Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
2000-05-10
2001-12-25
Arana, Louis (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S320000
Reexamination Certificate
active
06333630
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magnetic field generating apparatus for a magnetic resonance imaging (MRI) system, in which permanent magnets and a steel element are arranged to produce a controlled magnetic field within a specified volume between magnetic pole surfaces.
2. Description of the Related Art
An important aspect in the medical field and applications of a magnetic resonance imaging (MRI) system is the uniformity of a magnetic field within the separation between two magnetic pole surfaces facing each other. As defined herein, the magnetic pole is a north or south pole of a magnet, such as an electromagnet, a permanent magnet or a superconductive magnet, or any magnet surface where the field flux lines are to be controlled. The magnetic field emanates from one magnetic pole surface and terminates at the other magnetic pole surface. Typically, the magnetic pole surfaces are flat in the central portion which is close to an object to be photographed, for example, the body of a patent.
The magnetic flux density is commonly labeled B and the magnetic field strength is conventionally labeled H (each being vector quantities having direction). The uniformity of the magnetic field depends in part upon the uniformity of the separation between the magnetic pole surfaces, the homogeneity and permeability of the pole material, and the pole correction (shim) method, i.e., adding or subtracting small rings or buttons of ions on the pole surface. For cylindrical or polygonal poles with flat surfaces the concentricity of the cylinders (or polygons), and the separation and parallelism of the pole surfaces, must be precisely controlled to produce a uniform field. Thus, the pole holding structure must be controlled to hold the two magnetic pole surfaces at a precise distance apart, at a precise concentricity, and at a precise angular orientation (usually parallel).
Also, such a pole holding structure must be constructed to accommodate other auxiliary magnets used to shim and confine the various magnetic fields. One such feature concerns the flux return path. There is a need for an efficient design of a structure which minimizes the volumes of permanent magnets and the material (such as steel) surrounding them while still providing a given uniform field. Another need is a structure that allows adjustment of the field strength of the individual poles, or “pole strength matching”. Since the flux (B) lines have no end points—the lines form closed loops—use of materials, relative physical sizes and orientations of structures affecting any part of the flux lines must be controlled to provide a given field strength and homogeneity. The support and positioning structures are adjustable, allowing fine adjustment of the separation and parallelism of the pole surfaces. This is necessary, for example, to compensate for manufacturing tolerances.
Table 1 illustrates the comparison of three types of permanent magnetic circuits for MRI systems. In particular, characteristics of a tunnel-like magnetic system of the prior art, an ordinary magnetic system of the prior art and an embodiment of a magnetic system according to the present invention are tabulated in Table 1. From Table 1, it can be understood that the configuration becomes complicated to meet the requirements according to the development of a new technique.
TABLE 1
Type of magnet system
Tunnel-like
Ordinary magnet
Novel magnet
Characteristcs
magnet system
system
system
Magnetic circuit
Size of mag-
Small
Medium
Small
netic circuit
Tolerance for
Small
Large
Medium
magnetic
properties
Easiness for
Difficult
Easy
Easy
assembly
Flux leakage
Small
Small
Very small
Temperature
Large
Medium
Medium
dependence
Magnetic field
No shielding
Medium
Large
shielding
Sensitivity for
Large
Medium
Small
steel
Tuning of
Impossible
Easy
Easy
homogeneity
at hospital
In magnetic systems where the field distribution is determined by the magnetic pieces alone, it is difficult to achieve accuracy with field errors in the order of 1/100 or 1/1000. Thus, methods and techniques which achieve these small errors use high permeability material together with “tuning” or adjusting the assemblies to the required degree of precision.
In order to create a uniform magnetic field, a permanent magnet, steel (or other materials) surrounding the permanent magnet, a shim and a support structure must be effectively designed. A patient must also be able easily to access the magnetic field generating device, and therefore the size of a structure which surrounds the body of the patient must be reduced. A method must also be provided for raising the uniformity of the magnetic field and for matching the magnetic field strength from each magnetic pole with a smaller permanent magnet.
A need exists for a magnetic field generating apparatus for a magnetic resonance imaging (MRI) system with an improved structure in which a permanent magnet shim and a booster shim are included.
SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention, there is provided a magnetic field generating apparatus for an MRI system adopting a C-shaped open magnet support structure. The apparatus includes a pair of polygonal permanent magnets for creating a primary magnetic field, disposed parallel to each other in the horizontal plane, with a predetermined imaging area therebetween. A pair of polygonal magnetic pole plates are stacked to face each other on the inner sides of the pair of permanent magnets, respectively, for making the magnetic field produced from the permanent magnets uniform in the imaging area. The permanent magnets are fixed to a pair of yokes. At least one column connects the two yokes at one of their respective ends, with the imaging area being interposed between the yokes, wherein the at least one column together with the pair of yokes forms a closed path of the magnetic field. The apparatus further includes shims, having a polygonal shape, disposed at the vertexes of each of the polygonal magnetic pole plates, wherein the shims of one of the polygonal magnetic pole plates face those of the other polygonal magnetic pole plate. Peripheral permanent magnets are disposed along the edges of the polygonal magnetic pole plates, for inducing a magnetic field produced from the permanent magnets into the imaging area, wherein the peripheral permanent magnets of one of the polygonal magnetic pole plates face those of the other polygonal magnetic pole plate. Booster shims, which assist the function of the peripheral permanent magnets, are arranged in the spaces between adjacent peripheral permanent magnets disposed at the vertexes of the polygonal magnetic pole plates.
Preferably, the permanent magnets are formed of an alloy of neodymium-iron-boron (Nd—Fe—B) and the magnetic pole plates are formed of steel.
In more particular embodiments, the shims comprise positive and negative shim rings. The positive shim rings have a polygonal shape and are disposed at the edges of the polygonal magnetic pole plates, wherein the positive shim ring of one of the magnetic pole plates faces that of the other magnetic pole plate. The negative shim rings are embedded at the edges of the polygonal magnetic pole plates, at the inner sides of the positive shim rings, wherein the negative shim ring of one of the magnetic pole plates faces that of the other magnetic pole plate. Preferably, the positive shim rings are formed of low-carbon steel.
In another preferred embodiment, the magnetic field generating apparatus further comprises magnetic clamps disposed around the peripheral permanent magnets, being fixed to the yokes, for preventing the magnetic flux from leaking through both the sides of the magnetic pole plates, together with the peripheral permanent magnets. The magnetic field generating apparatus comprises, in additional preferred embodiments, permanent magnet shim bricks on the positive shim rings. According to still other preferred embodiments, the magnetic field generating apparatus further comprises a plurality of cap screws for fixing
Holsinger Ronald F.
Kim Jung-hoe
Lee Kang-suk
Moon Chang-wook
Mun Chi-woong
Arana Louis
Law Offices of Eugene M. Lee, PLLC
Samsung Electronics Co,. Ltd.
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