1. Field of the Invention
This invention relates to an apparatus and method for providing passive shimming for a superconducting magnet of the type that has a relatively small imaging volume. Such structures of this type, generally, provide the proper amount of passive shimming for a relatively small imaging volume.
2. Description of the Related Art
In magnetic resonance (MR) imaging systems, the magnetic field inhomogeneities have to be less than a few parts per million (ppm) for producing good images. Therefore, a superconducting magnet is designed for a homogeneous field distribution of only a few (ppm) within the imaging volume. However, once built, due to the manufacturing tolerances, lead routings, and other manufacturing difficulties, the ppm inhomogeneities are usually in the range of hundreds. Active or passive shimming needs to be used to homogenize the magnetic field.
In recent years, due to its lower cost, simplicity, and capabilities, passive shimming has been the preferred method of field inhomogeneity correction. Exemplary of such passive shimming systems are U.S. Pat. Nos. 4,698,611 and 4,771,244 entitled "Passive Shimming Assembly for MR Magnet" and "Method for Passively Shimming Magnetic Resonance Magnet", respectively, both to M. E. Vermilyea and both assigned to the same assignee as the present invention. While these prior passive shimming assemblies have met with a modicum of success, due to the compact nature of an imaging system which images only the limbs, the space available for passive shimming is extremely limited. In order to keep the length of the MR system down and the inside diameter of the MR system large, the magnet coils and the gradient coil need to be physically very close. This is in fact the area where passive shims need to be placed. Therefore, a passive shimming systems needs to be designed which will occupy the least amount of physical space.
It is apparent from the above that there exists a need in the art for a passive shim/gradient coil assembly which is light weight and compact through simplicity of parts and uniqueness of structure, and which at least equals the shimming and gradient coil characteristics of known shimming and gradient coil systems, but which at the same time is capable of providing passive shimming and a gradient coil for a relatively small imaging volume. It is a purpose of this invention to fulfill this and other needs in the art in a manner more apparent to the skilled artisan once given the following disclosure.