Magnetic assembly for a nuclear magnetic resonance imaging syste

Details Magnetic assembly for a nuclear magnetic resonance imaging syste Magnetic assembly for a nuclear magnetic resonance imaging syste

1994-12-07

1996-10-15

Brown, Brian W.

Electricity: magnetically operated switches, magnets, and electr

Magnets and electromagnets

Magnet structure or material

335301, 335306, 324319, H01F 100, H01F 700, G01R 3328

Patent

active

055658342

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
The invention relates to magnet assemblies particularly for use in magnetic resonance imaging (MRI).
2. Description of the Related Art
Conventional MRI systems are large superconducting systems and generally have to be carefully mounted in special areas. Although this is acceptable for the purposes of laboratory applications such as spectroscopy and human body imaging, it is not so useful for recently developed applications of MRI.
We have realised that a step towards solving this problem would be achieved by providing a magnet assembly based on permanent magnets rather than the usual superconducting magnets which require associated cryostats and the like to achieve a superconducting condition. Permanent magnet structures for magnetic resonance imaging (MRI) have been previously described. The most compact and economical in materials usage are those described by E. Potenziani and H. A. Leupold (IEEE Transactions on Magnetics Mag-22, 1078-1080, 1986). These make use of "cladding magnets" to oppose the magnetomotive force (mmf) around the outside of the structure. This has the effect of preventing flux leakage so as to most effectively use the permanent magnet material and obtain good field uniformity in the working volume.
A disadvantage of this type of structure is that the working volume is substantially enclosed. This means that the magnet must be large enough to accommodate all the object to be examined, and that there are difficulties when the subject is large. Also when it is required to examine a relatively small volume of a large object, this type of "whole-body" magnet may not be the most economical.


SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, a magnet assembly has a generally U-shape, wherein the bight of the U includes a permanent magnet which generates a magnetic field whose flux passes through a working volume defined between the arms of the U, and wherein the dimensions of the assembly are chosen such that: ##EQU2## where: L.sub.g is the length of the gap between the arms,
Magnets made in accordance with the first aspect of the invention have an open side enabling parts of a body to be positioned within the working volume so enabling bodies which are much larger than the magnet itself to be examined. Furthermore, by constructing the magnet assembly from one or more permanent magnets, the assembly can be sited locally to the body to be examined, for example near the well head in the case of rock cores.
In the case of a magnet assembly for use in MRI, the arrangement of the assembly will be such that the magnetic field within the working volume will have a uniformity sufficient for the purposes of performing a MRI experiment.
The uniformity of the field within the working volume can be improved by adding one or more cladding magnets which compensate for the flux leakage which will occur inevitably due to the open sided nature of the assembly. The cladding magnets may be provided in any conventional form as for example described in the paper mentioned above.
The following mathematical analysis illustrates the derivation of the formula set out above for minimising the volume of magnet material.


______________________________________ Definitions ______________________________________ Units: Magnetic field, H Oersteds H.sub.c coercive force Flux density, B Gauss B.sub.r remnant field Magnetisation M Gauss Length L centimeters Area A square centimeters Magnetomotiveforce F = .intg.HdL oersted-cm Flux .phi. = .intg.BdA Maxwells Subscripts: m magnet g gap i iron ______________________________________
Flux continuity
B.sub.g A.sub.g =B.sub.m A.sub.m
In practice we must allow for flux leakage. The cladding technique should minimise this but useful access to the field region will cause some leakage. We allow for this writing:
For a "U-shaped magnet" e might be 0.5.
Formulae can be found in the literature for estimating leakage, alternatively, comparison of finite-element

REFERENCES:
patent: 2355298 (1944-08-01), Horlacher
patent: 3067366 (1962-12-01), Hofman
patent: 3419832 (1968-12-01), Baermann
patent: 3651283 (1972-03-01), Doschek
patent: 4339853 (1982-07-01), Lipschitz
patent: 4738824 (1988-11-01), Kobayashi
patent: 4816796 (1989-03-01), Miyajima et al.
patent: 4985678 (1991-01-01), Gangarosa et al.
8108 IEEE Transactions on Magnetics, vol. MAG-22, No. 5, Sep. 1986, New York, USA.

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