Magnets used for MRI  are of three types: permanent, resistive and superconductive.

Permanent MRI magnets use permanently magnetized iron like a large bar magnet that has been twisted into a "C" shape where the two poles are close together and parallel. In the space between the poles the magnetic field is uniform enough for imaging. Up to 30 tons of iron may be needed, restricting their placement to rooms with a strong enough floor. Their low-field strength of about 0.15-4 T restrict their use to imaging; being impractical for spectroscopy, chemical shift and susceptibility imaging such as function brain imaging. Their magnetic field homogeneity is also sensitive to ambient temperature so room temperature must be controlled carefully. The initial purchase price and operating costs are low compared to superconductive magnets. These magnets can also be made with alloys such as neodymium markedly reducing the weight of the magnet but at significant additional cost.

Resistive (air core) MRI magnets operate at room temperature using standard conductors such as copper in the shape of a solenoid or Helmholtz pair coil. A solenoid is a cylindrical shape coil of wire. The uniform magnetic field is found inside the coil, especially in the center. These magnets are relatively inexpensive to make but require a large constant flow of current while magnetized and imaging. The coil has electrical resistance that requires cooling of the magnet. The operating costs are high because of the large power requirements of the magnetic coils and associated cooling system.

Both permanent and resistive MRI scanners are limited to low-field applications, primarily open MRI and extremity scanners. These magnets are useful for claustrophobic patients.

Superconductive MRI magnets use a solenoid shaped coil made of alloys such as niobium/titanium or niobium/tin surrounded by copper. These alloys have the property of zero resistance to electrical current when cooled down to about 10o Kelvin. The coil is kept below this temperature with liquid He. 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 is always present. 

The surrounding copper acts as an insulator at low temperatures compare to the zero resistance of the alloy. The copper also protects the alloy coil from being destroyed in case of a quench of the magnet. A quench can occur if the helium levels drop too low or if a large ferromagnetic object is brought into the fringe field of the magnet. A quench results in loss of superconductivity with a large amount of heat produced by the current and rapid boiling off of cryogens. The gas produced is vented out of the room but can occasionally enter the scanner room with life-threatening consequences. Quenches and the constant magnetic field are a couple of the safety issues that are discussed elsewhere.

 The cost of cryogen replacement is reduced on modern magnets that incorporate a refrigeration system called a "cold head" to condense the cryogen gas. Startup cost for the scanner can run up to about $1.5 million for a 1.5 T MRI. Site preparation can frequently run several $100,000's that includes room RF shielding, possible magnetic shielding, floor reinforcement, vibration mitigation and adequate power supply.

Superconducting magnets at 1.5 T and above allow functional brain imaging, MR spectroscopy and superior SNR and/or improved time and spatial resolution. Magnets above 1.5 T have additional challenges from RF heating of the subject, and increased artifacts from susceptibility and RF penetration among others.

MRI physics
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Article Information

rID: 22573
Section: Physics
Tag: mri
Synonyms or Alternate Spellings:
  • Resistive MRI magnets
  • Resistive air core MRI magnets
  • Superconductive MRI magnets

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