METHOD AND APPARATUS TO IMPROVE FIELD HOMOGENEITY FOR SUPERCONDUCTING MRI MAGNETS
Publication Date: 2010-Aug-09
The IP.com Prior Art Database
The invention provides an auxiliary method and apparatus to regulate inhomogeneity of superconducting magnet with iron shielding. The auxiliary method and apparatus includes a superconducting magnet with iron shielding instead of bulking coil. The auxiliary method and apparatus focuses on the low orders of the field harmonics and specifically on magnets with iron shielding and unshielded gradient coils. Unshielded gradient coils do not allow addition of iron material inside the cryogenic region due to the eddy current or heat issues.
METHOD AND APPARATUS TO IMPROVE FIELD HOMOGENEITY FOR
SUPERCONDUCTING MRI MAGNETS
FIELD OF INVENTION
The invention relates generally to techniques of improving field homogeneity for superconducting MRI magnets and more particularly to regulate inhomogeneity of superconducting magnet with iron shielding.
BACKGROUND OF THE INVENTION
Generally, two techniques are employed to improve the homogeneity of an MRI superconducting magnet. The two techniques include active shimming methods and passive shimming methods for superconducting magnets with bulking coil.
The active shimming method includes some kinds of correction coil or shimming coils. Usually, the active shimming methods are designed to generate those low order field harmonics by injecting predetermined quantities of current into the shimming coils and generating the field to compensate the original unwanted field harmonics by main coil which are generated by the manufacture error.
The passive shimming method usually includes an array of iron chips near warm bore or gradient coil on a cylindrical surface to improve the magnet homogeneity. The locations of the iron chips and quantities of chips at each location are usually determined using an appropriate algorithm. The algorithm is designed to minimize the target inhomogeneity while keeping the added iron material also at a minimal level at some certain level.
However, the active shimming method increases the cost of the magnet and is not suitable for high order field harmonics. The passive shimming methods are usually suitable to reduce the high order field harmonics since for low order harmonics, quite a lot iron materials need to be added into the magnet. Adding lot of iron materials is quite
difficult or challenging in the modern design of superconducting magnets, as the space in the modern design of superconducting magnets is usually very limited for the chips.
Another method employed is an iron shielding method used for the main field. The iron shielding method substitutes the bulking coil as magnetic flux return loop and has the merit of saving superconductive wires. However, the original homogeneity of this kind of magnet is impacted by the shielding iron's permeability and has a quite bigger original inhomogeneity variance from magnet to magnet. The original homogeneity of this kind of magnet is compared with traditional magnets with bulking coils, whose original homogeneity is mainly determined and minimized by design. This kind of magnet is slightly disturbed by manufacture dimensional control accuracy.
Another conventional method involves various kinds of iron rings in superconducting magnets with bulking coil. The superconducting magnet with bulking coil is positioned in the magnet warm bore or in the gradient coil assembly. The position in the magnet warm bore or in the gradient coil assembly is near the Field of view (FOV) and is aimed at compensating or reducing those high order field harmonics.