"Permanent MRI magnets are used in cheaper "economy" systems and involve permanently magnetised iron acting like a bar magnet that has been twisted into a C-shape with the two poles 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 in diagnostic imaging, being impractical for spectroscopy, chemical shift, and susceptibility 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."
"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."
Expected headings
"Permanent MRI magnets"
"Resistive (air core) MRI magnets"
"Superconductive MRI magnets"
"Magnets used for MRI are of three types: permanent, resistive and superconductive."