"There are 3 types of magnets used in MRI: Permanent: These are rare to almost nonexistent today. They are made up of Brick sized magnets that, when placed in the right pattern, they combine to make a magnetic field strength large enough to image with. These magnets are large, heavy and have a weak B b0 s fields so SNR is at a premium. They can't be turned off. Resistive: Are a large coil of wire with a lot of electricity passing through it. The amount of electricity circulating is sufficient to make a magnetic field strong ...
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"There are 3 types of magnets used in MRI: Permanent: These are rare to almost nonexistent today. They are made up of Brick sized magnets that, when placed in the right pattern, they combine to make a magnetic field strength large enough to image with. These magnets are large, heavy and have a weak B b0 s fields so SNR is at a premium. They can't be turned off. Resistive: Are a large coil of wire with a lot of electricity passing through it. The amount of electricity circulating is sufficient to make a magnetic field strong enough to image. These are usually .5, .7. They also generate a lot of heat from electrical resistance in the wire. Superconducting: These are Resistive magnets with one major difference. The magnet is cooled with liquid Helium (Cryogen) that decreases electrical resistance in the wires that make up the actual magnet. The cooling and loss of resistance enables more current to be applied so stronger magnetic fields are possible. Typical field strengths are 1 to 4T. Research magnets get as strong as 7 - 9 Tesla. Stronger magnetic fields also come with a larger Fringe field compared to low fields. Think of the Fringe field like "Scatter Radiation" in X-ray. The magnetic field is not solely confined to the scanner. Magnetism picks up rapidly as you get closer to the bore. And I do mean quickly..."--
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