MRI magnet system having shield and method of manufacture

DETAILED DESCRIPTION Among the several aspects and features of the present invention may be noted the provision of an improved MRI magnet system and its incorporation in a unique method of siting.
The system provides shielding to limit the strength of the magnetic field outside the magnet and has a single coil disposed inside the vacuum vessel which provides a high strength field with a high degree of homogeneity notwithstanding the presence of the shield because the coil is configured to cancel the effect of the presence of the iron shield.
As the magnet system of the present invention avoids the need to place additional coils inside the magnet bore, the diameter of the bore can be reduced with the accompanying advantage that the size of the single coil is reduced because less flux is needed to provide the required flux density in the smaller bore.
This results in a small, lighter (than other shielded magnet systems) and less expensive magnet system.
The present system can also be assembled at the site in that the largest single component is the magnet which could weigh about 4500 lbs.
The various iron pieces forming the shield can be transported to the site separately and assembled to form the shield at the site.
The magnet system is reliable in use, has long service life and is relatively easy and economical to manufacture.
Other aspects and features of the present invention will be, in part, apparent and, in part, pointed out hereinafter in the following specification and in the accompanying claims and drawings.
Briefly, a magnet system embodying various aspects of the present invention includes a vacuum vessel defining a bore for receiving the patient with a cryogenic containment vessel supported inside the vacuum vessel.
A single coil is positioned in the containment vessel for providing the magnetic field.
The system also includes a shield assembly of ferromagnetic material disposed closely adjacent the outside of the vacuum vessel.
This shield assembly includes first and second end assemblies and a plurality of axially extending beam-like members interconnecting the end assemblies