Magnets (types)

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

Permanent MRI magnets

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 tonnes of iron may be needed, restricting their placement to rooms with a strong-enough floor. Their low-field strength of about 0.15 - 0.4 T restrict their use to diagnostic imaging; being impractical for spectroscopy, chemical shift and susceptibility imaging such as functional 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 containing metals such as neodymium, markedly reducing the weight of the magnet but at significant additional cost.

Resistive (air core) MRI magnets

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-shaped 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 an electrical resistance that requires cooling of the magnet. The operating costs are high because of the large power requirements of the magnetic coils and the 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

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 10 kelvin. The coil is kept below this temperature 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 is always present.

The surrounding copper acts as an insulator at low temperatures compared 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 the cryogen. 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, which incorporate a refrigeration system called a "cold head" to condense the cryogen gas. Startup costs for the scanner can run up to about $1.5 million for a 1.5 T MRI. Site preparation can frequently run into several $100,000s including room radiofrequency (RF) shielding, possible magnetic shielding, floor reinforcement, vibration mitigation and a very reliable uninterruptible power supply (UPS).

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.

Ultrahigh field MRI refers to systems employing magnetic field strengths of seven Tesla or higher.