they are still very expensive to build.
New MRI units constructed with rare-earth permanent magnets are the so called low-field ‚Äúopen‚ÄĚ MRI scanners (0.2 ‚Äď 0.3 T) and they are used as an alternative to the superconducting MRIs. Rare earth permanent magnets generate a high strength permanent magnetic field that does not require electricity, so there is no cost to maintain the field. These magnets are temperature sensitive as any change in temperature will affect the magnetization therefore permanent magnets have a high temperature control requirement. Closed loop temperature control systems with a precision of 0.1 ¬ļC often accompany these MRI units.
Open MRI scanners are available in C-shaped and two-legged designs which can be beneficial at the time of installation and provide a more confortable experience for patients (i.e. less claustrophobic and more quiet).
Low-field MRI scanners have decreased image quality and require a longer scan time compared to high-field MRI scanners. Another drawback is the weight of the currently produced systems for whole-body imaging although the use of Neodymium-Iron-Boron (NdFeB) permanent magnets have cut down the weight of permanent systems from 100 tons to less than 20 tons. These new MRI units are getting lighter with each new generation.
 Rakesh K. Gupta and Sunil Kumar, Ed. Magnetic Resonance Imaging of Neurological Diseases in Tropics, Jaypee Brothers Medical Publishers(P) Ltd., New Delhi, India 2014;