Cryogenic Magnets for Whole-Body Magnetic Resonance Systems
2012 - Volume 1 eMagRes
Volume 1, Issue 2
Published Online: 15 JUN 2012
Copyright © 2013 John Wiley & Sons, Ltd
How to Cite
Rayner, D. L., Feenan, P. J. and Warner, R. J. 2012. Cryogenic Magnets for Whole-Body Magnetic Resonance Systems. eMagRes. .
- Published Online: 15 JUN 2012
Superconducting magnets have been used in magnetic resonance imaging (MRI) since the early 1980s when the first low-field 0.35 and 0.5 T systems were introduced. Since then, field strengths have gradually increased and, today, 3.0 T is recognized as a standard field strength for clinical diagnostic imaging while magnets are also now available with field strengths up to 11.74 T for research in biomedical imaging applications.
Superconductivity is the physical phenomenon in which materials have zero electrical resistance below a critical temperature and a critical field. Most MRI magnets are constructed from niobium–titanium (NbTi) superconductor and sit in a bath of liquid helium boiling at 4.2 K. The critical current and magnetic field characteristics of the superconductor determine the maximum field strength a magnet can achieve, although operating at temperatures below 4.2 K provides enhancement of the superconductor properties allowing for higher field strengths to be achieved.
Controlling the extent of the stray fields around MRI magnets has always been an issue, and various techniques have been employed utilizing both steel shielding (passive) and active shielding (reverse wound coils). Today, virtually all MRI magnets up to 7.0 T use active shielding technology to reduce the stray fields and help with magnet-siting issues.
The liquid helium reservoir containing the magnet is housed in a cryostat that is designed to minimize the heat loads reaching the reservoir. The cryostat is an evacuated vessel with one or more radiation shields cooled by a cryorefrigerator. Today, most systems use a cryorefrigerator that provides sufficient cooling power at 4.2 K to allow the evaporated gas to be recondensed and operate with no loss of liquid helium.
This article describes the current state of superconducting magnet technology used in MRI systems.
- high field