The procedure for correcting the lack of homogeneity is called shimming. It consists of two steps:
(i) mapping the magnetic field at certain points lying on the surface of a sphere centered at the magnet center, and
(ii) correcting field inhomogeneities by driving certain currents through individual shimming coils to generate new magnetic fields that correct for the lack of homogeneity.
Shimming the NMR magnets to improve the magnetic homogeneity presently is accomplished semi-manually. More particularly, the magnetic field of the NMR imaging systems is measured for homogeneity using probes in particular regions of the static magnetic field or by imaging phantoms in particular regions. Ideally of course, the static magnetic field should be universally homogeneous within the bore of th magnet. In practice there is a small but not neglectabe lack of homogeneity in all NMR magnets. Shim coils are used to correct for this lack of homogeneity. When the shimming coil currents are applied, new mapping is done and compared. Finally the current intensities are adjusted. The measuring and shimming steps are repeated until sufficient homogeneity is obtained.
This is understandably a very time consuming process. NMR systems are costly, therefore, in order to be cost effective for hospitals and clinics, the systems require minimum down time and maximum operating time. Accordingly, it is in the interest of manufacturers of the NMR systems to provide for shimming the magnetic field automatically to correct inhomogeneities in as short a time as possible and with the use of the least amount of manpower as possible.
In the prior art, attempts have been made to use phantoms within the central region of the magnet to map the magnetic field there. Such field mapping by imaging is, for example, disclosed in a publication of the Society of Magnetic Resonance Imaging in Medicine at the second annual meeting of the society in San Francisco on Aug. 19, 1983 in a paper entitled "Field Measurement by Fourier Imaging" authored by A. G. Simon et al of the Columbia University of Physicians and Surgeons. In that article the authors suggest use of phantoms that comprise a single straight tube on an array of tubes in a circular disc. The article describes a phantom that is a thin water filled disc.
The use of the thin water filled disc measures the field homogeneity in only a single plane. A plurality of such water filled discs would test the homogeneity in a plurality of planes but would require a rather complicated and time consuming three dimensional computation to obtain a map of the magnetic field intensities. Thus the requirement for efficient inhomogeneity corrections persists.