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These questions are concerned MRI hardware and quality control of images.

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Magnet Types

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Describe the three main magnet types, including the pros and cons of each.

There are three main magnet types; permanent, resistive (air cored or iron yoke) and superconducting. Resistive magnets have a limited field strength and require a very stable power supply. Water cooling is necessary to remove the heat. Permanent magnets have advantages - they are relatively cheap, the fringe field is small (in the interests of safety, although the magnetic field gradient close to the magnet is high) and no power consumption is required. However they cannot be "switched off" and can be very heavy. Superconducting magnets enable high field strengths to be produced but with the need to house cryogenic liquid around the superconducting coils. These and further considerations are summed up in the following list:

 SuperconductingResistivePermanent
Field Strength0.15T-7.0T (high field possible)0.02T-0.2T (limited)~0.3T (limited)
HomogeneityGood (<5ppm/50cm dia.)Moderate (<5ppm/20cm dia.)Moderate (40ppm/40cm dia.)
StabilityGoodModerateTemp. dependent
Fringe FieldLarge (without shielding)SmallNegligible
WeightModerate (without shielding)LowHigh
Emergency ShutdownQuench (expensive)Switch offNot possible
Power ConsumptionNegligibleHighNone
CoolingCryogen liquidsChilled waterNone
Manufacturing CostsHighLowMedium

Further reading on this topic:
Books: MRI From Picture to Proton p166-169

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